The Latitudinal and Yearly Variation in the Timing of Microsporogenesis in Alms, Re- Tula and Corylus

Hereditus 104: 231-243 (1986)

The latitudinal and yearly variation in the timing of microsporogenesis in Alnus, Betula and Corylus

ALP0 LUOMAJOKI

The Finnish Forest Research Institute, Department of Forest Genetics, Helsinki, Finland

LUOMAJOKI, A. 1986. The latitudinal and yearly variation in the timing of microsporogenesis in Alms, Re- tula and Corylus. - Hereditas 104: 231-243. Lund, Sweden. ISSN 0018-0661. Received May 10,1985

The timing of the tetrad phase of inicrosporogenesis in five Betulaceae species growing in natural condition5 in Finland was studied by calendar days and by period unit and degree-day heat sums. Latitudinal variation and variation between successive years were examined. To account for the variations found, models using heat sums. critical daylength and joint effects of daylength and temperature sums were tested. The total lengths of meioses and tetrad phases were measured in four tree species. Variation in the phenology of tree species and the underlying different timing systems are discussed with reference to the annual cycle of the generative development in trees. The tetrad phase of the microsporogenesis was usually reached in August in all the species studied. There were no significant differences between the species studied, but latitudinal differences were significant. In contrast to previously studied species (the meioses of which occur in the spring), the meiosis on site occurred carlier the higher the latitude. Within Finland a latitudinal range of about 10 days was observed. Moderate variation between years and high dispersion within trees and populations were also characteristic of Be- tulaceae. Neither a simple temperature-sum or critical-day timing model nor a model on the joint effect of daylight and the temperature could account for all the variation found. Duration of daylight is found to be the most important factor in the timing of Betulaceae meioses. Temperature is considered to be a modifying factor here in contrast to meioses in previously studied species in which the temperature was the main factor. This difference in timing systems is found to exist in adaptative strategies between different parts of the annual cycle rather than between tree species or genera.

Alpo Luomajoki, The Finnish Forest Research Institute, Department of Forest Genetics, Unioninkatu 40 A, SF-001 70 Helsinki 17, Finland (present address)

In the northern temperature climate there are large 1969). Some conifer meioses also begin in the au- differences in the timing of meioses between various tumn but the meioses are completed no earlier than tree genera. The most essential feature of the timing the next spring. The PMCs have thus to overwinter is the existence of different timing patterns. In a re- dormant in pachytene or diplotene. This type of view by ANDERSONet al. (1969) most of the early lit- timing was recently studied in Larix (ERIKSSONet al. erature on conifer meioses can be found. The con- 1970b; OWENSand MOLDER1971, 1979b; SARVAS ception of the timing of Pseudotsugu meiosis has 1972; HALLand BROWN1976; HALL1979; LUOMAJOKI since changed, and more detailed information of the 1982, 1984), in Pseudotsuga (OWENSand MOLDER other species is now available. 1971), in Thuju (OWENSand MOLDER1971; SIMAKet Most conifer meioses take place in the spring. Re- al. 1974), and in Tsugu (OWENSand MOLDER1971, cent literature is available of Abies (LUOMAJOKI1975). 1982, 1984), of Pica (ERIKSSON~~al. 1970a; SARVAS Following the suit of most conifers, the meioses in 1972; MOIRand Fox 1976; OWENSand MOLDER many deciduous species also occur in the spring. 1979a, 1980; ROZHDESTVENSKII1981; SINGHand Populus has been observed by POSPISIL(1966), EK- OWENS1981; LUOMAJOKI1982, 1984), of Pinus (EK- BERG et al. (1967) and LUOMAJOKI(1984), Quercus by Btmet al. 1972; Ho and OWENS1974; OWENSet al. TucoviC and JOVANOVIC(1970) and Urnus by 1981; LUOMAJOKI1982, 1984) and of Pseudolarix DAHLGREN(1915) and REDENBAUGHet al. (1980). (MFKGEN1977). In several deciduous genera the meioses occur Meiosis in a few species both begins and is com- after mid-summer, i.e., in July or August, or even pleted during the autumn (see ANDERSSONet al. later: This has been observed in Betula by WOOD- 232 A. LUOMAJOKI Hereditns 104 (1986)

WORTH (1929), SOKENSEN (1941) and LUOMAJOKIature an equally dominant factor for the timing of (1977,1982) and in Ahus and Corylus by LUOMAJOKImeioses in Alnus, Betulu, Corylus as it was for the (1977, 1982). meioses that occur in the spring (e.g., Abies, Picea There have been few attempts to find a quantita- and Pinus)? tive basis to the timing of tree meioses. One of the The lengths of meioses in the species under study reasons for this was the lack of a suitable heat-sum had also to be appraised, because there is probably system for simulation of development according to no information of these available. This is also the temperatures. Such simulation was much facilitated situation concerning the length of the tctrad stage. after SAKVAS(1972) created his period unit system. Knowing both variables mentioned, the relative du- The definition of the "period unit" is presented in ration of thc tetrad stage could be calculated the footnote of Table 2. For a more complete de- scription see SARVAS(1972). Careful simulation under natural conditions also calls for monitoring temperatures locally at treetop-level and frequent 2'0" 25" 30" sampling of generative buds (SARVAS1972; 1 LUOMAJOKI1977, 1982). 69' The timing of the successive stages of tree meioses was studied in calendar time by LUOMAJOKI (1982). A more physioecological view was then 68" taken by LUOMAJOKI(1984) in terms of simulation the meiotic development in degree-day and period 67" unit heat sums. It was not only found that the male 67" generative development in the spring followed closely the temperatures, but that the so called zero 66" point problem was the worst source of error in the 66"- simulation of development. While the simulation of development itself was rather accurate, the yearly p.u. (period unit) sums covered periods of winter 6 5' dormancy as well as the "active period" (see SAKVAS 1972) of development sought after. The photoperiod has been shown to have an in- 64' creasing influence on the timing of various vegetative phenomena towards the end of the growth period (DORMLINGet al. 1968; SMITHBERGand WEISER 1968; ROCHE1970; ANDERSON1974; HABJ0RG 1978; FUCHIGAMIet al. 1982; JUNT~ILA1982; KOSKIand SEL- KAINAHO 1982; KOSKIand SIEVANEN1985). It is therefore reasonable to consider the possible effects of the photoperiod on the generative development and on the timing of male meiosis in Betulaceae in par- 61' { ticular. The geographical and yearly variations in the timing of male meioses in the spring were found to be very large (LUOMAJOKI1984). In the same year the meioses in northern stands of Finland lagged the meioses of southern stands by as much as four Fig. 1. The localities where microsporogenesis materials weeks. were collected. One to six experimental stands (see Table In this paper the relevant temporal variations in 1) were studied at each locality. These places were in lati- Betulaceae will be appraised and compared to tudinal order: 1 Bromarv (60'02'; annexed in 1977 toTen- hola parish), 2 Tuusula (60"21'), 3 Punkaharju (61"48'), 4 species previously studied. Is the variation equally Kerimaki (61"51'), 5 Viippula (62"03'), 6 Rovaniemi rnlk. large, and are the geographical correlations similar (66"21') and 7 Inari (69"04'). The distance between locali- to meioses that occur in the spring? The relevant ties 1 and 7 is ca'. 1050 krn. Localities 1-5 were grouped to- physioecological factors, daylength and tempera- gether as southern Finland and localities 6 and 7 as ture, are evaluated as far as possible. Is the temper- northern Finland. Hereditas 104 (1986) VARIATIONIN TIMINGOF MICROSPOROGENESIS 233

Tab/? 1. Origin, age and elevation of the sample stands

Species Site, plot Origin Age in 1970 Elevation (years) (4

A/nus glic/rnosa Tuusula3 Local 100 50 A. iticana Inari, Toivoniemi' Local Ca. 50 148 Kerimaki 540' Local 52 101 Punkaharju LXII Local 44 81 Tuusula XLI Local 34 40 Betuh pendula Bromarv VI Local Ca. 40 5 Kerimaki 5432 Local 66 86 Punkaharju LIV Local 59 90 Rovaniemi XXVlIl Local 44 251 VilppulaV Local 38 150 Inari. Toivoniemi' Local Ca. 40 156 Punkaharju XIV Local 43 85 Punkaharju L" Local 70 90 Punkaharju LX Local 38 83 Rovaniemi XVII Local 123 170 Tuusula 12 Local 52 45 Vilppula 153 Local 68 120 Corylrc uveNaria Bromarv v Local Variable 7 Punkaharju' Finland, Bromarv Unknown 90

I Nut it regular, numhered plot. The numbered plots are sample stands of Dept. Silviculture. Finnish Forest Research Institute 'The srnnd wacut down: Kerimaki 540 in 197S77. Kerimhki 543 in 1978, Punkaharju L in 1974-75

Material and methods The material was mainly collected between July meters below the treetop-level, or in all from a 1964 and August 1971 and it was extended in July population of ten trees, daily, 1-6 times a day, de- and August 1983. The seven localities where collec- pending on air temperatures. At Punkaharju 21 tions were made range from Bromarv (60'02') in the meter hydraulic aerial ladders on a tractor were very south to Inari (69'04') in the north (Fig. 1). This useful. At other locations conventional ladders study concentrates on Betula pendula Roth and Be- were used. tulapubescens Ehrh. Ahus incana (L.) Moench was The catkins were sliced (one half being discarded) also studied both in the north and in the south of and put in a fresh fixative of 25 vol. Yo of glacial ace- Finland. Alnus glutinosa (L.) Gaertner and Corylus tic acid and 75 vol. % of absolute ethanol. (This fix- avellana L. were studied only locally (names are ac- ative contains less acetic acid than that used for con- cording to Flora Europaea, TUTINet al. 1964). The ifers.) A pooled sample from each fixation was sub- southern and northern individual trees studied be- sequently prepared using about ten anthers from long broadly to the same species. the middle part of each catkin. The anthers were The origin, age and elevation of the 19 sample macerated in 1N HC1 at 58°C for 5 min, transferred stands are shown in Table 1. Four of the trees into distilled water, and squashed into acetic orcein. studied in this paper were also used in author's two 400 PMCs were inspected under the microscope previous papers (LUOMAJOKI1977, 1982). Alnus from each sample of 4 slides. glutinosa was not involved in the previous studies. Sometimes every stage of meiosis was identified The material consists of two parts, the material but sometimes only the PMCs in pre-tetrad and tet- collected between 1964 and 1967 and that collected rad phases were observed. In that latter case macer- between 1968 and 1971 and supplemented in 1983. ation could be dispensed with. In this study mainly The newer part of the material was collected as de- the tetrad stage is pursued. To measure the duration ccribed earlier (LIJOMAJOKI1977,1982) and the older of the tetrad stage, also falling apart of tetrads (i.e., part as described by LUOMAJOKI(1984). In short, in rupture of the callose wall) into immature pollen the newer method 10-30 male catkins were excised was mostly observed. Four pollen grains were either from each of the individual trees within a few counted as equivalent to one PMC. 234 A. LUOMAJOKI Hereditus 104 (1986)

In the older part of the material (1964-1967) 5 991% Alnus incana catkins were excised from each of the trees in a Inarl. Toivoniemi: tree 1 population of 5-15 trees, daily, and fixed as de- 95 Tetrads: 8-14055 P.U. / scribed above. Each catkin was studied separately and the presence of PMCs in the tetrad phase was checked. The results were indicated with either a O/ minus or a plus sign. The overall result from one 0 tree ranged from no positive observations (015) to 80 0 five (5/S). 50 / The older method tends to give too high percentages of PMCs in the tetrad stage, because not all PMCs in the catkins where tetrads were observed had actually reached the tetrad stage. Too high percentages meant too small heat sums as read from the probability paper, and consequently too early 3-1 / dates. That is why a part of the old material was reinspected and average corrections were calculated for all of the species involved, as described by LUOMAJOKI (1984). The relevant correctons applied to the old results were: 110 period units for Betula + Fig. 2. Progress of the onset of the tetrad phase in Alnus in- species, +140 p.u.'s for Alnus species and +200 cana at Inari, Toivoniemi (69"04') in 1983. Cumulative p.u.'s for Corylus. These corrections in p.u.'s are normal distribution is represented as a straight line on usually temporally equivalent to half a day or less. Gauss integral/linear scale. Note the unequal daily p.u.- As described by LUOMAJOKI(1982) each set of data sums. The relatively large distribution visible already wit- on individual meioses (i.e., the number of PMCs in hin one tree is typical of Betulaceae. each meiotic phase in each sample) was transformed into a series of cumulative percentages. The percentages indicate also the proportion of PMCs in the tetrad phase including those PMCs that already had fallen apart to pollen. Similar percentages were also obtained for (immature) pollen stage. T~I~~~~:x-26060 P.u From each meiosis a series of ascending percent- S0.550 p.u ages of PMCs in the tetrad stage was obtained. These percentages were placed on probability paper, where the ordinate is a Gaussian integral 50 40 while the abscissa is linear. The abscissa was utilized 30 as a scale of the period unit heat sums, which simu- 20 late the progress of meiosis. The normal distribution is represented as a straight line on the probability paper. A line was consequently fitted to the dot cluster of percentages of PMCs in tetrad phase on the frequency net. The mean could then be read on the abscissa at the SO YO point, and further, the standard deviation could be read as the difference between 84 % and 50 YO Fig. 3. Progress of the onset of the pachytene and tetrad points along the abscissa (see Fig. 2 and 3). phases in Betula pubescens at Punkaharju, stand L, tree The p.u. heat sum value that coincides with the no. 40 (61"48') in 1970. Gauss integral/linear scale. The in- terval pachytene-tetrads is in this example 1180 p.u.'s, 50 O/O point of the PMCs in the tetrad stage was taken as the beginning of the tetrad phase, and the while the average length of the period leptotene-tetrads (i.e., the whole meiosis) was estimated at 1350 p.u.'s (both point where % of tetrads had fallen apart to (im- 50 without direct radiation correction; cf. Table 3). The stan- mature) pollen was taken as the end of the tetrad dard deviations can be appraised only approximately stage. The definitions of the beginning and ending owing to the considerable dispersion of observations, but of a phase were thus similar to those used previ- the variation seems to grow slightly towards the tetrad ously. phase. Herditus 104 (1986) VARIATION IN TIMING OF MICROSPOROGENESIS 235

Table 2. riming of microsporogeneses

Onset of the tetradstaeel Number of Calendar Period units Degree days (over +ST) Species Region observations timerange Mean SD CV(%) Mean SD CV(%) (standdyears) (dayslmonth)

Afiiirsglutinosu South 1 26.8 23 140 - - 874 - - A. incana North 1 6.8 14055 - ~ 495 - - South 5 7 .-20.8 24536 1893 7.7 914.8 71.3 7.8 Total 6 6.-20.8 22789 4602 20.2 844.8 182.9 21.7 Hetrrlapendrrla North 3 4.-12.8 17600 1151 6.5 666.0 50.5 7.6 South 10 8.-16.8 23854 1877 7.9 894.6 73.1 8.2 Total 13 4.-16.8 2241 1 3222 14.4 841.8 120.3 14.3 8.pithescens North 4 28.7- 6.8 15969 2766 17.3 586.3 121.7 20.8 South 10 1.-17.8 25127 1483 5.9 946.2 68.0 7.2 Total 14 28.7-17.8 22510 4661 20.7 843.4 187.3 22.2 Corvius uvelIunu South 4 5.-26.8 24923 2833 11.4 935.8 105.2 11.2

’ 511 ‘in ot the PMCs have reached the tetrad stage Now: tivc period units (P.u.) are equivalent to the progress made in (generative) development at 10°C in one huur (SAKVAS 1972). The relevant regression (development rateltemperature) is non-linear (S-shaped) SD = \tandard deviation: CV = coefficient of variation

In this study, bi-hourly data from a network of The meioses proceed similarly without any con- mostly treetop-level meteorological measuring spicuous diffuse phases. The interphase (inter- points (123 stations with thermographs at best) kinesis) between reductional and somatic divisions crcctcd by the Dept. of Silviculture of the Finnish is relatively short. The microspores formed after the Forest Research Institute were used. At Toivo- tetrad stage also resemble each other in all thc niemi, Inari this asset was not available, and ther- species studied. There were no large differences in niograph data from an official meteorological sta- the timing of microsporogeneses among the species tion 1.4 km away were used. However, the trees in- studied. Accordingly, an alphabetical order of pre- spectcd as well as the meteorological station were sentation was chosen. situated on the same watercourse at comparable elevations. This leads to expect only minor devia- 1.1 Alnus species tions in temperatures between the place of the study and the meteorological station. Ahus glutinosa was studied only once, in 1965 at Thc duration of daylight was determined as the Tuusula. The population reached the tetrad stage in time span when the upper edge of the sun is over the average (50 O/O of the PMCs in the tetrad phase) Au- horizon (sunhours). The data were obtained from gust 26th at 23140 period units (p.u.’s in short) or the yearly almanac published by the University of 874 degree days (d.d.’s in short). While the two heat Hclsinki (ANONYMOUS1982) and from the Smithso- sums are of typical magnitude for Betulaceae nian tables (LIST1966). species in southern Finland, the date is the latest re- corded in any species together with one observation of Corylus. Ahus incana was more thoroughly covered with Results six yeadstand combinations, including one from northernmost Finland. This observed date from 1. Phenology of the microsporogenesis Inari is also the earliest in this species. The tetrad There is a strong resemblance in the microsporo- phase was reached at Inari August 6th in 1983 at genesis between the genera of the family Betulaceae 14055 p.u.’s or 495 d.d.’s (Fig. 2). The relevant ob- studied. The PMCs are small and initially stick rela- servations from southern Finland range temporally tively firmly together. For this reason the reduction from August 7th to 20th and in heat sums 22040 to part of meiosis had to be studied using maceration 26410 p.u.’s or 829 to 999 d.d.’s (more data in Table techniques. 2). The differences in the relevant heat sums be- 236 A. LUOMAJOKI Heredims 104 (1986)

Table 3. Total lengths of meioses

~ ~ ~~~ Species Actually measured Approximation of Estimated equivalent The equivalent part of meiosis until thelengthofthe in 1aboratoryp.u.’~’ in hoursat the tetrad stage period leptotene 20°C in starting from -tetrads in air laboratory’ temperature p.u.’s

AInus incana Pachytene 1275 1400 87 Betula pendulu Pachytene 1200 1320 82 B. pubescens Pachytene 1350 1480 91 Corylus avellana Pachytene 1690 1860 115

’ With direct radiation correction. Necessary correction in August was estimated at + 10 Yo according to comparative temperature measurements in buds and in air hy SARVAS (1972) This estimate servrs as a comparison for the many experiments made at this temperature

Tublr 4. Length of the tetrad stage

Species Number of Average length of the tetrad stage Estimated The equivalent measurements in air temperature p.u.’s equivalent in hoursat 20°C in laboratory in laboratory Mean SD CV(%) p.u.’sl

Alnus incana 2 565 92 16.3 620 38 Betulu pendulu 5 931 303 32.3 1030 64 B. pubescens 4 970 390 40.2 1065 66 Corylus avellana I 1000 - - 1100 68 Betulaceae2 12 89 1 315 35.4 980 61

I With direct radiation correction ’ Total of all ahove species tween northern and southern Finland are so consid- similar (not unlike Alnus incana), and the micro- crable. sporogenesis was somewhat earlier in the north, even if the temporal ranges did overlap. In heat sums the differences were more clearcut. The heat 1.2 Betula species sums in the north were much smaller, and the two A total of 13 meioses (yearslstands) were observed ranges were clearly differentiated. The d.d.-sum for in Betulapendula including three of Rovaniemi near B. pubescens at Inari was also relatively smaller, the Artic Circle. The Rovaniemi stands reached the being only 61 % of the local long-term yearly aver- tetrad stage August 4th to 12th at 16300-18490 age d.d.-sum, while the relevant figures in southern p.u.’s or 608 to 700 d.d.’s. The southern stands Finland ranged between 71 and 86 YO,with an avcr- reached the same developmental stage August 8th age of 79 %. to 16th at 19910 to 26100 p.u.’s or 742 to 986 d.d.’s (more parameters in Table 2; see also Fig. 3). 1.3 Corylus In Betulapubescens there are 14 observation periods (years/stands) available, including one from Corylus avellana thrives only in southern Finland. lnari. (Incidentally, the geographical limit of B. The four stands studied are all of the same origin, pendula is not quite as high and, therefore, com- the Punkaharju specimens having been transferred parative studies were not possible.) At Inari the tet- from Bromarv on the southern coast of Finland. rad phase in R. pubescens was reached early, July Corylus reached the tetrad stage August 5th to 26th, 28th, at only 12405 p.u.’s or 429 d.d.’s. At while the relevant heat sums ranged from 21500 to Rovaniemi (66”21’), the tetrad stage was reached 27900 p.u.’s or from 807 to 1052 d.d.’s. August 4th to 6th at 15760 to 19100 p.u.’s or 584 to 725 d.d.’s. In the south of Finland the relevant dates 2. The lengths of meiosis and of the tetrad phases ranged from August 1st to 17th at 23060 to 26910 p.u.’s or 850 to 1031 d.d.’s (see Table 2). The approximate lengths of meioses were also eval- In both Betula species the geographic trends were uated, even though the meioses were only measured Herediras 104 (1986) VARIATION IN TIMING OF MICROSPOROGENESIS 237

Betula pubescens Date of onset of the tetrad phase

\ \ linear regresslon \ __

0 \ --__critical daylength model \ \ (1st yearly 15 h day + 116 days) "'168y \ \

4' 200 204 208 212 216 220 224 228 232 236 240th sequential day . , , 2b I , I i,I '2'8 r I r , , , , I , 16 5 'd"'ij'"ii"'2i"'2~'"2$thmonthlyday July August

Fig. 4. Date of onset of the tetrad stage in relation to latitude in Betulu pubescens. The figure was compiled according to the sequential day scale; the monthly day scale is valid in non-leap years. The years involved were 1964,1966-67,1970-71 and 1983 (only 1964 was a leap year). Effects of altitudes (45-170 m) on geographic relationships have been ignored. Of the species studied, B.pubescens has the widest latitudinal distribution in Finland. The linear regression fitted into the observations (Y = 137.448 - 0.33715 X) exaggerates the timing of the tetrad phase at latitudinal extremes. Also shown is the critical daylength model: impulse at 15 hours of daylight and 116 days of subsequent development. -The two observations at the extreme left originate form 1983, an exceptionally early year. For a two-factor model, see text. from pachytene onwards, leaving the leptotene- being inequally divided into various species, the zygotene part of the meioses to be approximated by comparison had to be performed with data from presumed proportionality with other species. The southern Finland. In the south of Finland the differ- lengths of meioses in Alnus incana and Betula ences between species in heat sum averages were species were of the same order, while the meiosis in not significant, either in terms of period units or in Corylus was only slightly longer. In all of the Be- degree days. The species being so similar, all the tulaccae, meioses ranged from 1320 to 1860 p.u.'s species studied could be treated as statistically including the 10 Yo direct radiation correction homogeneous. (Table 3). These figures are comparatively high and For comparison purposes, the same parameters are nearly of the same order as for Pinus sylvestris. as calculated for all of the Betulaceae, were also cal- As could be expected from earlier experiences, culated separately for Betula pubescens. This the lengths of tetrad phases measured in this study species was chosen because it is well represented in also varied considerably (see Table 4). While the the material both for the south and north of Finland. measurements under natural conditions in individu- In the combined material for Betulaceac (38 al meioses ranged from 500 to 1350 p.u.'s, the aver- species/stands/years -combinations) the correlation age for all species, i.e., 890 p.u.'s, seems quite rep- between latitude and the period unit sum measured resentative. That is equivalent to 53-74 YO of the at the tetrad stage was significant and negative (r = lengths of the preceding meioses, while the most -0.8777), while the relevant figure for Betulu typical figure was 69 Yo. pubescens was still higher (r = -0.9468). The correlations between latitude and degree-day sums were of the same order (also significant). Thus in the all 3. Statistical evaluation of the timing species group, the correlation was only slightly Thc temperature sums at the onset of the tetrad lower (r = -0.8654), but significant and for Betula phase in the species studied were compared. The pubescens the same trend continued (r = -0.9306). northern heat sums being in all species smaller than The similarity in behaviour of p.u. and d.d. sums in the south and the observations from the north was not surprising as the correlation between the 238 A. LUOMAJOKI Hereditus 104 (1986) two kinds of heat sum was found as high as r = employing joint variables, i.e., daylengths and 0.9966 for all of the species and r = 0.9957 for B. temperature sums. Simple multiplying of local de- pubescens. gree days with local daylengths did not work, as the These calculations prove that the tetrad phase resultant sums were always much smaller in the was attained at much lower heat sums in the north. north. However, squaring local daylengths (sun- The latitudinal correlations were somewhat hours) and then multiplying them with the degree stronger for Betula than in the all species group. days or period units measured gave roughly equal The correlations between latitude and the date of sums for the whole of Finland. The subperiods on the onset of the tetrad phase were also calculated. which the calculations were based were pentads. The datcs had to be expressed sequentially from the The simple equation is beginning of the year. In the leap years 1964 and Y = Z h,2. t,, where 1968 the sequential day numbers were higher by Y = the photothermal sum one. This latitudinal-temporal correlation was h, = average duration of daylight within each found to be r = -0.6702 for all of the Betulaceae pentad and r = -0.7824 for Betulapubescens (both significant). This means that the completion of meiosis re- t, = degree days or period units cumulated within each pentad ally occurred earlier in the north (cf. Fig. 4).

The remaining few odd days were calculated separately and added to the total. Starting the calcula- 4. Modeling the timing of Betulaceae meioses tion from May 22th yielded a smaller variation than The large geographical differences found in heat longer or shorter total periods also tried. The period sums prevented simulation by just one average May 22th to each tetrad phase studied in Betula temperature sum. Fair simulation could be achieved pubescens was consequently 295400 “d.d.-based by introducing temperature sum as a function of squared photothermal units” on average, with a re- latitude. However, the regressions of required heat latively large coefficient of variation of 10.1 %. sums (degree days or period units) on latitude are Using period units instead of d.d.’s in the calcula- actually not linear. Linear models exaggerate the tions improved the accuracy. The relevant average difference between north and south, predicting sum of products obtained by using period units was dates too early in the north and too late in the south 7437000 “p.u.-based photothermal units” with a of Finland (see Fig. 4). coefficient of variation of 8.9 YO. Assuming that simple models can be instructive, modeling was tried starting from a critical daylength and counting then a standard number of days. The model is not linear, the latitudinal changes in the du- Discussion ration of daylight not being linear to begin with. 1. General inspection of the results This kind of impulse-type model allows for the earli- ness found in the northern latitudes. Critical day- The phenological peculiarities of Betulaceac mei- lcngths (sunhours) from 12 to 18 hours, in half-hour oses are that the geographical differences in the tim- increments, were tried in combined Betida data, ing of meioses are rather small, that the northern and with 15 hours the best fit was achieved (the least stands are earlier than the southern ones, and that sum of squared differences of days to observed on- the between-years variation is only moderate. set of the tetrad phase). In this way the onset of the These characteristics can be compared with those of tetrad phase can be expressed simply as the 116th previously studied meioses occurring in the spring. day after the day when daylight is a full 15 hours Thus in Picea abies (L.) Karsten, for example, the long for the first time. This occurs, e.g., April 9th at northern stands lagged the southern ones by 3-4 Inari (69’04’) and April 20th at Bromarv (60’02’). weeks (LuoMAJoKi 1984), while in Betula, to the The model so predicts the tetrad phase to be at- contrary, the southern stands were the slowest to tained in Betula on August 3rd at Inari and on Au- reach the tetrad stage. The relevant difference is in gust 14th at Bromarv (see Fig. 4). This is very realis- Betula about one weck, but to the opposite direc- tic, but the inevitable effects of temperature as a tion in comparison to Picea. However, when consi- modifying factor are absent. dering the northernmost stand of Betula pubescens The trial with the above model based on day- (no Picea stands were studied norther than at length encouraged calculations with models 68”01’), the nationwide geographical range in Betu- Hrndrfa\ 104 (lY86) VARIATION IN TIMING OF MICROSPOROGENFSIS 239 la was about ten days. WOODWORTH(1929) studied in the south did not allow simulation with one plain Betulaceae meioses, also including Betula pendula, heat sum. The largest absolute latitudinal differ- at Arnold Arboretum (U.S.A., Mass., 42”19’N.) in ences during spring were previously found in Pinus the first half of September. The timing pattern sylvestris L., in which the average difference be- seems perfectly logical: the lower the latitude, the tween northern and southern stands was less than later the meiosis. 500 p.u.’s or about 13.5 YO of the southern average. The between-years local variation in Picea can be (In Larix russica the small south-north difference of as large as three weeks (the range was 40 days) and 63 p.u.’s is equivalent to 18 YO of the southern in Lark russica (Endl.) Sabine ex Trautv. locally mean.) In comparison, the relevant average differ- more than four weeks (the range was 35 days; see ence in Betula pubescens was nearly 9200 p.u.’s or LCIOMAJOKI1984). In comparison, the local variation over 36 70 of the southern mean. in Betula pubescens is about two weeks (the range An impulse-type effect of a critical daylength was was 21 days). Betula pubescens had, together with also considered. The location of such an impulse to Corylus, the largest total variation, i.e., 21 days. In the beginning of meiosis as a threshold must be dis- the other species studied, considerably smaller carded, because the day is fully 24 hours long in the ranges were measured. north at the time of onset of Betula meiosis (i.e., no dark period at all). Moreover, this model could not explain the differences found between years, nor 2. Evaluation of methods the large variation in the timing of meioses within a 2.1 Heat sums population or within one tree (LUOMAJOKI1977; also Fig. 2 and 3). The location of the impulse in the In the simulation of the timing of meioses occurring springtime, very much prior to the meiosis, was during the spring the degree-day system (with a helpful in explaining the latitudinal differences and +S”C threshold) was found inferior to the period the lack of precise synchrony in the stands, but not unit system (LUOMAJOKI1984). Degree days were to- in explaining the differences between years. The tally inefficient in the simulation of the earliest model is only good in thermally average years (see springtime meioses because no d.d.’s had accumula- Fig. 4). Moreover, this model is hardly physiologi- ted before the tetrad phase was reached. Towards cally realistic owing to location of impulse as early as summer the accuracy of d.d.’s as simulators im- in late winter in the northernmost latitudes. proved owing to higher temperatures, but the ex- The models based on temperature alone or on the tension of ranges and variation were always in favor duration of daylight alone were not fully satisfac- of thc period units. tory, and so joining the two known principal factors In the present study the yearly heat sums are com- had to be attempted. The joint effects of daylight paratively large, and the practical difference be- and temperature sums were suggested by Nu’rroN tween the two heat sum methods was found to be SON (1948); FUCHIGAMIet al. (1982); KOSKIand SEL- small. If only yearly heat sums were needed, the KAINAHO 1982; and KOSKIand SIEVANEN(1985). The mort: laborious p.u. sums could be dispensed with. simple “Nuttonson-type” product sums could not However, when measuring short periods within simulate the early occurrence of meioses in the microsporogenesis, e.g., the duration of meiosis or north even if the long duration of daylight there thc tetrad phase, period units were found to serve helped to narrow the differences. However, squar- better. The p.u.’s were calculated bi-hourly (and ing the daylength (sunhours) before multiplying the sums so obtained can be interpolated) while with local degree day or period unit-sum produced d.d.’s are daily values. Low temperatures are also more equal photothermal sums nationwide. The takcn in account in the p.u. system. model was tried in Betula pubescens data because this species was observed in the widest latitudinal range. The large temporal variation of 21 days also warranted testing the model properly. 2.2 Timing models The starting point of May 22th is arbitrary, and Thc purpose of modeling Betulaceae microsporo- the equation Y = Zh,2.tp (see Results) does not have geneses was to compare effects of two known poten- an explicit physiological meaning. But the fair fit of tial factors. that of temperature and that of daylight. the formula indicates that the light factor is very Period unit sums were fairly good overall simulators likely more important for the timing of Betula of springtime meioses (LUOMAJOKI1984), but the in- meiosis than the temperature sum, which seems to equalities of Betulaceae heat sums in the north and be just a modifying factor. Because the relative sig- 240 A. IJJOMAJOKI Hereditas 104 (1986) nificance of the two factors utilized in the model yearly d.d.-sums. The relevant figures according to may change with time, no further conclusions on the yearly d.d.-sums given by KOLKKI(1966) are: an- effects of daylight or temperature can be made. This thesis in southern Finland (Tuusula) at ca. 19 YO and model broadly fits the microsporogenesis data avail- at Utsjoki at ca. 38 % of the yearly d.d.-sum. In Be- able, but the unexpectably large variation met tula pubescens the opposite was found to be true, as hardly justifies use of these two-factor models in the relative heat sum at tetrad phase diminished to preference to the simple critical daylength model, 61 YO at Inari (in the single case observed) from an for example. If the time of meiosis in either of the average of 79 Yo measured in southern Finland. Betula species has to be predicted, the critical day- (These latter figures are in KOLKKI'S(1966) scale.) lcngth model of 116th day after the first day of full It is evident that a sufficient heat sum require- 15 hours can be employed (cf. Fig. 4). In Betula ment protects early developing cells and organs pubescens data the average error between the pre- from spring frosts. Betulaceae meioses, for exam- dicted day and the actual day of onset of the tetrad ple, seem at first sight to be little affected by frost stage was 2.6 days. (In Betulapendula, respectively, problems. However, the immature pollen formed only 1.8 days.) In addition to random variation, the after Betula meiosis has to undergo two pollen temperatures contribute to the between-years vari- mitoses, has to mature and harden against frost (cf. ation. SARVAS1972) because the pollen has to overwinter The latitudinal linear correlation for Betula before anthesis in the next spring. This means that pubescens shown in Fig. 4 predicted the onset of tet- the Betulaceae meioses have to be completed early rad stage at an average error of 3.5 days, but the er- enough especially in the north. rors would rapidly increase when data outside the The systems that are based on duration of day- present latitudinal range were introduced. The two- light seem to be capable in taking care of the devel- factor model using duration of daylight and period opment to complete early enough, also in the north. unit sums reached a comparatively low accuracy, The simple models tried demonstrated the signifi- with an average error of 7.6 days. cance of the light factor. In experiments with tree seedlings the effect of shortening days seemed to increase towards the au- 3. Phenological symmetry the summer of tumn (KOSKIand SELKAINAHO1982; KOSKIand SIEVA- The conifer meioses studied previously (LUOMAJOKINEN 1985). This is not necessarily so when meioses 1984) behaved uniformly; the southern stands were are concerned, because limiting quantitative growth temporally considerably earlier. Accordingly, in the is not needed. Instead, the microsporogenesis has north of Finland the anthesis (which follows meiosis to be completed with all its necessary subphases. later in the course of microsporogenesis) in Scots The process itself is rather predictable because ma- pine regularly extends into July and exceptionally ture, fertile trees are involved. Consequently less into August. adaptiveness is needed, but the system has still to Some other phenological events,e.g., Betulaceae cope with unequal latitudinal growth periods. meioses, occur earlier in the north than in the south. The late summer development being earlier in the This means that two phenological events that are north is presently less self-evident than the lagging temporally well separated in the south, can come of the development in the north during the spring. rather close each other in the north. In fact, there However, early development in the north has been are likely two different timing systems that cause the observed also in conifer megasporangiate strobili inherently different time-tables of events to con- (KUPILA-AHVENNIEMIet al. 1980). verge in the north. A further measure of this convergence is the rela- 4. Reflections on photoperiodism in trees tive heat sum at an event at each geographical latitude. SARVAS(1966) found that the anthesis in In his interesting paper, Mi~ov(1956)found no ef- Scots pine occurred usually at nearly the same rela- fects of photoperiod in the flowering of pines even if tive d.d.-sum, i.e., the percentage of the d.d.-sum also very northern and southern species were measured at anthesis of the local yearly average studied. As far as the generative cycle was con- d.d.-sum, was largely invariable. However, at cerned he considered pines day-neutral plants. northernmost latitudes the relative d.d.-sum grew Similarly, SARVAS(1972,1974) concentrated into the rapidly from the ca. 17 O/O measured in southern generative cycle of trees (of which a large quotient Finland (and also in Central Europe) to ca. 32 % at were conifers) and did not pay much attention to Utsjoki. SARVAS(1966) did not give the source of his photoperiodic effects. In his model of the annual Hereditas 104 (1986) VARIATION IN TIMINGOF MICROSPOROGENESIS 241 cycle of forest trees he accordingly dispensed with kind of differences are weak or undetectable. Also, such effects and relied on simple thermoregulation. all provenancial differences found are not latitudi- LUOMAJOKI(1984) found no evidence of photo- nal (cf. KRUTZSCH,cit. by ERIKSON1982). periodic effects on springtime meioses of 11 tree In shrubs latitudinal correlations have emerged in species, of which 10 were conifers. This conclusion garden experiments in which different origins of the parallels experiences from tree anthesis studies by shrub species were under study. BANNISTER(1978) BASStTet al. (1961). found significant negative latitudinal correlation in Others, e.g., DORMLINGet al. (1968) and DORM- Calluna vulgaris flowering. The correlations found imci (1979) have studied the vegetative cycle of trees in Erica species varied and were less conspicuous. and found the photoperiodic effects to dominate in READER(1983) also found latitudinal correlations in ceasing of the growth. (Joint effects of light and Kalmia polifolia and in Ledum groenlandicum. temperature have been already discussed.) However, the northern origins being earlier in the The photoperiodic effects of light sometimes transplant garden did not mean that the northern seem to be absent, and at times they are found to origins had flowered on site earlier: in fact on the dominate. Most often the photoperiodic effects southern sites the flowering occurred earlier. were found in the vegetative cycle while they were As far as Betula is concerned, HABJBRG (1978) absent from generative development. However, found latitudinal, photoperiodic ecotypes in birch this condition can generate distorted ideas of real- seedlings. However, when the joint effects of temp- ity. The microsporogenetic development in those erature sum and daylight are considered, these veg- Betulaceae studied seemed to be governed by the etative birch ecotypes seem to be rather different joint effects of light and temperature. However, stages of photoperiodically almost similarly reacting after the microspores have overwintered, the an- geographical ecotypes than genuine photoperiodic theses in the following spring are probably only con- ecotypes. This is so because the photoperiodic reac- trolled by temperature (see SARVAS1972 on opening tions of seedlings change as a function of tempera- of Betula catkins). ture sums received and because the inherent temp- If the differently timed tree microsporogeneses erature sum requirements of the northern proveni- are considered it seems that the developmental ences are smaller than those of the southern prove- phases occurring in late winter or early spring do not niences (cf. KOSKIand SIEVANEN1985). need any kind of photoperiodic control. On the con- The latitudinal, provenancial characteristics of trary most generative processes that are initiated in the generative cycle of Betula are not known. How- late spring or early summer, seem to keep pace with ever, considering the facts just discussed, ecotypic the joint effects of daylight and temperature. After differences could probably not alone explain the the winter dormancy, generative development pre- conspicuous, inverse latitudinal development pat- viously controlled by the joint effects of light and tern found in microsporogenesis of Betulaceae temperature, seems to attain full autonomy and species. proceed according to temperatures. Most ovenvin- There were no clear differences in the timing of tering PMCs and also matured pollen cones and the microsporogenesis between the northern Be- male catkins are good examples of this. Further, any tulaceae species studied. Kinship of the genera development that has started autonomously in the Alnus, Betula and Corylus can be only a partial ex- spring, i.e., controlled only by temperature, seems planation. Very coherent timing was previously ob- to retain its autonomy into late summer. Scots pine served also within the genus Abies (LUOMAJOKI microsporogenesis from meiosis onwards is an 1984). Similarity in the timing of meioses in related example of this behaviour. species may lead to simultaneous or partly overlapping antheses and, therefore, has biological signifi- cance. If other premises for crossing of species are favourable, hybridization can be abundant, as is ex- 5. Concluding remarks perienced when different species of Abies are grown The probable effects of tree provenances should close to each other. In conifers, foreign pollen may, also be considered. In the present study it was not according to SARVAS(1970), also disturb fertilization possible to arrange an experiment to test the influ- by clogging up the micropylar cavities. Owing also ence of the provenance. As discussed earlier to differences in dormancy characteristics, antheses (LUOMAJOKI1984), latitudinal provenancial differ- of Alms, Betula and Corylus do not coincide. Both ences in the timing of development are evident in Betula species reach anthesis considerably later some tree species and in some other species this than the other species studied. Overlapping within 242 A. LUOMAJOKI Hereditas 104 (1986) the genus Betula is only slight, and hybridization is acclimation in temperate woody plants. - In Plant Cold Hardi- relatively rare owing to partial incompatibility ness and Freezing Stress. Vol. 2. Mechanisms and Crop implica- tions (Eds. P. LI and A. SAKAI),Academic Press, New York (HAGMAN1971). p. 93-1 16 HAGMAN, M. 1971. On self- and cross-incompatibility shown by Acknowledgements. -The laboratory work involved was done at Betula verrucosa Ehrh. and Betulapubescens Ehrh. - Commun. the Punkaharju Breeding Station of the Finnish Forest Research Inst. For. Fenn. 7.3(6):1-125 Institute, and also in 1983 at the Muddusjarvi experimental farm of HALL, J. 1979. Microsporogenesis in Larix laricina in eastern University of Helsinki, Inari. Newfoundland. -Bimonthly Res. Not. Can. For. Serv. 35: 19- I am deeply indebted to Mrs. Marja Lampisaari, Mr. Pentti Man- 20 ninen and Miss K. Elina Maattanen for their skilful technical assis- HALL, J. and BROWN, 1. 1976. Microsporogenesis, pollination tance. Mr. TimoYlitalo, ForestTechnician, has helped me with the and potential yield of seed of Larix in NE Scotland. - Silvae meteorological data. Mr. Veikko Silander, Forest Technician, has Genet. 25: 132-137 provided me with stand characteristics. The figures were drawn by H,&BJ0RG,A. 1978. Photoperiodic ecotypes in Scandinavian trees Miss Sisko Salminen. Typing was performed by Mrs. Sinikka and shrubs. - Meldinger Norges landbruksh@gskole57(33): 1- Luomajoki. Professor Max. Hagman gave me valuable advice. Do- 20 cent Veikko Koski, Ph. D. and Professor P. M. A. Tigerstedt have HO, R. and OWENS, J. 1974. Microsporogenesisand pollen forma- read the manuscript and have made many valuable suggestions. I tion in lodgepole pine. -Can. J. Bot. 52: 1669-1674 offer my thanks to them all. -This study was supported by a grant trom The Academy of Finland. JUNTTILA, 0. 1982. Thc cessation of apical growth in latitudinal ecotypes and ecotype crosses of Salix pentandra L. - J. Exp. Bot. 33: 1021-1029 KOLKKI, 0. 1966. Taulukoita ja karttoja Suomen Iampooloista kaudelta 1931-1960. - Liire Suomrn meteorol. vuosik. 65(Ia): 1-42 Literature cited KOSKI, V. and SELKAINAHO, J. 1982. Experiments on the joint effect of heat sum and photoperiod on seedlings of Retulu pen- ANDERSON. R. 1974. Seasonality in terrestrial primary producers. dula. - Commun. Inst. For. Fenn. 105: 1-34 - In Phenology and Seasonality Modeling (Ed. H. LIETH), KOSKI, V.and SIEVANEN, R. 1985. Timing of growth cessation in Springer, Berlin, p. 103-111 relation to variations in the growing scason. -Proceedings ofun ANDERSSON. E., EKBERC, 1. and ERIKSSON. G. 1969. A sum- international conference on miinaging forest tree.v us cultivated mary of meiotic investigations in conifers. -Stud. For. Suec. 70: plants. IUFRO division 2. S2.0100 (physiology), Finland 1984. 1-20 Crop physiology of forest trees, p. 167-193 ANONYMOUS 1982. Almanakka vuodeksi 1983. - Weilin and KUPILA-AHVENNIEMI, S., TAANILA, A. and HOHTOLA, A. Goos, Espoo 1980. Structure of the strobili of Scotch pine from initiation to BANNISTER, P. 1978. Flowering and shoot extension in heath opening. - Aquilo, Ser. 501. 17: 1-10 plants of different geographical origin. -J. Ecol. 66: 117-131 LIST, R. (Ed.)1966. Smithsonian meteorological tables. -Smitli- BASSET. I., HOLMES, R. and MACKAY, K. 1961. Phenology of sonian Misc. Coll. 114: 1-527 several plant species at Ottawa, Ontario, and an examination of LUOMAJOKI. A. 1977. Effects of temperature on spermatophyte the influence of air temperatures. - Can. J. Plant Sci. 41: 643- male meiosis. - Heredifas 85: 33-47 652 LUOMAJOKI, A. 1982. Temperature and dates of male meiosis in DAHLGREN, K. 1915. Uber die Uberwinterungsstadien der Pol- trees. - Hereditas 97: 167-178 lensacke und der Samenanlagen bei einigen Angiospermen. - LUOMAJOKI, A. 1984. The tetrad phase of microbporogenesis in Svensk. Bat. Tidskr. 9: 1-12 trees with reference to the annual cycle. - Hereditas 101: 179- DORMLING, I. 1979. Influence of light intensity and temperature 197 on photoperiodic response of Norway spruce provenances. - MERGEN, F. 1977. 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