Reprinted from , Vol. 48, No. 2. Early 1967. Purchased by the Forest Service, U.S. Department of Agriculture, for official use.

PATTERNS OF UNDER NATURAL ENVIRONMENTAL CONDITIONS JOHN D. HODGES1 Ulli'Vcrsity of rVashington, Seattle, T,Vas/lillgton (Accepted for publication August 6, 1966)

Abstract. Net photosynthesis in six different conifers was studied under various natural environmental conditions. Changes in the pattern of photosynthesis on clear days, especially the midday decrease, are apparently primarily controlled by changes in water potential. In noble fir and Scots pine, water potential probably acts mainly through its influence on stomatal movement. In grand fir, Douglas-fir, hemlock, and Sitka spruce, however, some other mechanism, probably mesophyll resistance to CO2 diffusion, seems to play a more important role. Both mechanisms probably operate concurrently in all species. Daily variations in leaf water potential seem to occur primarily in response to changes in atmospheric moisture, or, more precisely, vapor pressure gradient from leaf to atmosphere. Variation in carbohydrate content, through its influence on solute concentration, may also influence leaf water potential.

INTRODUCTION tion in explaining ecological differences between The pattern of assimilation in forest dif­ species. Results of the measurements of photo­ fers markedly on days with different local synthetic rates and efficiency will be presented in conditions and, most often, the highest rates of a subsequent paper. assimilation occur on cloudy or overcast days MATERIALS AND METHODS (Polster 1950, Pisek and Tranquillini 1954, Gentle 1963, Helms 1963). On overcast days, material and environment net photosynthesis normally reaches a peak about Seedlings (2-0 stock) of six species-Douglas­ midday and then gradually decreases throughout fir (Pseudotsuga menziesii (Mirb.) Franco) , the remainder of the day. On clear days, there is grand fir (Abies grandis (Lindley)), western normally a morning peak followed by a marked hemlock (Tsuga heterophylla. (Rafinesque) Sar­ depression in photosynthesis and a secoGd, lower gent), Sitka spruce (Picea sitchensis (Bongard)), peak (Rabinowitch 1945, Polster 1950, Stocker noble fir (Abies procera), and Scots pine (Pinus 1960). The objective of this research was to silvestris (L.) )-were out-planted in plots along study intensively the pattern of net photosynthe­ north-south transects extending from deep within sis in conifer seedlings of the Pacific Northwest a 35- to 40-year-old Douglas-fir stand into an and, if possible, to explain these patterns by mea­ adjacent open area cleared of all in­ surement of various plant and environmental fac­ cluding herbaceous . The four most inter­ tors. The research was part of a larger investiga­ esting environmental situations were those des­ tion which was also designed to study and compare ignated as open (full exposure), outside stand photosynthetic rates and efficiency of seedlings of border, inside stand border, and deep inside stand west coast conifers under a wide range of natural (deep shade). The location designated as "out­ environmental conditions and to use the informa­ side stand border" was immediately adjacent to l Present address: Southern Forest Experiment Sta­ the outer margin of the Douglas-fir stand, while tion, Alexandria, Louisiana. the one designated as "inside stand border" was

Early Spring 1967 PATTERNS OF PHOTOSYNTHESIS 235 approximately 20 ft inside the stand. Thus the sample. The total needle mass sampled was kept envirollment at both locations was influenced by at about 0.5 g, ovendry weight. For all samples, the Douglas-fir stand, but the influence of the a single branch from the uppermost of stand was mllch greater at the location just inside branches which developed during the second grow­ the stand border. The location deep inside the ing season in the was used. Thus, seedlings stand was approximately 125 ft fr0111 the margin llsed fr0111 June 1963 to about June 1964 were on the stand. planted in January 1962 and those used from June Light intensity was often greater than 11,000 1964 until the end of the study were planted in ft-c in the open on clear summer days. Intensity January 1963. In all cases, the needles were fully at the stand border (outside) was normally in the developed and, depending on the species, 2-4 range of 1,000- 2,000 ft-c except in late afternoon inches of the branch tip were placed in the cuvette. when the area received direct for a short An automatic switching mechanism at the CO2 period. Inside the stand border the plants re­ analyzer permitted cyclic sampling at six separate ceived little direct sunlight and intensity was nor­ points. Since at least one point (line) was always mally less than 1,000 ft-c. Light intensity deep used to monitor ambient CO2 concentration, up inside the stand was normally less than 100 ft-c to five seedlings could be sampled at any . and seldom over 200 ft-c except during occasional The sampling procedure was varied so that at "sun flecks." Environmental factors other than all sample lines were placed in one location light were also modified by the Douglas-fir stand. in order to compare photosynthesis in different The effect of the stand on air temperature, as com­ species under the same environment. At other pared to the open area, was to reduce the maxi­ times the sample lines were placed along the en­ mum temperatures and to slightly increase the vironmental transect, and rates and patterns of minimum temperatures. At locations influenced assimilation were compared for the same species by the stand, relative humidity, especially on clear over a range of environments. Net photosynthe­ days, was higher than in the open. A more de­ sis was normally determined for a period of 3 tailed description of the environment at the dif­ days on each sample seedling after which the ferent locations will be presented in a subsequent foliage sample was removed and needle dry weight paper dealing with ecological differences between determined. Seedlings were normally used only the six species. once except at the location deep inside the stand Seedlings were also planted at locations inter­ where survival was poor. The investigation ex­ mediate in position between the open area and tended over a 2-year period beginning in June the stand border (outside), and between the 10­ 1963. Thus measurements were made in all sea­ cation inside the stand border and deep shade. sons of the year. A total of 390 seedlings was These seedlings were not studied intensively, how­ ' used in the investigation. ever, since their environment and rates and pat­ Plant factors which were measured in the terns of net photosynthesis were almost identical spring, summer, and fall of 1964, included leaf either to seedlings in the open area or in deep water potentiaJ,2 content of the foliage, leaf shade. temperature, and relative stomafal aperture. Leaf Experimental plantings were made in January water potential and sugar concentrations were 1962 and January 1963. Two transects of plots determined from foliage samples of the same.­ were established in 1962 and one transect in 1963. ling taken at periodic intervals throughout the The 1962 plots contained six seedlings of each day. An electric psychrometer, similar to the one species, while those established in 1963 contained described by Spanner (1951), was used for water 20 seedlings of each species. Thus 32 seedlings potential measurements. , reducing and of each species were planted at each experimental non-reducing, were determined colorimetrically by location. The study area was located on Charles a. modification of the method given by Bernfield Lathrop Pack Demonstration Forest which is (1955). Leaf temperature was measured con­ owned by the College of of the Univer­ tinttously by a thermocouple attached to the under sity of Washington and located near LaGrande, surface of the needle. On two occasions, field Washington. measurements of relative stomatal aperture were Measu,remen! of plant factors and processes made throughout the day and into the night. In addition, stomatal measurements were made in the An infrared gas analyzer was used for the mea­ surement of net photosynthesis during the day­ • Leaf water potential is the difference in free or chemical potential between pure water and the water light hours and of respiration at night. Gas contained in the leaf cells. The potential of pure water samples were collected from a small plastic (poly­ is zero. Therefore, the potential of water in leaf cells is propylene) cuvette which surrounded the foliage less than zero (a negative number). 236 JOHN D. HODGES Ecology, Vol. 48, No.2 laboratory under controlled conditions of light, 0 50 12 Light temperature, and humidity. All stol11atal measure­ ments were made by an infiltration technique 40 10 " described by Fry (1965). Measurements listed .... ., 8 above as well as the measurement of environmen­ Q. E tal factors were made concomitantly with mea­ ., 2 6 surements of CO2 assimilation. I- 4 MeaSlIre111ent of environmental factors Continuous measurements of light intensity, air 2 temperature, and relative humidity were made for the duration of the study. Light intensity was measured by means of selenium photocells and/or radiometers. Temperature and humidity were ... measured with wet-dry thermocouples and with hygrothermographs. All light intensity and co ;;. thermocouple temperature measurements were re­ E c corded on a 24-point millivolt recorder. Soil 0 moisture was determined by gravimetric sampling. 'E -I. RESULTS AND DISCUSSION 'iii .. < Patterns of net photosynthesis Under fully exposed conditions, the typical pat­ 12 2 4 6 8 10 12 2 4 6 8 10 12 tern of photosynthesis on overcast or cloudy days M N M was one in which net photosynthetic rates in­ Time creased to a maximum about noon, then either FIG. 2. CO2 assimilation for single seedlings under full exposure on a selected clear day in the summer months

Light (1964). Curves shown were chosen as typical for above = 2 conditions. V.P.D. vapor pressure deficit.

U 20 6 20 ;: decreased or remained constant' for an hour or e... .. Temp. 15 0 15 .. two and finally decreased (Fig. 1). On bright .., 10 10 '3:< sunny days in the growing season, assimilation i.. ... 3 :tJ .c§ x!" normally increased rapidly, reaching a peak at t 5 '" 5 u .c AM, i! '" about 9-12 then decreased until late after­ :::; : 6.0 0 noon when it again increased and reached a second r but much lower peak (Fig. 2). Patterns were Grand Fir 5.0 basically the same at all seasons of the year, but 4.0 in the colder months the period of photosynthesis was shorter, rates of net photosynthesis were 3.0 lower, and the depression on sunny days was less 2.0 than in the warmer months.

1.0 The patterns described above for exposed con­ ditions were typical for all six species. vVith ... "- 0.0 .. Scots pine, however, the midday decrease on clear E c -1·0 days was often not so marked as for the other 0 .!!;:: species (Fig. 2), and rates were much more vari­ ] -2'() able. Also with Scots pine, the average daily rate «:: -3.0 of photosynthesis increased after several days of sunny weather (Table 1), but with the other 12 .2 4 6 8 10 12 2 .. 6 .8 10 12 M N M species a progression of clear days resulted in a Time progressive decrease in total photosynthate pro­ FIG. 1. Typical patterns of net CO2 assimilation under duction. Furthermore, there was a noticeable full exposure on overcast days in the summer months. interaction between weather conditions and species Curves shown are for individual grand fir and noble fir in terms of assimilation rates. Species showing seedlings, but the pattern is representative of all six spe­ cies under similar conditions. V.P.D. = vapor pressure the highest rate on clear days (Scots pine and deficit. noble fir) did not necessarily show the highest Early Spring 1967 PATTERNS OF PHOTOSYNTHESIS 237 TARLE 1. Effect of successive days of clear weather on vironmental situations were invaluable in explain­ average rate of CO.) assimilation (same seedlings used ing the general patterns. Unless otherwise noted, throt1ghout-sumll1e , 1964) data which are cited are for the fully exposed environment. Av£'rnp;cdai lY rate of assimilation Average .\.vrrap;e (lOg! gUl/hr) No. days trmp<'raturo V.P.D. --------. --- with sun ('C) (10m H) Douglas-fir Noble fir Scot. pine Expla.na.tions of patterns --- 1...... 23 12.80 1.44 1.89 2.26 Much has been written concerning daily patterns 2...... 26 15.88 0.76 1.14 2.43 of photosynthesis and especially the midday de­ 3...... 28 20.13 0.59 1.04 2.60 4 ...... 26 15.63 0.52 1.07 2.76 crease in photosynthesis. The literature suggests at least nine possibilities that might account for the depression, as follows: (a) a daily decrease in rate on overcast days (compare noble fir and the CO2 content of the air; (b) bleaching of grand fir) Fig. 1 and 2). In general, net photo­ (Bohning 1949); (c) increased chlo­ synthesis in Scots pine, and to a lesser extent roplast temperature (Kozlowski 1957); (d) pho­ noble fir, appeared less adversely affected by tooxidative deactivation of enzymes (Rabinowitch sunny conditions than in the other species. 1945); (e) increased respiration (Kozlowski At the locations near the stand border (outside 1957); (f) accumulation of photosynthetic end and inside the stand) the basic patterns of net products (Thomas and Hill 1949); (g) stomatal photosynthesis were the same as for the open area. closure (Maskell 1928, StMelt 1935, N utman However, there were differences in rate of photo­ 1937) ; (h) increased mesophyll resistance to CO2 synthesis, timing of the peak rate of photosynthe­ diffusion (Gaastra 1959, Shimshi 1963, Bierhuizen sis, and in the magnitude of the depression on and Slatyer 1964); and (i) water deficit in the clear days. For all species except Scots pine and (Pisek and Tranquillini 1954). noble fir it was quite common for peak rates of In the present investigation, the first five of photosynthesis and total photosynthate production these possibilities could not account for the differ­ to be higher outside the border than in the open ences in pattern of photosynthesis on overcast and area even on overcast days. Also, the depression clear days. As for possibility (a), the CO2 con­ on clear days was usually not so great as in the tent of the atmosphere was remarkably constant open. Inside the border peak rates of photosyn­ and there was often a marked decrease in net thesis occurred later in the day and the depression photosynthesis even though there was no change on clear days, although still very apparent, was in CO2 concentration. The data for photosynthe­ much less than in the open. Daily photosynthate sis under various environmental conditions (along production by the seedlings was lower inside the the environmental transect extending from within stand border than in the open on overcast days the stand to the open field) indicate that sugges­ but, except for Scots pine and noble fir seedlings, tions (b), (c), (d), and (e) can also be eliminated was higher on bright sunny days. Short-term as major causes of the midday decrease. The fluctuations in rates of photosynthesis were much decrease was C0111mon at the location just inside more common at the stand border, especially in­ the stand border on clear days and sometimes oc­ side the border, than in the open due to greater curred deep inside the stand. This would strongly short-term variation in light intensity. As with suggest that bleaching of chlorophyll and photo­ the open area, daily weather conditions seemed to oxidative deactivation of enzymes were not factors control the basic pattern of net photosynthesis so in the depression because light at both locations that no great seasonal differences were apparent. was from a diffuse source, and intensity was less In the colder months, however, rates of net photo­ than 1,000 ft -c near the border and normally less synthesis were lower, the photosynthesizing period than 200 ft-c deep inside the stand. was shorter, and midday depression was less on \Vhen the data are considered in relationship to clear days. daily variation in temperature, there is little like­ Under deep shade the pattern of photosynthesis lihood that high temperature and in­ was controlled by variations in light intensity. creased rates of respiration (resulting from in­ Peaks of photosynthesis as well as rate were de­ creased temperature) could account for the ob­ termined by the occasional sun flecks which pene­ served decrease in assimilation. On August 17, trated the dense canopy. 1963, for example, at the location just inside the The discussion which follows is concerned with stand, there was a 5970 reduction in net assimila­ the general pattern of photosynthesis which was tion of Sitka spruce from 9 :30 AM (maximum common to all locations, except deep shade. As rate) until 3 :30 PM (minimum rate), after which will be seen, however, comparisons of rates and the rate again increased. During this period the pattern of photosynthesis under the different en­ air temperature increased only 3.0°C (15.0° to 238 JOHN D. HODGES Ecology, Vol. 48, No.2 18.00). Leaf tel11perature was never 111000e than centrations may affect photosynthesis through an 0.50 higher than air temperature. Uncler more influence on cell water relations. exposed conditions, increased temperature did re­ sult in some decrease in net photosynthesis due to Leaf water status and photosynthesis an increase in respiration. On several occasions, Of the possible explanations for the diurnal however, calculations were made of the reduction decrease in photosynthesis, (f), (g), and (h) are in net photosynthesis which could be expected all related in some way to the water status of the fr0111 the increase in temperature. Even under plant. They may either directly affect the status the most extreme conditions (hot summer days) of the water in the plant or be affected by it. The respiration could not account for all the reduction, last named possibility, water deficit, is a "catch and on cooler days increased respiration c0111monly all" in that the possible influence of the water accounted for less than 15% of the midday reduc­ status is recognized but no explanation is given as tion in assimilation. to how the water deficit acts to bring about a The possibility that reductions in the rate of reduction in photosynthesis. photosynthesis result from a clogging of the photo­ Since these possibilities are related to the status synthetic mechanism due to an accumulation of of the water in the plant, it was decided to first photosynthetic end products cannot be substan­ determine whether or not photosynthetic rates are tiated by the findings of this study. Sugar con­ related to water regime in the foliage. Vvater centration in the foliage did increase during the status was determined by periodic measurement period when rate of photosynthesis was decreas­ of leaf water potential with an electric psy­ ing; but normally, sugar content continued to chrometer. increase even after net photosynthesis began to In agreement with the work of Boyer (1964), increase in late afternoon (Fig. 3). Also, the rate photosynthesis was found to be well correlated of sugar accumulation was as high and sometimes with changes in leaf water potential. In Douglas­ higher on overcast mornings than on clear morn­ fir, grand fir, hemlock, and Sitka spruce correla­ ings. It is not implied, however, that sugar tion coefficients were 0.96, 0.95, 0.90, and 0.95 accumulation has no influence on patterns of pho­ respectively and were statistically significant in tosynthesis. As will be shown later, sugar con­ all cases. On sunny days, photosynthesis tended to decrease as leaf water potential decreased and 8 to increase when it increased (Fig. 4). The after­ c noon increase in water potential apparently began 0 7 += about an hour before the increase in 1:1 net photo­ ...... c 6 - 5 .. -.--,--.-,--.-,--,--,-,--.-.--, 4D U C e 0 .,[-10 (,) 5 -15 'I:., l-20 ... -25.. 4 c .0 D. Fir 4.0 o ;: .!!? 3.0 3.0 ·s .. 2.0.;; .s::. e2 .0 ",. 01 E 1.0 (\I' o c>1.0 g (,)1: 0.0 'E

6 ·:i-I.Oc(

12 FIG. 3. Photosynthesis and reducing-sugar content of M the foliage in single seedlings of Douglas-fir and grand Time fir on a selected representative day in July 1964. Pattern FIG. 4. Leaf water potential and CO2 assimilation in is basically the same for all species except that in Scots a Douglas-fir seedling on a clear summer day (1964). pine the change in sugar concentration appeared to be Relationship is basically the same for grand fir, hemlock, much less than for the other species. and Sitka spruce. Early Spring 1967 PATTERNS OF PHOTOSYNTHESIS 239 synthesis, but this could 1I0t he definitely estab­ in all species tested. However, this does not seem lished because of the limitatio1ls of the sampling probable. More likely, water potential is impor­ procedure. tant in the control of photosynthesis in all species The relationship of leaf water potential to photo­ tested, and the apparent difference between spe­ synthesis was not so obviolts in noble fir and cies is due to a difference in the mechanism by Scots pine as in the other species. Morning photo­ which water potential acts to control photosyn­ synthesis in noble fir decreased as water potential thesis. decreased (Fig. 5), after which there appeared to be an almost inverse relationship between the two. Stomalal and t11esophyll resistance \'Vith Scots pine there was normally a decrease to CO2 diffusion Leaf water potential could conceivably influence

e., patterns of photosynthesis through its influence !!! -15 on stomatal closure and/or mesophyll resistance

CI ;: to CO2 diffusion. Although the exact mechanism c: -20 of stomatal movement has not been definitely !0 II. -25 established, there is ample evidence that opening .. • of- and closing ultimately depend on increasing and 0 -30 decreasing the supply of water to the guard cells (Zelitch 1963). In the liquid phase of CO2 diffu­ 3.0 ... sion, control of photosynthesis could be exerted .t: 2.0 through resistance to gaseous movements into or 'E'" -- within the mesophyll cells. Changes in the water 1.0 regime of the mesophyll cells could have a marked c o influence on CO2 solubility and diffusion. Rawl­ 0.0 ings (1963) concluded that mesophyll resistance ] : -1.0 c was not constant, but was a function of water potential and could vary with . 12 2 4 6 8 10 12 2 To determine if either or both of these possible M N M Time resistances are involved in the midday reduction FIG. 5. Leaf water potential and CO2 assimilation in of photosynthesis, measurements of relative ­ single noble fir and Scots pine seedlings in early sum­ tal aperture were made in the field and in the mer, 1964. laboratory in a controlled environment chamber. Measurements were made in the summer and fall in photosynthesis during midday, and there ap­ of 1964 on foliage of the current year. In the peared to be a tendency for a slight diurnal de­ laboratory relative stomatal aperture was deter­ crease in water potential. A preliminary exami­ mined under conditions of decreasing relative nation, however, indicated no direct relationship humidity and increasing temperature. A marked between the two. difference in stomatal movemeilt was found be­ From these preliminary observations it might tween species. In one experiment the stomata of be concluded that plant water status, as evidenced grand fir were less responsive than those of noble by leaf water potential, is not an important factor fir (Table 2). Stomata of grand fir and Douglas­ in controlling the daily pattern of photosynthesis fir tended to close gradually while those of noble

TABLE 2. Relative stomatal aperture in four species as determined by infiltration and expressed in pounds of pres­ sure at various temperatures and levels of humidity (A single seedling of each species was used throughout the test. )

Relative Infiltration pressure (psi)h humidity Temperature Time" (OF) Grand fir Noble fir Douglas-fir Scots pine -----(%) 9:45...... 92 70 5.5 5.0 5.5 10.0 11:00...... 60 75 6.0 8.5 7.5 30.0 13:00...... 40 80 6.5 25.0 13.0 26.0 14:30...... 20 85 9.0 40.0 27.0 19.3 15:30...... 15 98 17.0 40.0 40.0+ 30.0 16:30 ...... 15 98 38.0 40.0+ 40.0+ 31.4

-Humidity and temperature were changed to next level immediately after the time shown. bA reading of 30-40 psi indicates that the stomata are effectively closed. 240 JOHN D. HODGES Ecology, Vol. 48, No.2

fir closed very rapidly once closure had begun. ., Bannister (1964) has also noted that all species :;.,'20 do not react in the same manner to internal water deficits in the leaves, stomatal closure occurring OJ C much sooner in some than in others. ..·: ., 't 30 CD:; ... - .. -. A second laboratory experiment on stomatal .. ::;: C .., " ... C> movement confirmed the findings of the first. In 7.0 :; OJ '" ... addition, grand fir showed a marked reduction in jlO :;; g' photosynthesis before there was any appreciable 6.0 ol!! change in stomatal aperture. In noble fir, how­

e\'er, the pattern of photosynthesis appeared to be ...... :: 5.0 more strongly related to stomatal movements E 4.0go (Fig. 6). E R.H.96"/t R.H.75% r·H.45°t R.H.20% :;::g 3.0 40 e ::: Nobl Fi r :§.. 2.0 .. (1)3 « x.-I( Grand Fir c0.: 0 1.0 o CII _D""" 2... '" 0 8 9 0 II 12 2 3 4 5 Grand Fir N ;: : Time ...... !:a. 10 FIG. 7. Photosynthesis, leaf water potential, and sto­ matal aperture in Douglas-fir and grand fir under field ------o conditions. Curves shown are for single seedlings on a selected representative day in late summer, 1964. 40 r: very little change in stomatal aperture (Fig. 7). Concomitant measurements of photosynthesis and ::::: ::: stomatal aperture were not made for the other . species, but the stomata of noble firand Scots pine ------ E I. __------2500 ft. c. . ft. c. were much more responsive to closure than were those of the other species. At night the stomata it: = of Scots pine and noble fir showed complete Ti me closure, but stomatal closure was not observed in FIG. 6. CO2 assimilation and relative stomatal aperture as determined by infiltration in noble fir and grand fir. grand fir. Douglas-fir was intermediate between A reading of 30-40 psi indicates that the stomata are the two extremes. Hemlock and Sitka spruce effectively closed. Results are for a controlled laboratory stomata showed some closure but not so much as test conducted in September 1964. Current-year needles Douglas-fir. from a single seedling of each species were used through­ out. These findings are consistent with the observed patterns of change in both water potential and Laboratory measurements of stomatal aper­ photosynthesis. In species where stomatal closure ture in Scots pine indicated that, under condi­ is of primary importance, it would be expected tions of decreasing relative humidity, the stomata that there would first be a decline in water poten­ may close and later open again. This possibly tial until the stomata closed. After closure, water occurs many times in the course of a day. Other loss by the plant would then be reduced so that researchers (Ehrler, Nakayama, and van Bave! water potential would again increase. Later, the 1965) have recently reported opening and closing stomata might open, followed by a second decline cycles of less than 30 min. Hemlock and Sitka in water potential due to increased transpiration. spruce seedlings were not used in the laboratory Fig. 5 shows that this is consistent with the pat­ experiments. Based on field observations, how­ tern of change of water potential in noble fir. In ever, it appears that stomatal movements in these Scots pine there is evidence that the same pattern species more closely resemble those of grand fir is followed, but that the stomata open and close and Douglas-fir than those of noble fir or Scots much more rapidly than in noble fir. This would pine. account for the more rapid short-term fluctuation Field tests of stomatal movement confirm the in photosynthesis in Scots pine as compared to laboratory observations. In Douglas-fir and grand the other species. It might also account for the fir there was a reduction in photosynthesis with net diurnal decrease in water potential being less Early Spring 1967 PATTERNS OF PHOTOSYNTHESIS 241 than in other species, and for the rate of photo­ -- Doullioa Fir synthesis being less affected by a progression of It---IIGra nd FI r 12 warm, clear days. During periods of stomatal Tolal closure transpiration would be reduced and partial Toiol 10 recharge might occur. To thoroughly investigate Red uclnll the relationship between water potential and pho­ ., 8 tosynthesis in this species woulcl require short CIt 2c term (5-10 min) sampling of both water potential '" 6 Reduclnll u ... and stomatal aperture throughout the photosyn­

Boon-Long, T. S. 1941. Transpiration as influenced (f';CCCl I'.ree/sa Link) und Rotbuchenbiiumcn (Fagus hy osmotic concentratiol\ and cell permeability. S;/7'1lIicn L.). Flora (Jena) 141: 237-270. Amer. J. Bot. 28: 333-343. Polster, H. 1950. Die physiologischen Grundlagcl\ del' Boyer, J. S. 19M. Effects of watcr stress on meta­ Stofferzcngung in Walde. Untersuchungen iiber As­ holic rates of cotton plants with open stomates. similation, H.cspiration und Transpiration unserer Plant Physiol. 39 (Supp.): XLIII. Hauptholzartcn. 13ayerischer Landwirtschaftsverlag Ehrler, W. L., F. S. Nakayama, and C. H. van Bavel. Gmbh. Miinchcn. 96 p. 1965. Cyclic changes in watcr halance and transpi­ --. 1955. Vergleichende Untersuchungen iibcr die ration of cotton leaves in a steady environment. Kohlcndioxyd-Assimilation und Atmung rler Doug­ l'hysiol. Plant. 18: 766-775. lasie, Fichte und WeYll1outhskiefer. Arch. Forstw. Fry, K. E. 1%5. A study of transpiration and photo­ 4: ()89-·714. synthesis in relation to stomatal resistance and in­ Rabinowitch, E. I. 1945. Photosynthesis and related ternal water potential in Douglas-iiI'. Ph. D. Thesis, processes. Vol. 1. Chemistry of photosynthesis, University of W:ashington. 192 p. chemosynthesis and related processes in vitro and Gaastra, P. 1959. Photosynthcsis of crop plants as in vivo. Interscience Publishers, New York. 599 p. influenced by light, carbon dioxidc, temperature, and --. 1951. Photosynthesis and related processes. stomatal diffusion resistance. Medd. Landbou. Wag. Vol. II, Part I. Interscience Publishers, N. Y. 1208 p. Nederland 59: 1-68. Rawlings, S. L. 1963. Resistance to water flow in the Gentle, S. W. 1963. The effect of local weather con­ transpiration stream, p. 69-84. In Conn. Agr. Exp. ditions on the assimilation of carbon dioxide by Sta. Bull. 664. Douglas-fir (Pscudotsllga 1II1'1I'::;I'S;;) CMirb.) (Fran­ Shimshi, D. 1963. Effect of soil moisture and phe·· co) in western Washington. Ph.D. Thesis, University nylmercuric acetate upon stomatal aperture, tran­ of \'\Tashington. 122 p. spira tion and photosynthesis. Plant. Physiol. 38: Helms, J. A. 1963. Seasonal patterns of apparent 713-721. photosynthesis in Psclldotsllga 1IIe/ldes;; (Mirb.) Fran­ Shreve, Edith B. 1931. Role of concentration in co, in relation to environment and silvicultural treat­ transpiration. Carnegie Inst. \'\Tash. Ann. Rpt. Year­ ment. Ph.D. Thesis, University of Washington. 237 p. book 30: 262-263. Kozlowski, T. T. 1957. Effect of continuous high light Spanner, D. C. 1951. The Peltier effect and its use intensity on photosynthesis of forest seedlings. in the measurement of suction pressure. J. Exp. Bot. Forest Sci. 3: 220-224. 2: 145-168. Maskell, E. J. 1928. Experimental researches on vege­ Stiifelt, M. G. 1935. Die Spaltiiffnungsweite als As­ table assimilation and respiration. XVIII. The re­ similations-faktor. Planta 23: 715-759. lation between stomatal opening and assimilation. Stocker, O. 1960. Die photosynthetischen Leistungen A critical study of assimilation rates in leaves of del' Steppen und Wustenpfianzen, p. 460-491. In W. cherry laurel. Proc. Roy. Soc. London B 102: 488­ Ruhland (ed.), Handbuch del' Pfianzenphysiologie, 5. 533. Thomas, M. D., and G. R. Hill. 1949. Photosynthesis Nutman, F. J. 1937. Studies in the physiology of under field conditions, p. 19-52. In J. Franck and Coffea arabaica II. Stomatal movements in relation to W. F. Loomis (eds.), Photosynthesis in plants. Ames, photosynthesis under natural conditions. Ann. Bot. 1: Iowa. 681-694. Yemm, E. W., and A. J. Willis. 1954. Stomatal move­ Philpot, C. W. 1965. Diurnal fluctuation in moisture ments and changes of carbohydrates in leaves of Chry­ content of ponderosa pine and whiteleaf manzanita santhemum maximu1Il. New Phytol. 53: 373-397. leaves. U. S. Forest Ser. Res. Note PSW 67. Zelitch, I. 1963. The control and mechanisms of sto­ Pisek, A., and W. Tranquillini. 1954. Assimilation matal movement, p. 18-42. In Stomata and water und Kohlenstoffhaushalt in del' Krone von Fichten relations in plants. Conn. Agr. Exp. Sta. Bull. 664.

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