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Influence of Fluctuating Temperatures and H2O2 Treatment on Germination of Leucospermum Cordifolium and Serruina Florida (Protea

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Influence of Fluctuating Temperatures and H2O2 Treatment on Germination of Leucospermum Cordifolium and Serruina Florida (Protea Influence of fluctuating temperatures and H202 treatment on germination of Leucospermum cordifolium and Serruria florida (Proteaceae) seeds G.J. Brits Vegetable and Ornamental Plants Research Institute, Pretoria Seeds of Leucospermum cordifolium (Salisb. ex Knight) Introduction Fourcade and Serruria florida Knight were germinated at 'Seeds' (achenes) of Leucospermum cordifolium (Salisb. ex constant temperatures of 4, 11, 18 and 25°C and also at Knight) Fourcade are dispersed annually at the end of the fluctuating temperatures that were made up of pairs of these temperatures over a 24 h period, the lower flowering season in mid-summer. They are released for many temperature of each pair being maintained for 16 h. An years after plants reach the flowering stage in their third or oxygenating treatment of soaking seeds in 1% H20 2 fourth year (Rourke 1972), apparently resulting in the accumu­ resulted in a slight increase in germination of L. cordifolium lation of a seed reserve in the soil. Seeds will apparently not seeds, without statistical interaction with temperature germinate in undisturbed vegetation until it is destroyed by conditions. Diurnal temperature amplitude had the strongest fire, after which dense, even-aged populations of seedlings effect on germination. An optimum low and high temperature of 9 and 24 oc respectively was calculated for emerge. This phenomenon has remained largely unexplained germinating seeds of L. cordifo/ium and 7 and 20°C for S. for L. cordijolium and other members of the Proteaceae with florida seeds. The daily surface soil temperature hard, nut-like seeds. The direct heat from fire apparently does fluctuations recorded in burnt mesic mountain fynbos not break dormancy in these seeds (unpublished results) unlike during winter was found to be of the same order as that in many members of the Leguminosae (Villiers 1972). required for germination. The temperature conditions Since seeds are insensitive to light (Horn 1962; Brown & Van recorded in unburnt, lightly or heavily shaded soils did not meet the germination requirements. A close relationship is Staden 1973), the improved illumination of burnt exposed soil therefore indicated between ecological conditions and would not influence germination. germination requirements in these two species. The breaking of dormancy in L. cordijolium seeds was S. Afr. J. Bot. 1986, 52: 286-290 shown to be strongly correlated with low seasonal air tempera­ ture (Brits & Van Niekerk 1986). Low temperature require­ Sade van Leucospermum cordifo/ium (Salisbury ex Knight) ment alone, however, does not explain the recruitment of Fourcade en Serruria florida Knight is by konstante temperature van 4, 11, 18 en 25°C en by wisselende seedlings en masse only after the occurrence of fire . temperature, opgemaak uit pare van hierdie temperature oor The seeds of some plant species are known to germinate 'n 24 h periode, ge"lnkubeer. By wisselende temperature is only under widely fluctuating diurnal temperatures (Koller die laer temperatuur vir 16 h gehandhaaf. Oksiginerende 1972). Proposed ecological functions for this requirement behandeling deur sade in 1% H20 2 te week het tot 'n include a sensor system to ensure that waterside plants would geringe toename in kieming by L. cordifolium sade gelei, germinate only when stranded after a period of floating sonder statistiese wisselwerking met temperatuur-toestande. Daaglikse temperatuurskommeling het kieming die sterkste (Thompson 1974) and for recognizing newly formed light gaps be"lnvloed. 'n Optimum lae en hoe temperatuur van 9 en in the forest canopy (Vazques-Yanes & Orozco-Segovia 1982). 24 oc respektiewelik is vir kiemende sade van L. cordifolium Temperatures of 10°C for 16 h alternated by 20°C for 8 h bereken en 7 en 20°C vir sade van S. florida. Die daaglikse were found to be a more suitable regime for germinating the wisseling in oppervlakgrondtemperature, gemeet in seeds of Proteaceae than either constant temperatures or gebrande mesiese bergfynbos, was gedurende die winter higher alternating temperatures (Brown & Van Staden 1971, van dieselfde orde as die benodig vir kieming. Temperatuur­ toestande gemeet in nie-gebrande, lig- of swaar-beskadude 1973). The purpose of this study was to determine whether grond het nie aan die temperatuurvereistes voldoen nie. 'n seeds of L. cordijolium and Serruria florida Knight required Noue verband bestaan dus tussen ekologiese toestande en high temperature in addition to the known requirement for kiemingvereistes by hierdie twee spesies. low temperature in order to break dormancy. An answer to S.-Afr. Tydskr. Plantk. 1986, 52: 286-290 the problem would also have commercial implications since these two species are widely cultivated from seed for the Keywords: Diurnal thermoperiodicity, dormancy, ecology, production of cut flowers (Vogts 1976). germination, Proteaceae Hydrogen peroxide may have an oxygenating effect when Submitted in partial fulfilment of an M.Sc. degree at the imbibed by achenes of Proteaceae species (Brown & Dix 1985). University of Stellenbosch (Botany Department) It was found to have an effect on germination in L. cordi­ folium similar to that of other oxygenating treatments, e.g. G.J. Brits Vegetable and Ornamental Research Institute, Private Bag X293, scarification (Brits & Van Niekerk 1976). Increasing the Pretoria, 0001 Republic of South Africa oxygen tension with H202 in terminating L. cordifolium seeds strongly influenced germination, independently from seasonal Accepted 14 February 1986 air temperature (Brits & Van Niekerk 1986). The interaction S. Afr. J . Bot., 1986, 52(4) 287 between various temperature regimes and increased oxygen sunshine in midsummer was measured at: tension was also investigated in the present study. burnt site: 128,69 103 lux lightly shaded site: 28,12 103 ± 11 ,71 103 lux Materials and Methods heavily shaded site: 5,72 103 ± 2,07 103 lux Germination experiment Thermometers were placed at a depth of 5 mm in the soil Seeds harvested freshly from cultivated L. cordifo!ium and of the burnt exposed fynbos or in leaf litter under plants. S. florida plants were treated with hot water (55°C) for 20 Maximum and minimum soil temperatures were recorded min to reduce microbial growth on seeds (Bertie & Knox­ during five days per week in each of the sites from April1983 Davies 1983). Seeds were soaked in either distilled water or to July 1984. a 1OJo H20 2 solution for 24 h at 22°C before incubation (Brits & Van Niekerk 1976). After soaking, the gelatinous outer Results and Discussion pericarp of L. cordifo!ium seeds was rubbed off and seeds Analysis of transformed [angular transformation to correct were washed for 30 s in running tap water before incubation. non-normality in proportions (Snedcor & Cochran 1967)] Seeds were germinated at ten different temperature regimes. percentage data showed that, for seeds of L. cordifolium, Where temperatures were alternated the lower and higher treatment with H202 led to significantly (P < 0,05) improved temperature was maintained for 16 and 8 h respectively. The germination over untreated seeds. Mean germination in treated regimes were: and untreated seeds was 28,6 and 22,8% respectively (compare 1. 4°C constant 6. l1 °C x 18°C Figure 1A and 1B). The statistical interaction of H202 x 2. 11 oc constant 7. 18°C x 25 °C temperature treatments was not significant (P > 0,25). For 3. 18°C constant 8. 4°C x 18°C seeds of S. florida the difference between H202 treated and 4. 25°C constant 9. l1°C x 25 °C untreated seeds was non-significant (P= 0,44). Data for S. 5. 4°C x l1°C 10. 4°C X 25°C. florida were therefore pooled (Figure 1C) for the purpose of Alternating temperatures were obtained by transferring petri regression analysis. The dense fungal and bacterial growth on dishes of seeds from one to another of the four incubators seeds, observed in most petri dishes, may have contributed which were kept at constant temperatures. Seeds were incu­ to the difference in degree of reaction to H202 treatment of bated in petri dishes on a single sheet of Whatman no. 1 filter seeds germinated in petri dishes and in seed beds (Brits & Van paper which was kept damp with distilled water. A seed was Niekerk 1986). considered to have germinated when the radicle had emerged. The strong effect of diurnal temperature amplitude on Treatments were replicated three times with 50 seeds per germination percentage (Figure 1) suggests that seeds required replicate (petri dish) for L. cordifolium and 25 seeds per both low and high temperature to attain maximum germina­ replicate for S. florida. tion. It is therefore postulated that seeds of both species A viability test with tetrazolium (Brits & Van Niekerk 1976) require an optimum low (base) and an optimum high (ceiling) was carried out on L. cordifolium seeds, which indicated temperature for maximum germination. Regression analysis approximately 43% viability of these seeds. of data gave similar curved responses of germination in L. Mean daily temperatures of the regimes with alternating cordifo!ium and S. florida to varying mean daily temperatures temperatures were calculated using the following equation: and temperature amplitudes (Figure 2). Estimation of the Mean daily temperature = 2/ 3 (lower temperature) turning point (maximum) in each case allowed the calculation + 113 (higher temperature). of base and ceiling temperatures required for germination in The relationship between mean daily temperature, tempera­ the two species (Table 1). The high r2 values found indicate ture amplitude and germination percentage was determined close fits of regressions to data. by means of bivariate polynomial regression analysis. From It was assumed that L. cordifolium seeds in nature would these the turning points (maxima) were estimated of X1 (mean be covered only lightly by leaf litter or soil. This assumption daily temperature) and x2 (temperature amplitude) for the is supported by the known requirement of shallow sowing data of untreated L. cordifo!ium and the combined data of of seeds in seed beds to obtain maximum germination (V ogts S.
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