Frequencies, Microclimate and Root Properties for Three Codominant Perennials in the Northwestern Sonoran Desert on North-vs.South-facing Slopes

At a site in the northwestern Sonoran Desert the percent groundcover for the C₃subshrubEncelia farinosawas eight-times higheron more arid 20° south-facing slopes than on 20° north-facingslopes at 820 m elevation, and was six-times higher on north-facingslopes at a 300-m-lower elevation, also the more arid condition.The ground cover of the C₄bunchgrassPleuraphis rigidadecreasedover 50% from 20° north-facing slopes to the more arid conditionsof a 36° north-facing slope, a 20° south-facing slopeand a 20° north-facing slope at a 300-m-lower elevation.The CAM leaf succulentAgave desertialso had greater ground coverfor the 20° north-facing slopes at 820 m compared with 520m. For these three codominants that averaged 58% of the totalground cover, the key for the relative frequency ofE. farinosawasapparently its greater root growth on the warmer slopes duringthe winter. The key for the other two species was most likelysoil water availability, especially during the seedling stageforA. deserti. The wetter soil conditions on 20° north-facingslopes at 820 m apparently led to individual plants ofP. rigidathatwere twice as large as on south-facing slopes. Thus root propertiesmay exert the primary influence on relative plant frequencyin this desert ecosystem for which soil temperature and wateravailability are crucial.Copyright 1997 Annals of Botany Company Agave deserti; Encelia farinosa; Pleuraphis rigida; rooting patterns; soil temperature; Sonoran Desert; water availability     

Influence of Temperature and Ageing of Ovules and Pollen on Reproductive Success in Trifolium repens L.

Four clones of Trifolium repens L., originating from Iceland,Denmark and Israel, were cultivated in controlled environmentchambers at 10, 14, 18 and 22°C. Seed set at 10°C was,on average, 45% lower than at 18°C. Generally, seed frequencieswere higher in the stylar ovule positions than in the basal.This polarization became more pronounced at low temperatures.At least 30% of the decrease in seed set was caused by inadequatepollen tube growth. This was further supported by in vitro studieswhich showed that pollen developed significantly shorter pollentubes at low temperatures and a strong positive correlationbetween the capacity of male parents to induce seed set in basalovule positions and the length of pollen tubes in vitro. Femaleage, determined by days relative to anthesis, was also foundto have a significant influence on seed set, i.e. when pollinationwas delayed until the fifth day of flowering, seed set decreasedby an average of 60% when compared to florets pollinated onthe first day of anthesis. Pollen age did not influence seedset. Ageing of pollen decreased in vitro germinability, tubeelongation and the percentage of pollen tubes with sperm cells.Seed set, pollen germinability, pollen tube lengths, and abilityto form sperm cells in vitro varied significantly among clones.Copyright1994, 1999 Academic Press Trifolium repens, white clover, temperature, pollen tube, sperm cells, ovule fertility, seed set     

Physiology of Post-Pollination Exudate Production in Acacia

Marginson, R., Sedgley, M. and Knox, R. B. 1985. Physiologyof post-pollination exudate production in Acacia—J. exp.Bot. 36: 1660–1668. Stigmas of A. wattsiana produced 1260 nl of secretion by 90min after pollination. The optimum temperature for post-pollinationsecretion in A. baileyana, A. brownii and A. iteaphylla was20°C. Both self and cross intraspecific, interspecific andintergeneric pollinations produced a similar positive responsein A. iteaphylla and A. baileyana. In all cases aged pollenwas as effective as fresh pollen despite reduced fluorescencein the fluorescein diacetate test. Live yeast cells, fixed chickenerythrocytes, glass beads, talc and Biogel P150 did not stimulatesecretion, nor did pollen which had been washed in water orethanol despite high retention of viability in some cases Pollendiffusates dried on to glass beads produced a positive responsein A. iteaphylla and A. gracifolia. Intraspecific and interspecificpollinations involving fresh Acacia pollen resulted in pollengermination on the stigma and pollen tube growth in the styleof A. iteaphylla and A. baileyana. In contrast washed pollenfailed to germinate and pollen germination and tube growth werereduced at 35 °C as compared with 20 °C. Key words: Acacia, pollination, secretion     

Fruit Number in Relation to Pollen Production and Viability in Groundnut Exposed to Short Episodes of Heat Stress

Hot days and warm nights are important environmental factorslimiting fruit yields of groundnuts in the semi-arid tropics.The objective of the present research was to quantify the effectsof short episodes of heat stress on pollen production and viability,and fruit yield. Plants of cultivar ‘ICGV 86015’were grown at a day/night temperature of 28/22 °C from sowinguntil 9 d after flowering. Cohorts of plants were then exposedto a factorial combination of four day (28, 34, 42 and 48 °C)and two night (22 and 28 °C) temperatures for 6 d. Thereafter,all plants were maintained at 28/22 °C until final harvest9 d later. Number of flowers per plant (FN), the proportionof flowers setting pegs (fruit-set), the number of pegs andpods per plant (reproductive number, RN_t), pollen productionper flower and pollen viability were determined during the 6d stress period. There were strong negative linear relationsbetween day temperature over the range of 28 to 48 °C andFN (slope, -1.1 °C^-1), fruit-set (-2.8% °C^-1), RN_t(-0.90°C^-1), and pollen production (-390 °C^-1) and viability(-1.9% °C^-1). Warmer night temperature (28 vs. 22 °C)had no effect on FN, but reduced fruit-set (31 to 19%), RN_t(8to 5), and pollen production (4389 to 2800) and viability (49to 40%). There were no significant interactions between dayand night temperature. Reduced fruit-set was a consequence offewer pollen grains and reduced pollen viability. The thresholdday temperature for pollen production and viability was 34 °Cand there were strong negative linear relations between bothpollen production and pollen viability and accumulated temperature>34 °C. Copyright 1999 Annals of Botany Company Arachis hypogaea L., fruit-set, groundnut, heat-stress, peanut, pollen viability, pollen production, temperature.     

Effects of Temperature on Pollen Viability in Mango cv. 'Kensington'

The response of pollen development to low or high temperatureregimes was studied to determine the conditions suitable forthe formation of fertile pollen in the mango cv. 'Kensington'.The phase most sensitive to the degree and duration of temperaturestress was that from meiosis to the pre-vacuolate microspore(about 3 d duration at 25/20 °C) though vacuolated microsporeswere also sensitive to low temperature. Night temperatures below10 °C resulted in pollen grains with a low viability (<50%). A temperature between 15 and 33 °C during the phasefrom meiosis to the pre-vacuolate microspore was optimum forpollen development (70-85% pollen viability). Analysis of field records showed a linear negative correlationbetween percentage of pollen viability and number of days whichhad a mean night temperature lower than 10 °C during theperiod from meiosis to early mature stage (y = 77·7-3·4x,r² = 0·60). The temperature sensitive phase was estimatedto begin 155 degree days D = [(T_max + T_min)/2 - 10] before anthesisand to end 78 degree days before anthesis. This equation maybe useful as a means of predicting pollen viability in the fieldfrom temperature records and thus fruit set, date of maturityand yield. It may also aid in the selection of areas for growingmangoes in marginal climates.Copyright 1994, 1999 Academic Press Mangifera indica L. mango, microsporogenesis, pollen development, viability, sterility, temperature     

Effects of Temperature, Photoperiod and Seed Vernalization on Flowering in Faba Bean Vicia faba

Factorial combinations of three photoperiods (10, 13 and 16h), two day temperatures (18 and 28 °C) and two night temperatures(5 and 13 °C) were imposed on nodulated plants of six diversegenotypes of faba bean (Vicia faba L.). Plants were grown inpots in growth cabinets from both vernalized (1.5±0.5°C for 30 d) and non-vernalized seeds. The times from sowingto the appearance of first open flowers (f) were recorded. Seedvernalization decreased the subsequent time taken to flowerin almost all genotype x growing environment combinations (theexceptions were plants of the cv. Maris Bead grown in threecooler, short-day regimes). The influence of temperature andphotoperiod on the rate of flowering was quantified, using amodel applied previously to other long-day species of grainlegume in which positive linear relations between both temperatureand photoperiod and the rate of progress towards flowering areassumed to apply. A significant positive linear response ofrate of progress towards flowering to limited ranges of meandiurnal temperature was detected in all six genotypes, but inthree genotypes (Syrian Local Large, Aquadulce and Maris Bead)the 28 °C day temperature reduced the rate of progress towardsflowering - suggesting that the optimum temperature for floweringin these genotypes is below 28 °C. In four genotypes (MarisBead, Giza-4, Aquadulce and BPL 1722) a significant positiveresponse to photoperiod, typical of quantitative long-day plants,was observed only in plants grown from vernalized seeds. Incontrast, plants of the genotype Zeidab Local grown from bothnon-vernalized and vernalized seeds showed the same positiveresponse to photoperiod, whereas plants of the land-race SyrianLocal Large were consistently unresponsive to photoperiod. Theimplications of this range of responses amongst diverse genotypesare discussed in relation to screening germplasm. Vicia faba, faba bean, flowering, photoperiod, temperature, seed vernalization, germplasm screening     

The Effect of Temperature Change on Leakage from Pea Seeds

Intact seeds of P. sativum and P. elatius leaked electrolytes,sugars and proteins for as long as 48 h after the beginningof imbibition. Initial leakage was higher at 25 °C thanat 5 °C, but its extent was much greater at the lower temperature.Transfer of seed from 5 °C to 25 °C after 5 h imbibitionresulted in leakage for 48 h at the initial rate at 5 °C.The transfer from 25 °C to 5 °C gave leakage at a rateequivalent to the initial rate at 25 °C. The results arediscussed in relation to behaviour of membranes and to the sensingof the initial temperature. Key words: Pisum sativum, P. elatius, Leakage, Electrolytes, Sugars, Proteins, Temperature     

Abnormal Sporogenesis in Wheat (Triticum aestivum L.) Induced by Short Periods of High Temperature

Plants of Triticum aestivum L. cv. Gabo, grown at 20 °C,were exposed to 30 °C for short periods during the timebetween the beginning of meiosis in the pollen mother cellsand anthesis. Plant water deficit at this temperature was avoidedby maintaining a high atmospheric relative humidity and tissuewater potential did not change. This temperature treatment appliedfor 3 days, at the time of reduction division and tetrad breakup in the male tissue, lowered grain yield through a drasticreduction in grain set, but was without effect at other stagesof development. Grain set was also reduced by exposing plantsto 30 °C for 1 day only or to a 30 °C day, 20 °Cnight (16 h photoperiod) regime for 3 days during the sensitiveperiod. A reduction in grain set did not result in a compensatoryincrease in the weight of remaining grains. The female fertility of previously heat-stressed plants wasassessed by pollinating with pollen from plants grown at a lowertemperature (20 °C). Grain set in such plants was less thanthat in plants grown at the lower temperature and hand pollinatedwith similar pollen, indicating that female fertility was reducedby high temperature. This was not the sole reason for reducedgrain set, however, as some anthers on heat-stressed plantswere small and neither extruded nor dehisced normally. Suchanthers contained pollen grains that were mostly shrivelled,had abnormal cytoplasm and showed no reaction to 2, 3, 5-triphenyltetrazolium chloride. Similar effects were also noted in pollenfrom apparently normal anthers on heat-stressed plants. Triticum aestivum, wheat, heat stress, pollen, sporogenesis, grain set, male sterility, female sterility     

Photoperiod and Temperature Effects on Late Developmental Stages of Stylosanthes guianensis

Seedlings of Stylosanthes guianensis var. guianensis cv. Cookand S. guianensis var. pauciflora cv. Bandeirante were defoliatedand placed in a naturally lit glasshouse at 23/18 °C, 28/23°C or 33/28 °C (day/night). After exposure to 14 h daysand after floral induction with 30 cycles of 11 h, plants wereallocated to 11, 12, 13 or 14 h during flowering and seed formation. Floral initiation occurred after 10–15 short-day cycles.Flower appearance was hastened by warm temperatures and spikenumber per plant at 20 d after flower appearance was negativelyrelated to temperature and greater in Cook than in Bandeirante.Exposure to 13- and 14-h days reduced the continued differentiationof inflorescences in Bandeirante, and in Cook in warm temperatures.Floret number per spike was greatest at 23/18 °C and a higherproportion of florets aborted in Bandeirante at 33/ 28 °C.Variations in seed setting of the bi-articulate loment of Bandeiranteare described. Highest potential seed yield occurred if afterfloral induction 11 or 12 h days were maintained with 23/18°C or 28/23 °C temperatures. Photoperiod, temperature, development, Stylosanthes guianensis, flowering     

Flowering Responses of Contrasting Ecotypes of Poa annua and their Putative Ancestors Poa infirma andPoa supina

Flowering responses of two Australian and six Norwegian populationsof Poa annua and their putative ancestors P. infirma and P.supina were studied in controlled environments. The two Australianpopulations originating from suburban parks in Canberra hadopposite daylength flowering responses across the range of temperaturestested (9–21 °C), one being a quantitative short-day(SD) plant with no response to vernalization, the other a quantitativelong-day (LD) plant with a quantitative vernalization requirement(winter annual type). Variation in earliness of flowering withinthe former population was shown to be genetically determined,and testing of selfed progenies indicated that the populationis an aggregate of several largely homozygous lines with divergentflowering responses. Two lowland populations from southern Norwaywere both quantitative LD plants with no vernalization response,while two alpine snowbed populations from southern Norway andtwo high-latitude, subarctic populations from northern Norwaywere quantitative SD plants with an obligatory plant vernalizationor SD requirement for flowering. Two populations of P. supinaexhibited the same flowering responses as the alpine and high-latitudepopulations of P. annua with an obligatory plant vernalizationor SD requirement for flowering. A combination of SD and lowtemperature (9–12 °C) for 8–10 weeks was optimalfor induction and inflorescence initiation. On the other hand,P. infirma was found to be an early-flowering quantitative SDplant which flowered freely across the range of temperatures(9–21 °C) as a typical summer annual. The experimentsdemonstrate that virtually any kind of photoperiodic and vernalizationresponses can be found among populations of P. annua. Theseversatile flowering responses reflect the contrasting floweringresponses of P. supina and P. infirma, and add strong supportto the hypothesis that P. annua has originated from these species.Copyright 2001 Annals of Botany Company Adaptation, evolution, flowering, Poa annua, P. infirma, P. supina, photoperiod, vernalization     

The Interaction of Heat and Smoke in the Release of Seed Dormancy in Seven Species from Southwestern Western Australia

Exposure to high temperatures (40–60 °C) over severaldays to months (simulating summer soil conditions) or to extremelyhigh temperatures (80–120 °C) for periods rangingfrom minutes to several days (simulating the heat of an extremefire) promoted germination in five (including one legume species)of the seven species tested from southwestern Australia. Heat,a collective term for high temperature (HT) and extremely hightemperature (EHT), has previously been used to promote germinationin species with impermeable seed coats (many of which are legumes).If heat is found to promote germination in species with permeableseed coats, it could be of use in the release of dormancy forcommercial development of native plants. Here we report thatelevated storage temperatures (50 °C for 45 and 90 d or60 °C for 45 d) and EHT (100 °C for 3 h and 120 °Cfor 30 min) alone, and in addition to smoke, directly promotegermination of the refractory species Actinotus leucocephalus,Anigozanthos manglesii, Gompholobium knightianum, Loxocaryastriatus and Stylidium affine. Germination of Sowerbaea laxifloraseed was not improved with heat alone but was improved by thecombination of heat and smoke. Seed of Schoenus unispiculatusdid not germinate in response to any of the treatments. Interactionsbetween the effect of temperature, the duration of heat exposureor smoke were detected in all species. Whilst most species demonstratedan interaction between heat and time, not all species were affectedby smoke alone. This was illustrated by an interaction betweensmoke and HT found only in Actinotus leucocephalus, Anigozanthosmanglesii and Stylidium affine or smoke and EHT in A. manglesiiandS. affine . Smoke and exposure time to heat influenced thegermination of Actinotus leucocephalus,Anigozanthos manglesiiand L. striatus in response to HT and only S. affine appearedto demonstrate an interaction between smoke and EHT. The variationsfound in the interactions between treatments suggest that germinationevents occur over a range of conditions both spatially and temporallywhich may facilitate seedling survival and limit competitionbetween emergents. Copyright 2001 Annals of Botany Company Smoke, heat, fire, seedbank, seed dormancy, seed germination, Western Australia     

Germination ecology of the polycarpic grassland perennials Primula veris and Trollius europaeus

The seed germination behaviour of Primula veris and Trollius europaeus , both perennial, polycarpic grassland plants was compared The species have similar-sized seeds that are dormant at dispersal Seeds buried in soil and exhumed at regular intervals showed that for both species, primary seed dormancy was overcome by cold-stratification Hence, their germination in the field should occur in spring, following dispersal, or later Seeds of P veris became dormant again in the late spring/early summer, and dormancy was broken again in the second winter Seeds of T europaeus did not exhibit such changes in dormancy
Seeds of P veris did not germinate in darkness This suggests that P veris can accumulate a persistent seed bank because buried seeds are prevented from germinating Trollius europaeus , on the other hand, germinated equally well in darkness and in light which suggests that seeds might germinate even when they are too deep in the soil for seedlings to emerge Two lines of evidence confirm this difference in seed bank behaviour (1) Primula veris was detected in the persistent seed bank of a grassland site, whereas T europaeus was not (n) After 16 months burial, 85% of the P veris seeds but only 8% of the T europaeus seeds remained viable     

Viability and Germination of the Pollen of Sorghum [Sorghum bicolor (L.) Moench]

Previous studies have demonstrated that pollen of sorghum [Sorghumbicolor (L.) Moench] loses capacity to both germinate in vitroand to set seed in vivo soon after being shed. The current studyevaluates the capacity for dehydrated pollen to effect in vitrogermination, reduce tetrazolium chloride, and set seed on cytoplasmicmale sterile plants. Morphological changes during pollen germinationwere examined by scanning electron microscopy (SEM). Close to70% of the pollen germinated in 5 min, or less, when collectedat 80% relative humidity (RH) and stored in sealed glass vials.Pollen tubes elongated autotropically with atmospheric humidityapparently being a controlling factor in the process. Pollendehydrated at 50% RH and 25°C for 15-30 min neither germinatedin vitro, reduced tetrazolium chloride, nor set seed on malesterile plants. Rehydrating the pollen did not restore the capacityfor germination. SEM micrographs demonstrated that elongatingpollen tubes encircled the pollen grain and were contiguousto the surface. A fibrillar-like material existed on the exineof separated pollen grains at the point where the grains hadbeen previously attached.Copyright 1994, 1999 Academic Press Sorghum pollen, germination, seed-set, viability, scanning electron microscopy, Sorghum bicolor (L.) Moench     

A Comparative Study ofMiscanthus floridulus(Labill) Warb andM. transmorrisonensisHayata: Photosynthetic Gas Exchange, Leaf Characteristics and Growth in Controlled Environments

The relationship between temperature and the distribution ofMiscanthusfloridulus(Labill) Warb andM. transmorrisonensisHayata alongaltitudinal gradients in central Taiwan was examined. Responsesof biomass accumulation, leaf characteristics and photosyntheticgas exchange to growth temperature (from 10 to 30 °C) ofM.floridulusfrom an altitude of 390 m and ofM. transmorrisonensisfrom2700 m were determined. There were differences between the twospecies in above-ground biomass, CO₂uptake characteristics andleaf chlorophyll contents in response to growth temperature.The optimal temperatures for biomass accumulation were 30/25(day/night temperature) and 25/20 °C forM. floridulusandM.transmorrisonensis,respectively. Light saturated photosyntheticrates (A_max) were largest in plants grown at the optimal temperature.Growth at 15/10 and 10/10 °C compared to the optima reducedaccumulated biomass, leaf chlorophyll content and photosyntheticrate in both species with a greater reduction inM. floridulusthaninM. transmorrisonensis.We concluded that growth ofM. floridulusathigh altitude is limited by an inability to grow at temperatureslower than 15 °C, whileM. transmorrisonensisis able to growin chilling temperatures at higher altitudes.Copyright 1998Annals of Botany Company Miscanthus floridulus;M. transmorrisonensis; C₄plants; chlorophyll content; leaf growth; photosynthetic gas exchange; biomass accumulation; temperature response.     

Cold-pretreatment of Excised Flower Buds in Float Culture of Tobacco Anthers

Pretreatment of excised flower buds at 7–9 °C is shownto be more effective in float culture of Nicotiana tabacum anthersthan pretreatment at 5 °C. The small temperature differenceresults in greatly enhanced embryo yields when the treatmentis given just before (stage 3), during (4) or just after (5)the first pollen division. Anther productivity (total embryoyield per anther) increases with the time of pretreatment toa peak beyond which further pretreatment is deleterious. Thepeak is attained sooner at 9 than at 7 °C and sooner inanthers pretreated at stage 5 than at stage 3. Peak productivityappears always to be the same. Owing to batch variations, accuratepretreatment-times cannot be specified. Suggested mean timesfor buds of mixed stage are about 12–14 days at 7 °Cand about 7–9 days at 9 °C. Anthers dehisce in culture, the time to dehiscence decreasingwith increasing anther stage. Mid-stage-5 anthers open withinthe first few days, sooner with pretreatment at 9 than at 7°C, and high-yielding free pollen cultures can be obtainedwithout mechanical disruption of the anthers. Buds survive longest when harvested at the beginning of theearly flowering period and kept at 7 °C. It is proposedthat the pretreatment exerts its effect not by altering thecourse of the first pollen division, as suggested by others,but by delaying anther deterioration and thus assuring survivalof a greater proportion of the pollen grains switched into embryogenesis. Nicotiana tabacum L., tobacco, anther culture, embryogenesis     

Pollen flow inPrimula veris (Primulaceae)

Observations on two natural and one artificial distylous populations ofPrimula veris in Britain revealed very high proportions of intramorph pollen on pin stigmas and moderate proportions of intermorph pollen on thrum stigmas. The average ratio of pin: thrum pollen production per flower was 2.4:1. Large amounts of pollen remained in dehisced anthers, perhaps reflecting low levels of pollinator activity. Population size and density had no clear effect on pollen flow patterns, which are asymmetrical for the two morphs.     

Selection on floral display in insect-pollinated Primula farinosa: effects of vegetation height and litter accumulation

Grazing reduces litter thickness and vegetation height and may thereby indirectly affect reproductive success and selection on floral characters in plants with prostrate growth. Reductions in litter thickness and vegetation height should influence both the resource status of plants with leaves positioned close to the ground and the significance of inflorescence height for interactions with pollinators and seed predators. We experimentally examined how simulated grazing of surrounding vegetation affected pollen limitation, fruit predation and fecundity of short-scaped and long-scaped Primula farinosa, which differ markedly in floral display and therefore in expected attractiveness to pollinators. Litter removal and pruning of surrounding vegetation increased fruit and seed production per plant in the year of the treatment and the probability of flowering in the following year. Pollen limitation of fruit and seed production was stronger in the short-scaped morph than in the long-scaped morph, but was not significantly affected by litter removal and simulated grazing of surrounding vegetation. Supplemental hand-pollination reduced seed size in the year of the treatment and flowering probability in the second year, and these effects did not differ among scape morphs or grazing treatments. The results suggest that grazing indirectly favours seed production in P. farinosa, mainly because it increases the resource status of plants that escape damage. Contrary to expectation, there was no strong evidence that litter accumulation and tall vegetation increase the severity of pollen limitation or reduce the relative performance of the short-scaped morph.     

Seed Viability Constants for Lettuce

Seeds of two lettuce cultivars (Lactuca sativa L., cv. Meikoninginand cv. Grand Rapids) were hermetically stored with constantmoisture contents ranging between 3.6 and 17.9 per cent (freshweight basis) at constant temperatures ranging between 5 and75 °C. The decline with time in percentage germination andpercentage normal seedlings was determined for each storagetreatment. The data were fitted to an equation which containsthe constants: K₁, the probit of the initial percentage germinationor normal seedlings; K_E, a species constant; C_W, the constantof a logarithmic moisture term; C_H, the constant of a lineartemperature term and C_Q, the constant of a quadratic temperatureterm. Regression analysis of data from storage periods up to5.5 years at temperatures of 5–75 °C and seed moisturecontents of 3.6–13.6 per cent yielded the following values:K_E= 8.218, C_W=4.797±0.163, C_H=0.0489±0.0050 andC_Q=0.000365±0.000056. Although this equation consistentlyprovided a better fit, simplified equations, assuming eithera log-linear relationship between seed longevity and temperature,or a log-linear relationship between seed longevity and bothmoisture content and temperature, accounted for more than 94per cent of the variation at the restricted temperature rangeof 5–40 °C. Longevity of the same seed lots at sub-zero temperatures (–5,–10 and –20 °C) was studied in separate tests.Freezing damage, resulting in abnormal seedlings in the germinationtest, occurred at –20 °C when the moisture contentof the seeds exceeded 12 per cent. No decline in percentagenormal seedlings was observed after a storage period of 18 monthsor longer at –20 °C, provided the seed moisture contentdid not exceed 9.5 per cent. For seeds stored at –5 and–10 °C with 9.6–12.5 per cent moisture content,the observed rate of decline of percentage normal seedlingswas adequately predicted by the viability equation, using theabove values for the constants. This suggests that for low moisturecontents the viability equation can be applied to estimate longevityat sub-zero temperatures. Lettuce, Lactuca sativa (L.), seed longevity, seed storage, viability constants, storage conditions     

Growth and Partitioning in Pascopyrum smithii (C3) and Bouteloua gracilis(C4) as Influenced by Carbon Dioxide and Temperature

This study investigated how CO₂and temperature affect dry weight(d.wt) accumulation, total nonstructural carbohydrate (TNC)concentration, and partitioning of C and N among organs of twoimportant grasses of the shortgrass steppe,Pascopyrum smithiiRydb. (C₃) andBouteloua gracilis(H.B.K.) Lag. ex Steud. (C₄).Treatment combinations comprised two temperatures (20 and 35°C)at two concentrations of CO₂(380 and 750 µmol mol^-1),and two additional temperatures of 25 and 30°C at 750 µmolmol^-1CO₂. Plants were maintained under favourable nutrient andsoil moisture and harvested following 21, 35, and 49d of treatment.CO₂-induced growth enhancements were greatest at temperaturesconsidered favourable for growth of these grasses. Comparedto growth at 380 µmol mol^-1CO₂, final d.wt of CO₂-enrichedP.smithiiincreased 84% at 20°C, but only 4% at 35°C. Finald.wt ofB. graciliswas unaffected by CO₂at 20°C, but wasenhanced by 28% at 35°C. Root:shoot ratios remained relativelyconstant across CO₂levels, but increased inP. smithiiwith reductionin temperature. These partitioning results were adequately explainedby the theory of balanced root and shoot activity. Favourablegrowth temperatures led to CO₂-induced accumulations of TNCin leaves of both species, and in stems ofP. smithii, whichgenerally reflected responses of above-ground d.wt partitioningto CO₂. However, CO₂-induced decreases in plant tissue N concentrationswere more evident forP. smithii. Roots of CO₂-enrichedP. smithiihadgreater total N content at 20°C, an allocation of N below-groundthat may be an especially important adaptation for C₃plants.Tissue N contents ofB. graciliswere unaffected by CO₂. Resultssuggest CO₂enrichment may lead to reduced N requirements forgrowth in C₃plants and lower shoot N concentration, especiallyat favourable growth temperatures. Acclimation to CO₂; blue grama; Bouteloua gracilis ; carbohydrate; climate change; global change; grass; growth; growth temperature optima; nitrogen; N uptake; Pascopyrum smithii; western wheatgrass