Photosynthetic capacity and dry mass partitioning in dwarf and semi-dwarf wheat (Triticum aestivum L.).

Efficient use of space and high yields are critical for long-term food production aboard the International Space Station. The selection of a full dwarf wheat (less than 30 cm tall) with high photosynthetic and yield potential is a necessary prerequisite for growing wheat in the controlled, volume-limited environments available aboard long-term spaceflight missions. This study evaluated the photosynthetic capacity and carbon partitioning of a full-dwarf wheat cultivar, Super Dwarf, which is routinely used in spaceflight studies aboard U.S. space shuttle and NASA/Mir missions and made comparisons with other dwarf and semi-dwarf wheat cultivars utilized in other ground-based studies in plant space biology. Photosynthetic capacity of the flag leaf in two dwarf (Super Dwarf, BB-19), and three semi-dwarf (Veery-10, Yecora Rojo, IBWSN 199) wheat cultivars (Triticum aestivum L.) was assessed by measuring: net maximum photosynthetic rate, RuBP carboxylation efficiency, chlorophyll concentration and flag leaf area. Dry mass partitioning of carbohydrates to the leaves, sheaths, stems and ear was also assessed. Plants were grown under controlled environmental conditions in three replicate studies: slightly enriched CO2 (370 micromoles mol-1), high photosynthetic photon flux (1000 micromoles m-2 s-1; 58 mol m-2 d-1) for a 16 h photoperiod, 22/15 degrees C day/night temperatures, ample nutrients and water provided by one-half strength Hoagland's nutrient solution (Hoagland and Arnon, 1950). Photosynthetic capacity of the flag leaf was determined at anthesis using net CO2 exchange rate versus internal CO2 concentration curves measured under saturating light (2000 micromoles m-2 s-1) and CO2 (1000 micromoles mol-1). Dwarf wheat cultivars had greater photosynthetic capacities than the taller semi-dwarfs, they averaged 20% higher maximum net photosynthetic rates compared to the taller semi-dwarfs, but these higher rates occurred only at anthesis, had slightly greater carboxylation efficiencies and significantly increased chlorophyll concentrations per unit leaf area. The reduced-height wheat had significantly less dry mass fraction in the stem but greater dry mass partitioned to the ear than the taller semi-dwarfs (Yecora rojo, IBWSN-199). Studies with detached heads confirm that the head is a significant sink in the shorter wheat cultivars.     

Analysis of the spaceflight effects on growth and development of Super Dwarf wheat grown on the Space Station Mir.

The hypothesis being tested is that Super Dwarf wheat, Triticum aestivum L., plants in the Svet Greenhouse onboard the Russian Space Station Mir will complete a life cycle in spaceflight, providing that the environmental conditions necessary for adequate growth on Earth are supplied. Twenty six seeds of wheat were planted in each of 2 rows of 2 root compartments for a total of 104 seeds in Svet. Germination rate at 7 d was 56 and 73% on Mir and 75 and 90% in ground-based controls. Plants were grown throughout the whole cycle of ontogenesis (123 d) with samples gathered at different times to validate the morphological and reproductive stages of the plants. Young plants showed vigorous early seedling growth, with large biomass production, including the formation of 280 floral spikes. Upon return to Earth, comparative analyses showed that the number of tillers and flowers per spikelet were 63.2% and 40% greater, respectively, in Mir-grown plants than in the controls. By contrast, the stem length (52.4%), spike mass (49.2%) and length (23.1%), awn length (75.7%), number of spikelets per spike (42.8%) and number of seeds per spike (100% sterile) from Mir-grown plants were substantially less than the controls. Distribution of moisture and roots throughout the substrate was very good. All florets on Mir-grown spikes ceased development at the same stage of ontogeny. Lack of caryopses formation was attributed to male sterility occurring at different stages of staminal development. Anthers failed to dehisce and pollen grains were smaller and shriveled compared to the controls, suggesting a chronic stress had occurred in the Svet growth chamber. Recent ground-based studies indicated that ethylene, which was measured at 0.3 to 1.8 mg kg-1 in the Mir, almost certainly could have induced male sterility in the wheat plants grown on the Mir.     

Very high CO2 reduces photosynthesis, dark respiration and yield in wheat

Although terrestrial CO2 concentrations, [CO2] are not expected to reach 1000 micromoles mol-1 for many decades, CO2 levels in closed systems such as growth chambers and glasshouses, can easily exceed this concentration. CO2 levels in life support systems in space can exceed 10000 micromoles mol-1 (1%). Here we studied the effect of six CO2 concentrations, from ambient up to 10000 micromoles mol-1, on seed yield, growth and gas exchange of two wheat cultivars (USU-Apogee and Veery-l0). Elevating [CO2] from 350 to 1000 micromoles mol-1 increased seed yield (by 33%), vegetative biomass (by 25%) and number of heads m-2 (by 34%) of wheat plants. Elevation of [CO2] from 1000 to 10000 micromoles mol-1 decreased seed yield (by 37%), harvest index (by 14%), mass per seed (by 9%) and number of seeds per head (by 29%). This very high [CO2] had a negligible, non-significant effect on vegetative biomass, number of heads m-2 and seed mass per head. A sharp decrease in seed yield, harvest index and seeds per head occurred by elevating [CO2] from 1000 to 2600 micromoles mol-1. Further elevation of [CO2] from 2600 to 10000 micromoles mol-1 caused a further but smaller decrease. The effect of CO2 on both wheat cultivars was similar for all growth parameters. Similarly there were no differences in the response to high [CO2] between wheat grown hydroponically in growth chambers under fluorescent lights and those grown in soilless media in a glasshouse under sunlight and high pressure sodium lamps. There was no correlation between high [CO2] and ethylene production by flag leaves or by wheat heads. Therefore, the reduction in seed set in wheat plants is not mediated by ethylene. The photosynthetic rate of whole wheat plants was 8% lower and dark respiration of the wheat heads 25% lower when exposed to 2600 micromoles mol-1 CO2 compared to ambient [CO2]. It is concluded that the reduction in the seed set can be mainly explained by the reduction in the dark respiration in wheat heads, when most of the respiration is functional and is needed for seed development.     

Ethylene Is Responsible for a Disturbed Development of Plant Reproductive System under Conditions of Space Flight

The development of reproductive organs was studied on three dwarf cultivars of wheat Triticum aestivum and the fast-cycling Brassica rapa plants, grown under earth control conditions, during the space flight in the Mir orbital station, and in a earth experiment that simulated growth conditions during the space flight, including an elevated content of ethylene in the air (1 mg/m³ on average). We found that the embryological characteristics of the plants were not affected by space flight conditions. The elevated ethylene content in air resulted in some changes in the morphometric characteristics of inflorescences and a greater frequency of sterility similar under conditions of space flight and control earth experiment. We conclude that the abnormalities and modifications in the development of reproductive organs, induced by space flight conditions, were caused by a secondary factor, an elevated ethylene content in the cabin air, rather than by microgravity.     

Effects of CO2 on stomatal conductance: do stomata open at very high CO2 concentrations?

Potato and wheat plants were grown for 50 d at 400, 1000 and 10000 micromoles mol-1 carbon dioxide (CO2). and sweetpotato and soybean were grown at 1000 micromoles mol-1 CO2 in controlled environment chambers to study stomatal conductance and plant water use. Lighting was provided with fluorescent lamps as a 12 h photoperiod with 300 micromoles m-2 s-1 PAR. Mid-day stomatal conductances for potato were greatest at 400 and 10000 micromoles mol-1 and least at 1000 micromoles mol-1 CO2. Mid-day conductances for wheat were greatest at 400 micromoles mol-1 and least at 1000 and 10000 micromoles mol-1 CO2. Mid-dark period conductances for potato were significantly greater at 10000 micromoles mol-1 than at 400 or 1000 micromoles mol-1, whereas dark conductance for wheat was similar in all CO2 treatments. Temporarily changing the CO2 concentration from the native 1000 micromoles mol-1 to 400 micromoles mol-1 increased mid-day conductance for all species, while temporarily changing from 1000 to 10000 micromoles mol-1 also increased conductance for potato and sweetpotato. Temporarily changing the dark period CO2 from 1000 to 10000 micromoles mol-1 increased conductance for potato, soybean and sweetpotato. In all cases, the stomatal responses were reversible, i.e. conductances returned to original rates following temporary changes in CO2 concentration. Canopy water use for potato was greatest at 10000, intermediate at 400, and least at 1000 micromoles mol-1 CO2, whereas canopy water use for wheat was greatest at 400 and similar at 1000 and 10000 micromoles mol-1 CO2. Elevated CO2 treatments (i.e. 1000 and 10000 micromoles mol-1) resulted in increased plant biomass for both wheat and potato relative to 400 micromoles mol-1, and no injurious effects were apparent from the 10000 micromoles mol-1 treatment. Results indicate that super-elevated CO2 (i.e. 10000 micromoles mol-1) can increase stomatal conductance in some species, particularly during the dark period, resulting in increased water use and decreased water use efficiency.     

两个育性相关基因在BNS和YS小麦温敏雄性不育系中的表达差异分析

为研究雄性不育相关基因TA1和TA2在BNS和YS小麦温敏雄性不育系732A花粉发育时期的表达特点,探讨这2个育性相关基因与温敏雄性不育小麦育性转换的联系,本研究利用荧光实时定量PCR方法,在BNS和YS型不育系732A花药发育四分体期、单核期、二核期和三核期定量检测基因TA1和TA2的mRNA表达水平。结果表明:(1)在732A和BNS花粉发育四分体时期至二核期,基因TA1相对表达量上调,在三核期相对表达量下降;(2)基因TA2相对表达量在BNS花粉发育的四分体时期至二核期逐渐下降,三核期上升;在732A花粉发育4个时期中的相对表达量变化刚好相反;(3)在BNS和732A花粉发育二核期,基因TA1和TA2均表现极值,推测二核期可能为BNS和YS型小麦温敏雄性不育系花粉发育最敏感时期;(4)在不育系BNS和732A花粉发育过程中,基因TA1的相对表达量变化幅度比TA2的高。推测TA1对不育系BNS和732A花粉败育影响程度强于TA2;(5)基因TA1和TA2相对表达量在BNS的花粉发育时期表达趋势相反,推测其对BNS花粉败育影响表现为拮抗作用,且2个基因不连锁;在732A花粉发育时期表达趋势相同,推测其对不育系732A花粉败育影响表现为协同作用。     

Water stress,CO2 and photoperiod influence hormone levels in wheat

'Super Dwarf' wheat (Triticum aestivum L.) plants have been grown from seed to maturity in the Mir space station where they were periodically exposed, because of microgravity and other constraints, to water deficit, waterlogging, high CO2 levels, and low light intensities. The plants produced many tillers, but none of them produced viable seed. Studies have been initiated to determine why the plants responded in these ways. In the present study, effects of the listed stresses on abscisic acid (ABA), indole-3-acetic acid (IAA) and isopentenyl adenosine ([9R]iP) levels in roots and leaves of plants grown under otherwise near optimal conditions on earth were measured. Hormones were extracted, purified by HPLC, and quantified by noncompetitive indirect ELISA. In response to water deficit, ABA levels increased in roots and leaves, IAA levels decreased in roots and leaves, and [9R]iP levels increased in leaves but decreased in roots. In response to waterlogging, ABA, IAA and [9R]iP levels briefly increased in roots and leaves and then decreased. When portions of the root system were exposed to waterlogging and/or water deficit, ABA levels in leaves increased while [9R]iP and IAA levels decreased. These responses were correlated with the percentage of the root system stressed. At a low photosynthetic photon flux (100 micromoles m-2 s-1), plants grown in continuous light had higher leaf ABA levels than plants grown using an 18 or 21 h photoperiod.     

Impact of plant flowering phenology on the cost/benefit balance in a nursery pollination mutualism,with honest males and cheating females

This study documents the flowering phenology and its potential consequences on a nursery pollination mutualism between a dioecious plant, in which honest male plants, but not cheating females, allow the specific pollinator to reproduce within inflorescences. Very few pollinators were found to emerge during plant anthesis, leading to a low (if any) potential benefit through pollen dispersal. This opens the question why male plants do not also cheat their pollinators. Female plants flowered late in the season, when many males had just achieved their own anthesis, which increased the efficiency of pollen transfer. Finally, some late‐flowering males reached their anthesis simultaneously with females, which open the possibility for pollinator to choose between honest males and cheating females. Nevertheless, female plants were found to produce fruits, even though fruit production was limited by pollen (and pollinator) supply, meaning that cheating was not entirely retaliated by the mutualistic partner.     

Growth and photosynthetic responses of wheat plants grown in space.

Growth and photosynthesis of wheat (Triticum aestivum L. cv Super Dwarf) plants grown onboard the space shuttle Discovery for 10 d were examined. Compared to ground control plants, the shoot fresh weight of space-grown seedlings decreased by 25%. Postflight measurements of the O2 evolution/photosynthetic photon flux density response curves of leaf samples revealed that the CO2-saturated photosynthetic rate at saturating light intensities in space-grown plants declined 25% relative to the rate in ground control plants. The relative quantum yield of CO2-saturated photosynthetic O2 evolution measured at limiting light intensities was not significantly affected. In space-grown plants, the light compensation point of the leaves increased by 33%, which likely was due to an increase (27%) in leaf dark-respiration rates. Related experiments with thylakoids isolated from space-grown plants showed that the light-saturated photosynthetic electron transport rate from H2O through photosystems II and I was reduced by 28%. These results demonstrate that photosynthetic functions are affected by the microgravity environment.     

THE DISTRIBUTION AND PHYLOGENETIC SIGNIFICANCE OF BINUCLEATE AND TRINUCLEATE POLLEN GRAINS IN THE ANGIOSPERMS

Studies of the cytology of angiosperm pollen which extend our knowledge of the distribution of binucleate and trinucleate pollen to almost 2,000 species of flowering plants are summarized. Approximately 70% of the species studied release pollen in a binucleate stage, and none of them shed both types of pollen as a constant varietal trait. All phylogenetically primitive taxa are binucleate. The trinucleate trait evidently has originated independently at many times during angiosperm evolution. In no instance must one infer the origin of binucleate taxa from trinucleate ancestors. Most genera are monotypic with respect to pollen cytology, and only 10 genera are known to include both binucleate and trinucleate species. Among the 265 families studied, 179 include only binucleate genera and 54 include only trinucleate genera, while 32 include both types. Nearly all aquatic species with submersed flowers shed pollen in a trinucleate stage. A group of physiological differences which distinguish binucleate and trinucleate pollen is reviewed in relation to the survival and possible selective advantage of trinucleate mutants, and the significance of the binucleate pollen grain in the origin and evolution of flowering plants is discussed.     

K型温敏雄性不育系KTP116A育性转换规律的研究

基于两系杂交小麦系统的K型（Aegilops kotshyi）细胞质小麦温敏雄性不育系,在中国北方小麦正常生长季节表现完全雄性不育,可用于杂交小麦种子生产,在反季节播种表现育性部分恢复,是一类非常有利用价值的小麦温敏雄性不育系。本研究以小麦温敏雄性不育系KTP116A为材料,采用人工气候箱控温和涂抹法压片的方法对其育性转换的关键时期及败育的花粉发育过程进行更为精细的研究。研究发现,KTP116A可育环境下能正常进行减数分裂并发育为成熟花粉粒;不育环境下只有少数能经过第二次有丝分裂产生两个精细胞,大部分为二核期花粉粒、异常花粉粒和没有原生质体的空壳花药不开裂,自然条件下不散粉;KTP116A育性敏感期为两个时期,减数分裂期至单核早期和二核晚期至三核期,在自然条件下温度感应历期为10d左右;从小麦外部发育进程来看,抽穗前的2-8d和扬花前的3-5d为其敏感时期。这对于进一步研究败育的机理,分析败育的原因和更好的利用温敏不育系奠定基础。     

Importance of pre‐anthesis anther sink strength for maintenance of grain number during reproductive stage water stress in wheat

Reproductive stage water stress leads to spikelet sterility in wheat. Whereas drought stress at anthesis affects mainly grain size, stress at the young microspore stage of pollen development is characterized by abortion of pollen development and reduction in grain number. We identified genetic variability for drought tolerance at the reproductive stage. Drought‐tolerant wheat germplasm is able to maintain carbohydrate accumulation in the reproductive organs throughout the stress treatment. Starch depletion in the ovary of drought‐sensitive wheat is reversible upon re‐watering and cross‐pollination experiments indicate that the ovary is more resilient than the anther. The effect on anthers and pollen fertility is irreversible, suggesting that pollen sterility is the main cause of grain loss during drought conditions in wheat. The difference in storage carbohydrate accumulation in drought‐sensitive and drought‐tolerant wheat is correlated with differences in sugar profiles, cell wall invertase gene expression and expression of fructan biosynthesis genes in anther and ovary (sucrose : sucrose 1‐fructosyl‐transferase, 1‐SST; sucrose : fructan 6‐fructosyl‐transferase, 6‐SFT). Our results indicate that the ability to control and maintain sink strength and carbohydrate supply to anthers may be the key to maintaining pollen fertility and grain number in wheat and this mechanism may also provide protection against other abiotic stresses.     

The coexistence of binucleate and trinucleate pollen in Mitrephora macclurei Weerasooriya & R. M. K (Annonaceae)

Pollen of angiosperms may be either binucleate or trinucleate at maturity. According to Schürhoff–Brewbaker law, the binucleate state is pleisomorphic occuring in about 70% of all angiosperms, while the trinucleate state is advanced occurring in about 30% of the angiosperms. The derivation of trinucleate pollen from binucleate pollen is irreversible, thus trinucleate or mixed pollen was considered uncommon in early lineage. In the present work, using conventional paraffin section method, the coexistence of both binucleate and trinucleate pollen in the same intact anther were observed in Mitrephora macclurei, a species belonging to the eumagnoliid family Annonaceae. The increasing number of plants with trinucleate or mixed pollen discovered in early lineages, as well as climatic introduced changes in nuclear number challenge analyses of ancestral state and evolution of pollen nuclear number in angiosperms.     

The effect of high temperature and high atmospheric CO2 on carbohydrate changes in bell pepper (Capsicum annuum) pollen in relation to its germination

Pollen viability and germination are known to be sensitive to high temperature (HT). However, the mode by which high temperature impairs pollen functioning is not yet clear. In the present study, we investigated the effect of high temperature on changes occurring in carbohydrate of bell pepper (Capsicum annuum L. cv. Mazurka) pollen in order to find possible relations between these changes and pollen germination under heat stress. When pepper plants were maintained under a moderate HT regime (32/26 degrees C, day/night) for 8 days before flowers have reached anthesis, pollen count at anthesis was similar to that found in plants grown under normal temperatures (NT 28/22 degrees C). However, the in vitro germination, carried out at 25 degrees C, of pollen from HT plants was greatly reduced. This effect matched the marked reduction in the number of seeds per fruit in the HT plants. Maintaining the plants at high air CO2 concentration (800 &mgr;mol mol-1 air) in both temperature treatments did not affect the in vitro germination of pollen from NT plants, but restored germination to near the normal level in pollen from HT plants. Under NT conditions, starch, which was negligible in pollen at meiosis (8 days before anthesis, A-8) started to accumulate at A-4 and continued to accumulate until A-2. From that stage until anthesis, starch was rapidly degraded. On the other hand, sucrose concentration rose from stage A-4 until anthesis. Acid invertase (EC 3.2.1.26) activity rose parallel with the increase of sucrose. In pollen from HT plants, sucrose and starch concentrations were significantly higher at A-1 pollen than in that of NT plants. Under high CO2 conditions, the sucrose concentration in the pollen of HT plants was reduced to levels similar to those in NT pollen. In accordance with the higher sucrose concentration in HT pollen, the acid invertase activity in these pollen grains was lower than in NT pollen. The results suggest that the higher concentrations of sucrose and starch in the pollen grains of HT plants may result from reduction in their metabolism under heat stress. Elevated CO2 concentration, presumably by increasing assimilate availability to the pollen grain, may alleviate the inhibition of sucrose and starch metabolism, thereby increasing their utilization for pollen germination under the HT stress. Acid invertase may have a regulatory role in this system.     

Peroxidase Activity in the Abscission Zone of Bean Leaves during Abscission

Peroxidase activity and localization in the abscission zone of bean leaves were studied histochemically and by gel electrophoresis. Deblading of bean leaves resulted in an increase in peroxidase activity in the abscission zone 2 to 4 days after deblading with highest activity just prior to separation. In debladed plants, the cell division in six to eight layers of cells preceded separation. An ethylene treatment (8 microliters per liter) induced separation of debladed petioles in approximately 24 hours and of intact plants in 36 to 48 hours. Ethylene treatment produced similar results in both debladed and intact plants. In ethylene-treated plants, whether debladed or not, enzyme localization was restricted to only two to three layers of cells with no cell division apparent prior to separation. Infrequent cell divisions were observed after treatment with 2-chloroethylphosphonic acid (1000 micrograms per liter) (Ethephon); however, other changes were similar to those observed with ethylene. Deblading and ethylene treatment resulted in changes in the six peroxidase isozymes observed in the abscission zone. Only four were observed in samples collected 2 centimeters below the abscission zone. Peroxidase bands IV and V increased significantly in debladed and ethylene-treated plants and peroxidase VI decreased only in debladed plants. The changes in peroxidase activity were invariably observed prior to separation in all treatments.     

Susceptibility of pollen to UV-B radiation: an assay of 34 taxa

Much of the ultraviolet-B radiation (UV-B) research on plants has concentrated on vegetative plant parts, and only a small fraction has dealt with the reproductive system. The present study analyzed pollen grains of 34 taxa germinated and grown under two levels of UV-B radiation (187 and 460 mW/m2) or no UV-B (control group). Visible radiation at 260 mmol/m/s was present in all treatments. Taxa included those with binucleate and trinucleate pollen types. We detected differences among species. A significant reduction in pollen germination occurred in only five species. Pollen tubes of >50% of the species showed significant reduction in length. Trinucleate pollen types were more likely to exhibit tube length reduction than the binucleate types. Proportionately more monocotyledonous species were sensitive to UV-B treatment than dicotyledonous species, and proportionately more wild species were sensitive than cultivated species and pollen collected from plants growing in the field were somewhat more sensitive than pollen collected from plants grown in the greenhouse. Species in which pollination occurred earlier in the season were more likely to be susceptible to UV-B radiation than those for which anthesis took place later in the season, suggesting a possible adaptation to UV-B radiation.     

Observation of Androgenesis in Cultured Wheat Anthers at Meiosis,Tetrad, Early Uninucleate and Trinucleate Stage

The obtaining of calluses and plantlets from cultured wheat anthersat the stages from pollen mother cell to trinucleate microspore has been reported previously. Haploids as well as diploids existed among the regenerated plantlets derivedfrom anthers at these stages. Present paper reports the study on androgenesis patter-ns of cultured anthers at meiosis, tetrad, early mid- and late uninucleate and trinucleate stage. Cytological evidence of pollen-origin of calluses produced by anthers atthese stages was given. Observation showed that meiosis of wheat anthers was able tocomplete under culture conditions, resulting in releasing microspores, from which multinucleate and multicellular pollen grains formed. In meiosis anthers, abnormal cells,including syncytium and two kinds of binueleate calls were sometimes observed. Theymight be products of abnormal meiosis and abnormal development of tapetum cells. Itwas noted that failure and/or uncomplction of forming callus wall and/or pollen wallin in vitro anthers at meiosis, tetrad and early uninucleate stage occured often. Itmight lead to the low frequency of callus induction. Mature wheat anthers (trinucleate stage) contained both normal and abnormal pollen grains (pollen dimorphism); onlythe abnormal pollen grains developed into embryoids while all the normai trinucleatepollen grains degenerated rapidly. However, the date of the frequency of equal divisionof microspores suggested that abnormal pollen (N pollen, small pollen) could not be theonly source of androgenic pollens in cultured anthers at late uninucleate and other earlier stages.     

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     

Dynamics of storage reserve deposition during Brassica rapa L. pollen and seed development in microgravity

Pollen and seeds share a developmental sequence characterized by intense metabolic activity during reserve deposition before drying to a cryptobiotic form. Neither pollen nor seed development has been well studied in the absence of gravity, despite the importance of these structures in supporting future long-duration manned habitation away from Earth. Using immature seeds (3-15 d postpollination) of Brassica rapa L. cv. Astroplants produced on the STS-87 flight of the space shuttle Columbia, we compared the progress of storage reserve deposition in cotyledon cells during early stages of seed development. Brassica pollen development was studied in flowers produced on plants grown entirely in microgravity on the Mir space station and fixed while on orbit. Cytochemical localization of storage reserves showed differences in starch accumulation between spaceflight and ground control plants in interior layers of the developing seed coat as early as 9 d after pollination. At this age, the embryo is in the cotyledon elongation stage, and there are numerous starch grains in the cotyledon cells in both flight and ground control seeds. In the spaceflight seeds, starch was retained after this stage, while starch grains decreased in size in the ground control seeds. Large and well-developed protein bodies were observed in cotyledon cells of ground control seeds at 15 d postpollination, but their development was delayed in the seeds produced during spaceflight. Like the developing cotyledonary tissues, cells of the anther wall and filaments from the spaceflight plants contained numerous large starch grains, while these were rarely seen in the ground controls. The tapetum remained swollen and persisted to a later developmental stage in the spaceflight plants than in the ground controls, even though most pollen grains appeared normal. These developmental markers indicate that Brassica seeds and pollen produced in microgravity were physiologically younger than those produced in 1 g. We hypothesize that microgravity limits mixing of the gaseous microenvironments inside the closed tissues and that the resulting gas composition surrounding the seeds and pollen retards their development.     

Male sterility in flowering plants: are plant growth substances involved?

Male sterility in flowering plants is of tremendous importance not only in molecular and developmental studies of stamen and pollen grains and evolutionary studies on the origin of dioecy, but also in its commercial application in hybrid seed production. This paper reviews the literature on the possible involvement of plant growth substances (PGSs) in male sterility, and in normal stamen and pollen development. Different experimental approaches on a number of male sterile systems and normal plants have shown that nearly all PGSs, i.e., gibberellins, cytokinins, auxin, abscisic acid, and ethylene, directly or indirectly influence the expression of male sterility. Analyses of endogenous PGSs have revealed that in male sterile plants the level and/or metabolism of more than one PGS is affected. These studies support the suggestion that it is the relative ratio of various PGSs, rather than any one substance, that is critical for normal stamen and pollen development. It is also proposed that gene-regulated male sterility is likely mediated through an altered balance of endogenous PGSs in developing flowers and stamens.