Male and female reproductive phenology and annual production of male cones in two natural populations of Cedrus deodara

Phenology of reproductive cones, microsporangium dehiscence and pollen production were studied in natural populations of Cedrus deodara at two different locations, the warmer Khirsu (1800  m a.s.l.) and the cooler Teka (1900  m a.s.l.) in the Pauri forest division of Garhwal Himalaya, India. The reproductive phenophases (cone bud initiation to pollination) goes up to four months with a blooming and pollination period of one month. The prolonged period of male cones during the immature phase was due to two months dormancy, and the dormancy, in turn, could be due to high precipitation during this phase. The pattern of opening of male cones was basipetal succession. Microsporangium dehiscence was dependent on air temperature and relative air humidity with a peak between noon and 2  pm. Flowering shows a high level of asynchrony among individuals within the population, leading to a possibility for geitonogamous selfing. Annual production of male cones varies greatly from one year to the next. A reduced number of pollen cones per tree in 2001 resulted in a decline in overall annual pollen production. In conclusion, the results revealed that the production of male cones represent only one factor that substantially influences the mating system, which will interact with the flowering phenology to influence the genetic composition of the population.     

Investigating Pollen and Gene Flow of WYMV-Resistant Transgenic Wheat N12-1 Using a Dwarf Male-Sterile Line as the Pollen Receptor

Pollen-mediated gene flow (PMGF) is the main mode of transgene flow in flowering plants. The study of pollen and gene flow of transgenic wheat can help to establish the corresponding strategy for preventing transgene escape and contamination between compatible genotypes in wheat. To investigate the pollen dispersal and gene flow frequency in various directions and distances around the pollen source and detect the association between frequency of transgene flow and pollen density from transgenic wheat, a concentric circle design was adopted to conduct a field experiment using transgenic wheat with resistance to wheat yellow mosaic virus (WYMV) as the pollen donor and dwarf male-sterile wheat as the pollen receptor. The results showed that the pollen and gene flow of transgenic wheat varied significantly among the different compass sectors. A higher pollen density and gene flow frequency was observed in the downwind SW and W sectors, with average frequencies of transgene flow of 26.37 and 23.69% respectively. The pollen and gene flow of transgenic wheat declined dramatically with increasing distance from its source. Most of the pollen grains concentrated within 5 m and only a few pollen grains were detected beyond 30 m. The percentage of transgene flow was the highest where adjacent to the pollen source, with an average of 48.24% for all eight compass directions at 0 m distance. Transgene flow was reduced to 50% and 95% between 1.61 to 3.15 m, and 10.71 to 20.93 m, respectively. Our results suggest that climate conditions, especially wind direction, may significantly affect pollen dispersal and gene flow of wheat. The isolation-by-distance model is one of the most effective methods for achieving stringent transgene confinement in wheat. The frequency of transgene flow is directly correlated with the relative density of GM pollen grains in air currents, and pollen competition may be a major factor influencing transgene flow.     

Pollen-mediated gene flow in Himalayan long needle pine (<Emphasis Type="Italic">Pinus roxburghii</Emphasis> Sargent)

Pollen migration in Pinus roxburghii has been estimated under two different conditions, viz. isolated single trees and at the fringe of natural forest. Trapping of pollen grains was conducted at geometrically increasing distances, i.e., 0, 5, 10, 20, 40, 80, 160, 320, 640, 1280 and 2560 m from the source. The observations revealed that the scattering of pollen grains was not uniform in all directions and that they travelled up to 2.5 km towards the downhill direction, while in the uphill direction dispersion of pollen grains was restricted to 320 m from the source. Pollen frequency declined rapidly as the distance from the source increased, with the highest densities within 50–100 m.     

Cyclic pollen production in Cedrus deodara

Based on a seven-year study of pollen production and release in two different natural populations of Cedrus deodara from Garhwal Himalaya, India, we determined that pollen output follows a two-year cycle. Pollen productivity determinations considered various sources of variability, including the number of pollen strobili per branch, strobili per tree, microsporophylls per tree and pollen grains per tree. Each of these parameters revealed significant year-to-year and population effects. Microsporangium dehiscence took from 2.5 to 3.5 days. Maximum dehiscence was observed between 12:00 and 14:00 h, which coincides with diurnal highest temperature and lowest relative humidity. Annual production of pollen per tree varied from averages of 4.7 × 10⁹, 7.2 × 10⁹ and 5.1 × 10⁹ in years of low production, with alternate high years, producing 12.6 × 10⁹, 14.1 × 10⁹, 13.3 × 10⁹ and 14.0 × 10⁹ pollen grains per tree. Annual pollen production in individual trees of C. deodara ranged from 1.4 × 10⁹ to 22.3 × 10⁹.     

13. Lake Saloio (Nazaré, western Portugal)

The possible impact of altitude and the related microclimatic conditions on the total production of fruiting branches, inflorescences, flowers and pollen grains of olive trees Olea europaea was analysed. A total of 90 Picual cultivar trees, the most extensive olive cultivar in the Iberian Peninsula, were studied for a three-year period (2007–2009). The study shows that production of flowers and pollen grains in a cultivar of the olive tree varies according to the microclimate. Our study also indicates that the olive trees frequently can have up to half a million flowers per tree. Moreover, the total flower production differs between years and study areas. In the Picual cultivar, the average production of pollen grains per anther is usually more than 60?000 grains. The total production of pollen per tree is around 72?000 million on average. The most favourable microclimatic conditions for reproduction in olive trees are found in years and olive growing areas with low temperature and high precipitation records during the months prior to flowering of the olive trees. We hypothesise that olive trees tend to increase their pollen production rate as altitude increases, which can be interpreted as a reproductive strategy to ensure fertilisation.     

Rapid Swelling of Pollen Grains in the Dehiscing Anther of Two-rowed Barley (Hordeum distichum L. emend. LAM .)

The role of rapid swelling of pollen grains in anther dehiscencein Hordeum distichum L. emend. L AM. and the mechanism of thisswelling were examined. Artificial opening of the floret inducedrapid swelling of pollen grains and thecae dehiscence. The thecadehisced as pollen grains became swollen and dehisced anthershad larger pollen grains than indehisced anthers. Septa in theanther segments dehisced as a result of water-induced pollenpressure. These results strongly support the theory that therapid swelling of pollen grains is a driving force for antherdehiscence. On the other hand, potassium was detected in pollengrains from dehisced anthers, but not in pollen grains in indehiscedanthers. This suggests that potassium ions function as a turgorregulator in the rapid swelling of pollen grains. The mechanismof anther dehiscence is discussed in relation to the swellingof pollen grains, as is the possible mechanism of this swelling.Copyright 2000 Annals of Botany Company Anther dehiscence, Hordeum distichum L. emend. L AM., pollen swelling, potassium ion, two-rowed barley     

肋果沙棘(Hippophae neurocarpa S.W.Liu et T.N.He)自然种群的风媒传粉特征

野外调查了青藏高原特有植物肋果沙棘的花期物候、开花式样以及风媒传粉特征,对其花粉生活力、柱头可授性及单花花粉量进行了检测.结果表明,肋果沙棘有许多适应风媒传粉的特征:雌雄异株,花小,无花冠,先叶开放,雄花萼片纵裂形成对流的"风洞",花粉近球形,单花花粉量大;雌花柱头细长捻状,伸出萼筒,授粉面大,密布乳突.花粉生活力可以持续16.5 d,柱头可授性为10～20d.肋果沙棘传粉多发生在昼夜温差大的5月中下旬,在多变的气候条件下,种群花期可持续约12～25d,但在晴朗的天气条件下具有爆发性的集中开花机制,是对恶劣条件的适应.重力玻片法检测表明,离花粉源15m左右远的地点花粉密度最大,135m仍能接收到花粉,夜间接收到的花粉数量也很可观,这些研究结果表明,该植物集中在有利的晴朗天气昼夜进行长距离的风媒散粉,能较好地适应青藏高原的复杂环境.     

Annual variations of air-borne pollen in the Coastal Plain of Israel

A survey of airborne pollen was conducted during 1984 in the coastal plain of Israel. The flora of that area is continuously changing due to urbanization and agriculture, thus affecting the airborne pollen spectrum.

Significant pollen counts were monitored throughout the year, with a seasonal peak during spring. Meteorological parameters, such as wind velocity, wind direction and temperature affected pollen content of the air. Under certain conditions, even pollen of insect-pollinated plant species was found in the air.

The most conspicuous among the airborne pollen were olive, cypress and pine trees as well as pollen grains from the Poaceae, Amaranthaccae, Chenopodiaceae, Asteraceae, Brassicaceae and Urticaccae.

Results indicate that most of the airborne pollen grains were of local origin and from cultivated trees. Thus, it is possible to reduce airborne pollen contaminants around human habitations by selection of the proper non-allergenic ornamental plants.     

Pollen flow of cultivated rice measured under experimental conditions

The pollen flow pattern of a cultivated rice variety, Minghui-63, was studied at horizontal and vertical levels under experimental conditions. Data obtained from pollen traps for six designed populations (as pollen sources) at different intervals showed that the dispersal of rice pollen decreased with the increase of distance from pollen sources and that the rice pollen flow was significantly influenced by weather conditions, particularly by wind direction and speed. For a mean wind speed of 2.52 m/s in a downwind direction, the observed distance of rice pollen dispersal was 38.4 m, indicating that rice pollen grains normally disperse at a relatively small range. However, the maximum distance of rice pollen flow could be up to 110 m, using regression analysis of pollen flow and wind speed, when the wind speed reached 10 m/s in this study. The frequency of pollen flow was positively correlated with pollen source size within a given range, suggesting that pollen flow will occur effectively at a considerable rate in rice fields with sufficiently large pollen sources. In addition, many more pollen grains were detected at the height of 1.0–1.5 m than at 2.0 m, indicating that rice pollen mainly disperses at relatively low heights. Results from this study are useful both for minimizing transgene escape from transgenic rice and in situ conservation of wild relatives of rice, as well as for hybrid seed production, where an effective isolation buffer zone needs to be established.     

Aeroallergens of plant origin: Molecular basis and aerobiological significance

This review presents an update on the sources and molecular basis of aeroallergens of plants, derived from pollen, seeds, leaf and stem detritus and their protein molecules. These aeroallergens are a natural component of the atmosphere, either because of their natural function or human activity. Pollen is a source of allergens within the 10–200 μm size range, and while most allergenic pollen types account for only 20–30% of total annual pollen catch, during their flowering season, they are usually the dominant type. Tree pollen commences the season in winter, with birch pollen counts in Scandinavia being the highest daily pollen counts yet reported and a major allergen, a 14-kDa protein, which is similar to pathogenesis-related proteins. Grass pollen follows in spring, and is unique as its two immunodominant allergens, a 35-kDa glycoprotein and 28–32-kDa protein, are in different cellular sites: the cytosol and surface of pollen grains; and in intracellular starch granules. The allergens at the pollen surface are not inhalable and can interact only with the eyes, nasal and oral cavities. Starch granules are released to the atmospheric aerosol when grains rupture in rainwater. These are a major source of allergen-containing micronic particles, which are important because they are inhalable. At the same time, allergen molecules are present in the aerosol, and these can bind to soot particles, and so be respired deep into the airways. The major Japanese cedar pollen allergen has been detected both within the pollen and in orbicules; particles less than 1 μm that line the anther cavity and can be released into the air when dehiscence occurs. Ragweed is the major cause of late summer hayfever in eastern North America, where its pollen accounts for up to 41% of the annual pollen catch. It is a major source of aeroallergens in both respirable and non-respirable size ranges. As a result of human activity, dusts derived from seeds and cereal grains during transport, storage and milling provide a source of micronic particles, containing potent allergens that can trigger allergic disease.     

The fluctuations of the allergenic pollen content of the air in Brussels (1982 to 1997)

The pollen content of the air was studied from1982 to 1997 in Brussels. Fluctuations weremainly observed in the tree pollen content whilethe amount of pollen originating from herbaceousplants remained more stable during theinvestigation period.The number of allergenic tree pollen grains didnot rise steadily during this period. We noted,however, the existence of years with high pollenproduction following the physiological biorhythmof the trees. Years with high annual sums for agiven tree pollen type did not coincide withyears with high annual sums for another treepollen type. Variations in annual sums of allergenicherbaceous pollen types were less important thanthose of tree pollen types. No rise, nor cyclicpatterns were observed. The most important allergenic pollen types(Betulaceae, Fagaceae, Gramineae and Artemisia) have been presented in histograms.     

白杨杂种三倍体小孢子母细胞减数分裂及花粉形态观察

通过醋酸洋红压片等方法观察毛新杨×银腺杨杂种三倍体小孢子母细胞减数分裂及花粉形态,以探讨杂种三倍体雄配子的育性及其细胞学机制.结果显示:(1)在小孢子母细胞减数分裂过程中可观察到部分染色体提前分向两极、落后染色体、微核等一系列染色体异常行为,表明该三倍体存在染色体的高度不平衡性.(2)在减数第二次分裂过程中存在大量胞质分裂提前现象;减数分裂产物中发现有少量二分体、三分体的存在,同时毛新杨×银腺杨杂种三倍体花粉粒的直径变化范围为22.6～62.6 μm,呈现双峰分布,由此推测有一定数量的具可育性的未减数花粉产生.     

Airborne pollen flora of Jabalur – the central India

The fortnightly aerobiological survey of pollengrains at five different sites at two differentheights (ground level and at 10 m) was carriedout at Jabalpur city during January toDecember, 1996. Air sampling was done for 30minutes at each sampling point by using aRotorod air sampler. A total of 84 pollen typesbelonging to 34 families of Angiospermae andone family of Gymnospermae were identified. Themaximum mean concentration (1.04/m³ air) ofpollen was contributed by Asteraceae followedby Amaranthaceae/Chenopodiaceae (0.77/m³ air)and the minimum (0.001/m³ air) byCucurbitaceae, Convolvulaceae and Sapotaceae.Maximum numbers of pollen grains werecontributed by herb species (62.07%) followedby shrubs (18.47%) and trees (14.96%). Asmall fraction, i.e., 4.50% of total pollenflora remained unidentified. Morphologicalanalysis revealed maximum concentration(1.81/m³) of triporate pollen grains. There wasa significant difference in the pollenconcentration at ground level and at 10 mheight. The ratio observed was 1.7 : 1.0.There was non-significant negative correlation(r = –0.196) between mean pollen concentrationand mean atmospheric temperature. May, thehottest month, showed minimum mean pollenconcentration (1.65/m³ air), whereas October,with moderate temperature, the maximum(10.86/m³ air).     

Pollination Biology of Cycas panzhihuaensis L. Zhou et S.Y.Yang

The population of cone-bearing cycad, Cycas panzhihuaensis L. Zhou et S. Y. Yang, was male-biased. Although the number of male individuals was almost equal to that of female individuals, the clonal and cone production of male individuals was 1.31 and 2.21 times as much as that of female individuals respectively. The sequence of pollen shedding was from the base to the top of the cone and microsporophyll. The peak of daily pollen shedding occurred at noon and in the afternoon. The airborne pollen concentration decreased quickly within 2.55 m and finally maintained at a low level along with the distance from emitting male cone. The megastrobilus was transformed into a receptive state at the beginning of pollination due to the regular morphological changes of megasporophylls. Two kinds of ants and one kind of cockroaches were found to be active within the megas- trobili during pollination, albeit causing no tissue damage. The microstrobili gave off a strong odor of fennel that could dispel all the insects nearby. The sterile foliar structure of the megasporophyll was able to secret aqueous droplets at dawn which might function as transporting adhered pollen grains by dislodging and accumulating them on or near micropyles during the process of droplets falling. Resuits from field observation showed that pollination of C. panzhihuaensis might be accomplished by different pollinators. Pollen grains were firstly wind-transported from microstrobili to megastrobili and then insects and secreted droplets on the megasporophyll either directly or indirectly carried the pollen grains to ovules within a megastrobilus. However, insects might play as a subsidiary pollinator due to the preferential concentration of airborne pollen grains transported to the megasporophylls.     

攀枝花苏铁传粉生物学研究

在攀枝花苏铁(Cycas panzhihuaensis L.Zhou et S.Y.Yang)自然群体中,雌雄株在数量上基本相等,但雄株的无性系产量是雌株的1.31倍,小孢子叶球的数量是大孢子叶球的2.21倍,呈现偏雄现象。小孢子叶球散粉次序是从轴基部向顶部和从小孢子叶的基部向顶部进行的,散粉高峰出现在午后,风传花粉浓度在2.55m内随着距离增加而迅速下降,而在2.55m以外维持在一个较低的水平上。在大孢子叶球内发现有两种蚂蚁和一种蜚蠊类昆虫在活动,没有发现这些昆虫对大孢子叶球的破坏,小孢子叶球则几乎没有昆虫探访。清晨在大孢子叶叶片上出现许多水样液滴,可能起着将大孢子叶叶片上沉积的花粉传递至胚珠的作用。研究表明,在攀枝花苏铁的传粉过程中,风是将花粉从小孢子叶球传至大孢子叶球的唯一媒介,大孢子叶球内活动的昆虫和大孢子叶叶片上的液滴同样起着传粉媒介作用,但不排除风传花粉一步到位的可能性。     

Evaluation of indoor passive pollen sedimentation over 1 year: a possible source of contamination?

The study aimed to evaluate the possible contamination by passive indoor pollen sedimentation in a laboratory where routine pollen analyses were conducted, but where no particular laboratory policies were adopted to limit contamination. Gravimetric pollen deposition was observed on traps (petri dishes soaked with glycerol) set in the palynology laboratory, under an extractor hood and on the bench beside it over 1 year (1995–1996), and in an air filtered room in a flow cabinet and on its roof over 1 month for comparison. Under the extractor hood, three types of airborne particles were deposited: pollen grains, spores and algae, representing 32.35, 67.28 and 0.37%, respectively, of the total sedimented particles over 1 year. The number of pollen grains deposited on the surface trap ranged from 0 (27 November to 4 December 1995) to 707 (10–18 April 1995). The highest number of taxa during a weekly collection was 23 (9–15 May 1995). The pollen flora represented by anemophilous pollen (>90% of the trapped pollen) was related to the vegetation next to the laboratory:Acer, Carpinus, Castanea, Corylus, Cupressaceae, Pinus, Quercus, Salix, Taxus for trees and shrubs andArtemisia, Brassicaceae, Plantago, Poaceae, andUrticaceae for grasses and weeds. Indoor pollen deposition corresponded to the period of the outdoor pollination (macroscopic field observation) which lasted from March to the beginning of August. However, some pollen were almost always present in the collection (Poaceae, Salix, Castanea, Betula), reflecting the occurrence of pollen grains in the atmosphere out of the pollination period. Moreover, about five times more entomophilous pollen was found under the extractor hood compared to the other area of the laboratory; even in the flow cabinet of the air filtered room, 237 particles were captured (versus only 15 on the roof). These data suggest a possible human contamination during operations under the extractor hood or in the flow cabinet. Although few airborne pollen were found, possible contamination has to be considered in investigations where even low pollen quantities are of interest.     

Microsporogenesis inPinus sylvestris L. VIII. Tapetal and late pollen grain development

This last portion of our developmental study ofPinus sylvestris L. pollen grains extends from just prior to the first microspore mitosis to the microsporangial dehiscence preparatory to pollen shedding. In nine years of collecting each day the duration of the above period was 7 to 11 days. Tapetal cells extended into the loculus and embraced microspores during the initial part of the above period. Thereafter tapetal cells receded, became parallel to parietal cells and so imbricated that there appeared to be two or three layers of tapetal cells. Tapetal cells were present up to the day before pollen shedding, but only rER and some mitochondria appeared to be in good condition at that time. A callosic layer (outer intine) was initiated under the endexine before microspore mitosis. After the first mitosis the first prothallial cell migrated to the proximal wall and was covered on the side next to the pollen cytoplasm by a thin wall joining the thick outer intine. There are plasmodesmata between pollen cytoplasm and the prothallial cell. After the second mitosis the second prothallial cell became enveloped by the outer intine. The inner intine appears after formation of the two prothallial cells but before the third mitosis. During this two-prothallial cell period before the third mitosis, plastids had large and complex fibrillar assemblies shown to be modified starch grains. After the third mitosis plastids of the pollen cytoplasm contained starch and the generative cell (antheridial initial), the product of that mitosis, is enveloped by the inner intine. On the day of pollen shedding cells are removed from the microsporangial wall by what appears to be focal autolysis. The tapetal and endothecial cells for 10–15 µm on each side of the dehiscence slit are completely removed. One or more epidermal cells are lysed, but both a thin cuticle and the very thin sporopollenin-containing peritapetal membrane remain attached to the undamaged epidermal cells bordering the dehiscence slit. Our study terminates on the day of pollen shedding with mature pollen still within the open microsporangium. At that time there is no longer a clear morphological distinction between the outer and inner intine but, judging by stain reactions, there is a chemical difference. The exine of shed pollen grains was found to be covered by small spinules on the inner surface of alveoli. These had the same spacing as the Sporopollenin Acceptor Particles (SAPs) associated with exine initiation and growth.     

Quantitative DNA Analyses for Airborne Birch Pollen

Birch trees produce large amounts of highly allergenic pollen grains that are distributed by wind and impact human health by causing seasonal hay fever, pollen-related asthma, and other allergic diseases. Traditionally, pollen forecasts are based on conventional microscopic counting techniques that are labor-intensive and limited in the reliable identification of species. Molecular biological techniques provide an alternative approach that is less labor-intensive and enables identification of any species by its genetic fingerprint. A particularly promising method is quantitative Real-Time polymerase chain reaction (qPCR), which can be used to determine the number of DNA copies and thus pollen grains in air filter samples. During the birch pollination season in 2010 in Mainz, Germany, we collected air filter samples of fine (<3 μm) and coarse air particulate matter. These were analyzed by qPCR using two different primer pairs: one for a single-copy gene (BP8) and the other for a multi-copy gene (ITS). The BP8 gene was better suitable for reliable qPCR results, and the qPCR results obtained for coarse particulate matter were well correlated with the birch pollen forecasting results of the regional air quality model COSMO-ART. As expected due to the size of birch pollen grains (~23 μm), the concentration of DNA in fine particulate matter was lower than in the coarse particle fraction. For the ITS region the factor was 64, while for the single-copy gene BP8 only 51. The possible presence of so-called sub-pollen particles in the fine particle fraction is, however, interesting even in low concentrations. These particles are known to be highly allergenic, reach deep into airways and cause often severe health problems. In conclusion, the results of this exploratory study open up the possibility of predicting and quantifying the pollen concentration in the atmosphere more precisely in the future.     

Ecological and selective effects of stigma-anther separation in the self-incompatible tropical tree Ipomoea wolcottiana (Convolvulaceae)

We evaluated the effect of stigma-anther separation (SAS) in flowers of the self-incompatible tropical tree Ipomoea wolcottiana (Convolvulaceae). We measured the deposition of self pollen on the stigma and the growth of pollen tubes. We also evaluated phenotypic selection on female fertility using univariate regressions of SAS with fruit production. SAS varied among individuals in two years, with a range of 0–6 mm. There was a negative correlation between SAS and the number of self pollen grains on the stigma; trees with SAS of 0–2 mm received averages of ca 80–130 self grains. The effect of loading the stigma with self-pollen was tested by allowing isolated flowers to self-pollinate, then cross-pollinating by hand: there was a negative correlation between the pollen load and the proportion of grains forming pollen tubes. The highest proportion of grains forming tubes was found with loads of <10 grains, while the minimum frequency of tubes occurred with loads of >50 grains. The results suggest that more self pollen deposition leads to fewer tubes from outcross pollen. There was a significant effect of SAS on fruit production in both years. The analyses suggested stabilizing selection; very small SAS is disadvantageous due to the high load of self pollen, while very large SAS probably led to damage to the style during pollinator visits. We suggest that herkogamy may be a key character for determining the success of fruit production in self-incompatible hermaphroditic species.     

Physiological factors limit fruit set of tomato (Lycopersicon esculentum Mill.) under chronic, mild heat stress

The effects of chronic, mild heat stress on fruit set, fruit production, release of pollen grains, photosynthesis, night respiration and anther dehiscence were examined in tomatoes (Lycopersicon esculentum Mill.) differing in high‐temperature sensitivity. Plants were grown under three temperature regimes: (1) 28/22 or 26/22 °C (optimal temperature); (2) 32/26 °C (high temperature); and (3) 32/26 °C day/night temperatures relieved at 28/22 °C for 10 d before anthesis, then returned to 32/26 °C (relieving treatment). FLA 7156 was the only cultivar with fruit set at 32/26 °C. All five cultivars, however, had fruit set under the relieving treatment (RT). The longer the relief, the higher the percentage of fruit set. Longer periods of relief also increased the number of pollen grains released, and linear regression analysis showed a significant relationship between the number of pollen grains released and the percentage of fruit set. Germination of pollen grains was also lowered in high‐temperature‐grown plants. The number of pollen grains produced, photosynthesis and night respiration did not limit fruit set under chronic, mild heat stress, however. This suggested that cultivar differences in pollen release and germination under heat stress are the most important factors determining their ability to set fruit.