Types and meaning of pollen carbohydrate reserves

During pollen development, soluble carbohydrates of sporophytic origin may be consumed immediately, polymerized to form starch reserves or intine, or transformed into other molecules. Disregarding intine, in mature pollen there are three different types of carbohydrates: (1) polysaccharides such as starch in amyloplasts or polysaccharides in cytoplasmic vesicles, (2) disaccharides such as sucrose and (3) monosaccharides such as glucose and fructose. At dispersal, pollen may be partly or slightly dehydrated, or not dehydrated at all. Partly dehydrated pollen has the capacity to lose or acquire water within limits without detriment to its viability. Slightly and non-dehydrated pollen is vulnerable to water loss and quickly becomes inviable. In partly dehydrated of pollen the carbohydrates consist of cytoplasmic polysacharides and sucrose; in slightly and non-dehydrated pollen these are absent or in low concentrations but there may be reserves of cytoplasmic callose. Starch, glucose and fructose are found in both types. It is postulated that cytoplasmic carbohydrates and sucrose are involved in protecting pollen viability during exposure and dispersal.     

Nectary Structure, Nectar Secretion Patterns and Nectar Composition in Two Helleborus Species

Abstract: The morphological and cytological characteristics of nectaries of Helleborus foetidus and H. bocconei during the secretory period are reported. The nectaries are derived from modified petals and secrete nectar continuously for about 20 days; they consist of a single layered epidermis, nectar-producing parenchyma and photosynthesizing parenchyma. Nectar secretion is holocrine and the nectar is released by rupture of the wall and cuticle of each epidermal cell. The nectaries of the two species differ in number and external morphology. In H. foetidus, secretion begins before anthesis and secretion rate decreases with nectary age. In H. bocconei it begins on the day of anthesis and proceeds at a constant rate. The nectar has a high sugar content, mainly sucrose, and also contains lipids and proteins.     

Ripe pollen carbohydrate changes in Trachycarpus fortunei: the effect of relative humidity

Pollen of the palm Trachycarpus fortunei was kept at 25°C and relative humidities (RH) of 20, 55 and 98%. Changes in viability, water content and carbohydrates were measured over 2–17 days. Water content remained almost constant at 20 and 50% RH and increased dramatically at 98%. Pollen viability and germination rate remained almost constant over 14 days at 20% RH and decreased to about 2% after 7–9 days at 55% and to even less at 98% RH. Although the three experimental conditions were constant, qualitative and quantitative variations in pollen carbohydrates were recorded, even after pollen had lost its viability. The quantities of mono-, di- and polysaccharides varied with the period of pollen storage at the various RH. The greatest changes in glucose, fructose and sucrose content were recorded at 55 and 98% RH. At these relative humidities, maximum glucose and fructose content and minimum sucrose content occurred at maximum water content. Starch was not present in mature pollen but appeared and peaked after 7–9 days of pollen storage at 55 and 98%. Appearance of starch coincided with an increase in pectin content. PAS-positive cytoplasmic polysaccharides showed an increasing trend at 20% RH. A relation was found between pollen viability, water content and monosaccharide content. Pollen viability and germination capacity remained high at 20% RH for 14 days. At this relative humidity, pollen water, glucose and fructose contents remained almost constant, while sucrose reached its maximum value. The fluctuations of more complex carbohydrates (starch, pectins and PAS-positive cytoplasmic polysaccharides) were less easy to interpret. Changes observed under experimental conditions could simulate processes occurring in nature during pollen presentation and dispersal.     

Occurrence of mono- or disaccharides and polysaccharide reserves in mature pollen grains

 Pollen from 13 species of gymnosperms and angiosperms was studied for soluble and insoluble carbohydrates at dispersal. Starch reserves stored during pollen development give rise to carbohydrates at maturity. Combinations of different types of carbohydrates in mature pollen may depend on the extent of starch hydrolysis. An inverse relationship was found between the extent of starch hydrolysis and sucrose content. If the starch was scarcely de-polymerized, the cytoplasm had very low levels of soluble sugars and none of the periodic acid-Schiff (PAS)-positive material as found in pollen not subject to high dehydration (Cucurbita pepo L., Zea mays L.). After total or partial starch hydrolysis, insoluble PAS-positive oligo/polysaccharides were found in the cytoplasm associated with much soluble sugar, and the pollen grains were dehydrated at dispersal as in Typha latifolia L., Chamaerops humilis L., Trachycarpus excelsa Wendl., and other specimens. Intermediate levels of starch and soluble sugars, together with cytoplasmic PAS-positive material, characterized species with dehydrated pollen such as Pinus halepensis Miller. Carbohydrates may be related to pollen longevity, which largely depends on the abundance of sucrose, which is known to protect membrane integrity. The relationship between PAS-positive material and pollen viability is unclear at present. Received: 30 July 1996 / Revision accepted: 18 December 1996     

From anther and pollen ripening to pollen presentation

The events and processes occurring between pollen maturation, opening of the anther and presentation of pollen to dispersing agents are described. In the final phases of pollen development, starch is always stored; this occurs before the anther opens. Depending on the species, this starch may be totally or partially transformed into: (a) other types of polysaccharides (fructans and rarely callose); (b) disaccharides (sucrose); (c) monosaccharides (glucose and fructose, all situated in the cytoplasm. While awaiting dispersing agents and during dispersal, polysaccharides, especially fructans, and sucrose may be interconverted to control osmotic pressure and prevent loss and uptake of water. Opening of the anther is preceded by disappearance of the locular fluid and in many cases by partial dehydration of the pollen. Pollen generally has a water content between 5 and 50%. Pollen with a high water content may or may not be able to control water retention during pollen exposure and dispersal. Pollen may be dispersed in monads or grouped in pollen dispersing units by the following mechanisms: (i). tangling of filamentous pollen; (ii). adhesion by viscous substances (pollenkitt, tryphine, elastoviscin) derived from the tapetum; (iii). common walls. When the anther opens, the pollen may be dispersed immediately, remain until dispersed (primary presentation), or be presented to pollinators in another part of the flower (secondary presentation).     

Pollination ecology in sympatric winter flowering Helleborus (Ranunculaceae)

We studied biotic and abiotic factors that influence pollination in two sympatric winter flowering species. Helleborus foetidus and Helleborus bocconei flower simultaneously in winter. Although climatic conditions are not favorable for biotic pollination both species rely mainly on large bees of the genus Bombus. At the beginning of flowering, harsh climatic conditions are restrictive for insect visits. As flowering continues and temperatures rise, pollinator activity increases. The two plant species share pollinators that visit them indiscriminately. The flowers of the two species differ in form and insects visit H. foetidus by inserting their heads and H. bocconei ventrally: pollen load on insects is highly specialized. With the arrival of spring, many other species start to bloom and in spite of the large number of flowers still on the plants insects abandon Helleborus species. At the end of spring increasing biotic interactions take away pollinators from the Helleborus species.     

Censusing natural microgametophyte populations: variable spatial mosaics and extreme fine-graininess in winter-flowering Helleborus foetidus (Ranunculaceae)

Little is known about patterns and correlates of variation in microgametophyte populations for naturally pollinated plants, yet this information is critical for evaluating the prevalence and potential evolutionary significance of gametophyte competition in the wild. This paper analyzes spatial and temporal variation in microgametophyte populations (= number of pollen tubes per style) for the winter-flowering, perennial herb Helleborus foetidus (Ranunculaceae), based on data from 29 populations in three regions of the Iberian Peninsula collected over two consecutive years. Mean size of microgametophyte populations varied significantly at a wide range of spatial scales, including among regions, among populations within regions, among individual plants within populations, among flowers of the same plant, and among pistils of the same flower (H. foetidus flowers are apocarpous). Differences between regions were quantitatively negligible. Differences between populations in the same region were moderate to low, and their sign and magnitude were inconsistent between years. Roughly half of total variance in microgametophyte numbers was accounted for by variation within individual plants, and the largest part of this component was due to differences between the pistils of the same flower. These results reveal extreme spatial fine-graininess and marked stochasticity in the spatial variation of H. foetidus microgametophyte populations and suggest that opportunities for consistent selection on male gametophyte competitive ability are probably negligible in this species.     

Pollen performance, cell number, and physiological state in the early-divergent angiosperm Annona cherimola Mill. (Annonaceae) are related to environmental conditions during the final stages of pollen development

Pollen performance is an important determinant for fertilization success, but high variability in pollen behavior both between and within species occurs in different years and under varying environmental conditions. Annona cherimola, an early-divergent angiosperm, is a species that releases a variable ratio of bicellular and tricellular hydrated pollen at anther dehiscence depending on temperature. The presence of both bi- and tricellular types of pollen is an uncommon characteristic in angiosperms and makes Annona cherimola an interesting model to study the effect of varying environmental conditions on subsequent pollen performance during the final stages of pollen development. In this work, we study the influence of changes in temperature and humidity during the final stages of pollen development on subsequent pollen performance, evaluating pollen germination, presence of carbohydrates, number of nuclei, and water content. At 25?°C, which is the average field temperature during the flowering period of this species, pollen had a viability of 60-70?%, starch hydrolyzed just prior to shedding, and pollen mitosis II was taking place, resulting in a mixture of bi- and tricellular pollen. This activity may be related to the pollen retaining 70?% water content at shedding. Temperatures above 30?°C resulted in a decrease in pollen germination, whereas lower temperatures did not have a clear influence on pollen germination, although they did have a clear effect on starch hydrolysis. On the other hand, slightly higher dehydration accelerated mitosis II, whereas strong dehydration arrested starch hydrolysis and reduced pollen germination. These results show a significant influence of environmental conditions on myriad pollen characteristics during the final stages of pollen development modifying subsequent pollen behavior and contributing to our understanding of the variability observed in pollen tube performance.     

Tomato pollen tube development and carbohydrate fluctuations in the autotrophic phase of growth

Ripe pollen has different soluble and insoluble carbohydrates in variable amounts. Pollen germination and pollen tube growth were studied in a tomato cultivar (Solanum lycopersicum L. cv. Platense) with atypical pollen among tomatoes due to its very low amount or absence of sucrose. In vitro assays were performed using a culture medium without carbohydrates to explore whether there is an autotrophic phase of pollen tube growth, and if there is, describe it, and to analyze the fluctuations of endogenous carbohydrates (soluble carbohydrates, starch, pectins, and callose). Pollen germination was fast (ca. 10 min) and a definite autotrophic phase was observed. Soluble carbohydrates and pectins showed the most substantial changes during this period, even after 10 min. A small amount of callose was observed in the ripe pollen and pollen tubes. Pectins were the most abundant pollen tube wall component. Pollen can be considered starchless; starch was not involved in the autotrophic phase of growth. Other types of substances must be connected with the carbohydrate metabolism, because the fluctuations of the different substances did not follow balanced stoichiometric relationships. Pollen germination and pollen tube elongation was sustained autotrophically, even though sucrose was absent and starch was negligible in pollen grains. The type of pollen reserves and the fast pollen tube formation could be selective advantages in this cultivar.     

Effects of relative humidity and temperature conditions on pollen fluorochromatic reaction of Rosmarinus officinalis L. (Lamiaceae)

Summary. Mediterranean ecosystems are characterized by seasonal and annual fluctuations in humidity and temperature which are considered limiting factors for plant growth and might have played a key role in the selection of species that compose the present vegetation. After anther release, pollen is generally exposed to various changes of temperature and humidity conditions, therefore its viability and consequently successful fruit set are strongly affected by these environmental parameters. The aim of this research was to study the effect of different combinations of humidity and temperature on pollen membrane integrity of Rosmarinus officinalis L. in order to investigate possible relations between pollen features and climatic conditions during flowering. This species is an evergreen perennial shrub, occurring among the sclerophyllous vegetation of Mediterranean maquis. In many areas of Southern Italy, it shows a flowering period starting from the beginning of winter and spreading throughout spring months. The analysis of data showed that duration of pollen viability of R. officinalis is enhanced by the co-occurrence of low temperature and high humidity. Moreover, these conditions are able to newly raise the viability of pollen kept at higher temperatures and lower humidity. These observations indicate that reactivation of the pollen membrane depends on low temperature apart from high humidity. Therefore “vernalization” of rosemary pollen enhances its viability, supporting that pollen behavior is adapted to winter condition and allows flowering in winter and early spring. Correspondence and reprints: Laboratorio di Botanica ed Ecologia Riproduttiva, Dipartimento di Arboricoltura, Botanica e Patologia Vegetale, Università degli Studi di Napoli Federico II, Via Università 100, 80055 Portici, Naples, Italy.     

Extreme intraplant variation in nectar sugar composition in an insect-pollinated perennial herb

Variation in nectar chemistry among plants, flowers, or individual nectaries of a given species has been only rarely explored, yet it is an essential aspect to our understanding of how pollinator-mediated selection might act on nectar traits. This paper describes variation in nectar sugar composition in a population of the perennial herb Helleborus foetidus (Ranunculaceae) and dissects it into components due to variation among plants, flowers of the same plant, and nectaries of the same flower. The proportions of sucrose, glucose, and fructose in single-nectary nectar samples collected at two times in the flowering season were determined using high performance liquid chromatography (HPLC). Sugar composition varied extensively among nectaries, and nearly all combinations of individual sugars were recorded. Population-wide variance was mainly accounted for by variation among flowers of the same plant (56% of total), nectaries of the same flower (30%), and only minimally by differences among plants (14%). In absolute terms, intraplant variation was similar to or greater than that ordinarily reported in interspecific comparisons. Results suggest that the prevailing notion of intraspecific constancy in nectar sugar composition may be unwarranted for some species and that more elaborate nectar sampling designs are required to detect and appropriately account for extensive within-plant variance. Within-plant variation in nectar sugar composition will limit the ability of pollinators to exert selection on nectar chemistry in H. foetidus and may be advantageous to plants by reducing the number of flowers visited per foraging bout by variance-sensitive, risk-averse pollinators.     

Plant traits, environmental factors, and pollinator visitation in winter-flowering Helleborus foetidus (Ranunculaceae)

BACKGROUND AND AIMS: This study examined the effect of plant traits and environmental factors on pollinator visitation in the winter-flowering Helleborus foetidus (Ranunculaceae) in three distant regions in the Iberian Peninsula. METHODS: Geographical variation in floral visitor assemblage, plant traits and environmental factors were analysed during the flowering season. KEY RESULTS: Differences were found in all plant traits measured (number of open flowers, flower size, number of stamens per flower, and number of nectaries) both within and among regions, and differences among regions in all the environmental factors considered (air temperature, exposure to sunlight, canopy cover, and distance to the nearest neighbour). Differences were also found among regions in the probability that plants would be visited by pollinators. CONCLUSIONS: The results show that, although floral display (i.e. number of open flowers on a plant on a given day) consistently explained among-plant differences in visitation rate in all regions, visitation rate was not significantly affected by any other biological or environmental variable. In Helleborus foetidus, then, 'how' the plant is would seem to be more important than 'where' is it.     

Effect of relative humidity on water content, viability and carbohydrate profile of Petunia hybrida and Cucurbita pepo pollen

Petunia hybrida and Cucurbita pepo pollen was exposed to 30 and 75% relative humidity (RH). Water content, viability and carbohydrate content (glucose, fructose, sucrose and starch) were measured at fixed intervals over 6 h. Water content of C. pepo pollen decreased drastically at both RHs, while P. hybrida pollen dehydrated slightly at RH 30% and hydrated at RH 75%. The pollen of the two species also showed very different sensitivity to dehydration. P. hybrida pollen was resistant to desiccation, with viability remaining around 80% throughout the experiment, whereas C. pepo was very sensitive to desiccation, showing an abrupt decrease in viability when its water content reached about 13% (fresh weight basis) at both RHs. Carbohydrate content was also different in the two species. Sucrose content increased soon after dehydration of P. hybrida pollen at RH 30%, whereas it remained almost constant in C. pepo pollen at the same RH. The results are discussed in the framework of basic pollen physiology and of plant reproductive strategy.     

The Effect of Storage Temperature on Shoot Growth, Flowering, and Carbohydrate Metabolism in Tulip Bulbs

Dry bulbs of the cvs. ‘Apeldoorn’ and ‘Paul Richter’ at stage G of flower development were stored at 5° or 21°C for 2, 4, 6, 8, 10, 12, and 14 weeks, respectively before being planted and forced at 18°C. Samples from each treatment were taken for carbohydrate analysis. The low temperature treatment (5°C) was necessary to obtain satisfactory shoot growth and flowering after planting. The rate of shoot growth and the percentage of flowering bulbs increased with increasing duration of the 5°C treatment. Time of flowering was also precipitated. 12–14 weeks of low temperature treatment seemed optimal. High temperature (21°C), or a short period at 5°C (2–6 weeks), resulted in many non-flowering bulbs, and a very slow shoot elongation when flowering occurred. In the latter case the tips or large areas of the perianths became white, the red pigmentation being prevented. Paper chromatographic analysis of oligosaccharides revealed a substantially increased content of sucrose and fructosyl sucrose (DP ≤ 5) during the first 2–4 weeks of cooling. At the end of 12 weeks at 5°C, the content of oligosaccharides decreased. The increase in the oligosaccharide content was accompanied by a corresponding starch decrease. High temperature storage (21°) led to comparatively slight changes in the sucrose and fructosyl sucrose content of the bulbs. The significance of carbohydrate metabolism in relation to shoot elongation and flowering is discussed.     

Seasonal Changes of the Metabolites in the Leaves, Bark and Xylem Tissues of Olive Tree (Olea europaea. L) II. Carbohydrates

Seasonal changes of starch and soluble carbohydrates in leaves,bark and xylem tissues of olive tree were examined during acomplete annual cycle. Leaf starch and soluble carbohydrateswere detected at high levels during the spring and autumn metabolicallyactive periods. The low level of leaf starch in combinationwith the drastic reduction of soluble carbohydrates and mannitol,defined the summer period of the low metabolic state of thetree. The low leaf starch level in conjunction with the risensoluble carbohydrate levels in leaves in winter were associatedwith cold acclimation processes. The bark and xylem tissueswere performing as starch deposition sites, and differencesin the extent of starch accumulation in these tissues were detectedduring the seasons. The starch fluctuations in bark and xylemwere discussed in relation to the translocation of metabolitesand other physiological processes. Mannitol, the most abundantleaf carbohydrate, was examined in relation to the reducingsugars exported to the bark. The bark mannitol was examinedin conjunction with the sucrose, glucose and starch levels inthe maturing bark tissues and was correlated to the low wintertemperatures. During the winter there was a drastic reductionof mannitol circulation from the bark to xylem. Olive tree, Olea europaea, L, carbohydrates     

Carbohydrate metabolism before and after dehiscence in the recalcitrant pollen of pumpkin (Cucurbita pepo L.)

Pumpkin (Cucurbita pepo L.) pollen is starchy, sucrose‐poor and recalcitrant, features opposite to those of several model species; therefore, some differences in carbohydrate metabolism could be expected in this species. By studying pumpkin recalcitrant pollen, the objective was to provide new biochemical evidence to improve understanding of how carbohydrate metabolism might be involved in pollen functioning in advanced stages. Four stages were analysed: immature pollen from 1 day before anthesis, mature pollen, mature pollen exposed to the environment for 7 h, and pollen rehydrated in a culture medium. Pollen viability, water and carbohydrate content and activity of enzymes involved in carbohydrate metabolism were quantified in each stage. Pollen viability and water content dropped quickly after dehiscence, as expected. The slight changes in carbohydrate concentration and enzyme activity during pollen maturation contrast with major changes recorded with ageing and rehydration. Pumpkin pollen seems highly active and closely related to its surrounding environment in all the stages analysed; the latter is particularly evident among insoluble sucrolytic enzymes, mainly wall‐bound acid invertase, which would be the most relevant for sucrose cleavage. Each stage was characterised by a particular metabolic/enzymatic profile; some particular features, such as the minor changes during maturation, fast sucrolysis upon rehydration or sharp decrease in insoluble sucrolytic activity with ageing seem to be related to the lack of dormancy and recalcitrant nature of pumpkin pollen.     

Effect of Carbohydrate Demand on the Remobilization of Starch in Stolons and Roots of White Clover (Trifolium repens L.) after Defoliation

White clover plants were grown from stolon tips in growth cabinetsand then defoliated. Thereafter, changes in the contents ofnon-structural carbohydrates such as starch, sucrose, glucose,fructose, maltose, and pinitol in stolons and roots were monitored.Initial contents of carbohydrate reserves, photosynthetic supplyof new carbohydrates and carbohydrate demand after defoliationwere varied by growing the plants at various CO₂ partial pressures,by varying the extent of defoliation and by removing eitherroots or stolon tips at the time of defoliation. Remobilization of carbohydrate reserves in stolons increasedproportionally to their initial contents and was greater whenplants had been severely defoliated, suggesting that carbohydrateswere remobilized according to availability and demand. Starchwas the predominant reserve carbohydrate. Starch degradationwas associated with decreased contents of sucrose, glucose andfructose in young stolon parts and roots but not in old stolonparts suggesting that starch degradation was not strictly controlledby the contents of these sugars. A decrease in the demand forcarbohydrates by removal of roots did not decrease starch degradationbut increased the contents of sucrose, glucose, and fructose.Removal of stolon tips decreased starch degradation and contentsof sucrose, glucose, and fructose. The results suggest thatstarch degradation was controlled by a factor other than sucrose,glucose, and fructose which was exported from stolon tips, e.g.gibberellin. Key words: White clover, storage carbohydrates, remobilization, regrowth     

Carbohydrates and carbohydrate enzymes in developing cotton ovules

Patterns of carbohydrates and carbohydrate enzymes were investigated in developing cotton ovules to establish which of these might be related to sink strength in developing bolls. Enzymatic analysis of extracted tissue indicated that beginning 1 week following anthesis, immature cotton seeds (Gossypium hirsutum L. cv. Coker 100A glandless) accumulated starch in the tissues which surround the embryo. Starting at 15 days post anthesis (DPA), this starch was depleted and starch simultaneously appeared in the embryo. Sucrose entering the tissues surrounding the embryo was rapidly degraded, apparently by sucrose synthase; the free hexose content of these tissues reached a peak at about 20 DPA. During the first few weeks of development these tissues contained substantial amounts of hexose but little sucrose; the reverse was true for cotton embryos. Embryo sucrose content rose sharply from the end of the first week until about 20 DPA; it then remained roughly constant during seed maturation. Galactinol synthase (EC 2.4.1.x) appeared in the embryos approximately 25 days after flowering. Subsequently, starch disappeared and the galactosides raffinose and stachyose appeared in the embryo. Except near maturity, sucrose synthase (EC 2.4.1.13) activity in the embryos predominated over that of both sucrose phosphate synthase (EC 2.4.1.14) and acid invertase (EC 3.2.1.26). Activities of the latter enzymes increased during the final stages of embryo maturation. The ratio of sucrose synthase to sucrose phosphate synthase was found to be high in young cotton embryos but the ratio reversed about 45 DPA, when developing ovules cease being assimilate sinks. Insoluble acid invertase was present in developing cotton embryos, but at very low activities; soluble acid invertase was present at significant activities only in nearly mature embryos. From these data it appears that sucrose synthase plays an important role in young cotton ovule carbohydrate partitioning and that sucrose phosphate synthase and the galactoside synthesizing enzymes assume the dominant roles in carbohydrate partitioning in nearly mature cotton seeds. Starch was found to be an important carbohydrate intermediate during the middle stages of cotton ovule development and raffinose and stachyose were found to be important carbohydrate pools in mature cotton seeds.     

Dynamics of carbohydrates in the embryo axes of horse chestnut seeds during their transition from dormancy to germination

It was shown that the content of carbohydrates and their composition in embryo axes of horse chestnut seeds changed as seeds acquired a capability of dormancy release and germination. Sucrose prevailed among carbohydrates, comprising to 150–160 mg/g dry wt. During the first half of the seed imbibition time, oligosaccharides, namely raffinose and stachyose, degraded, whereas the contents of glucose and fructose were very low. The second half of the imbibition period (until radicle protrusion) was characterized by a cessation of oligosaccharide breakdown and accumulation of monosaccharides. Carbohydrate balance showed that the contribution of oligosaccharide breakdown to sucrose and monosaccharide accumulation was rather small, and monosaccharides accumulated mostly at the expense of sucrose gradually coming from cotyledons during imbibition. The trend of carbohydrate metabolism in imbibing axial organs was similar during the entire period of a seed dormancy release in the course of stratification. A readiness for the commencement of these processes during the entire dormancy period implies that carbohydrate conversions in embryo axes are not a trigger for a dormancy release. Monosaccharide accumulation in embryo axes before radicle protrusion produces an increase in the osmotic pressure, as compared to that provided by sucrose, by approximately 20%. Recalcitrance of the horse chestnut seeds is discussed in relation to the role of carbohydrates and other endogenous osmotica in the establishment of osmotic properties.     

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.