Physiological aspects of Taxus brevifolia seeds in relation to seed storage characteristics

Water relations, desiccation tolerance and longevity of Taxus brevifolia (Nutt.) seeds were studied to determine the optimal stage of development and storage conditions for seeds of this species. Seeds equilibrated to a range of relative humidities (RHs) had unusually low water contents which can be accounted for by the high lipid content of gametophyte tissues (71% of the dry mass). Water relations of embryonic tissue were more typical of those reported for other seed species. The water content below which freezing transitions were not observable in the embryo was ca 0.24 g H₂O (g dry weight)⁻¹ (g g⁻¹) for all maturity classes studied. Embryos did not achieve significant levels of desiccation tolerance (survival to water contents less than 0.5 g g⁻¹) until the latter stages of development when dry matter was maximal. Mature embryos could be dried to 0.025 g g⁻¹ (seed water content of 0.010 g g⁻¹) with no loss of viability. Thus, at the latter stages of development, embryo water content could be optimized to avoid both desiccation and freezing damage. Survival of mature seeds declined over a 2-year period when seeds were stored at temperatures between 5 and 35°C and RHs between 14 and 75%, corresponding to seed water contents between 0.015 and 0.07 g g⁻¹. The deterioration rate was slowest for seeds stored at the lowest RH and temperature. Our data indicate that seeds of Taxus brevifolia show orthodox rather than recalcitrant storage characteristics, but that the optimum water content for storage was extremely low. The results suggest that even if stored at optimal water contents and low temperatures, T. brevifolia seeds will be relatively short lived. The high quantity of lipids or reducing sugars may be contributing factors in the poor storage characteristics.     

The survival of Salmonella enterica serovar Typhimurium DT104 and total viable counts on beef surfaces at different relative humidities and temperatures

Aims:  The aim of this study was to investigate changes in Salmonella and total viable count (TVC) survival on beef carcass surfaces stored for 72 h under different combinations of relative humidity (i.e. RH 75% or 96%) and temperature (5°C or 10°C).
Methods and Results:  The influence of low water activity ( a _w) and temperature on the survival and growth of Salmonella enterica serovar Typhimurium DT104 and the aerobic mesophilic flora on meat pieces from different sites on beef carcasses was investigated, under controlled conditions (75% or 96% RH; 5 or 10°C) in an environmental cabinet. Salmonella counts declined during storage at low a _w (75% RH) conditions at 5°C or 10°C. Salmonella counts increased during storage at high a _w (96% RH) at 10°C only. At 5°C, TVCs increased during storage at high a _w, but not at low a _w. TVCs increased on all samples from carcasses stored at high or low a _w at 10°C, except those samples taken from areas of surface fat.
Conclusions:  This suggests that substrate composition dictates growth rates under low a _w conditions. The results are discussed in terms of the possible protective effects of substrate osmolyte accumulation in bacterial survival and/or growth.
Significance and Impact of the Study:  The data obtained in this study provides useful insights on the influence of a _w and temperature on pathogen survival on meat surfaces at chill temperature.     

Peroxidase: Changes in soluble and bound forms during maturation and ripening of apples

Peroxidase (POD) enzymes, both soluble and bound, have been implicated in the ripening process of a number of fruits. However, their roles are poorly understood. The aim of this study was to define clearly the classes of POD activity in apple parenchyma tissues on the basis of their solubility in salt solutions, to determine the subcellular localisation of ionically‐bound POD activity, and to determine whether soluble‐ or ionically‐bound POD activities exhibit any relationships with maturation and ripening. The cortical parenchyma of apples ( Malus domestica Borkh cv. Cox's orange pippin) was investigated for changes in POD activity during maturation and post‐harvest storage at 0, 4, and 8°C at 100% relative humidity (RH), and controlled atmosphere (CA) storage (4°C in 1.25% O₂ and 0.5% CO₂). The parenchyma cells contained both soluble and ionically‐bound POD activity. The ionically‐bound POD was located in the cell wall in the middle lamella, plasmodesmata and cell corners, and could be extracted quantitatively with 0.4 M CaCl₂ at neutral pH to leave a POD‐free residue. During maturation, both forms of activity increased on a 'per apple' basis, but decreased as a function of fresh weight. Under all conditions of storage, soluble POD exhibited a peak of activity at approximately the mid‐point of ripening‐related softening; this event may constitute a biochemical marker. In contrast, ionically‐bound POD generally decreased during ripening, except under CA storage. CA storage inhibited ripening‐related softening after an initial loss of firmness. However, softening resumed after transfer of apples to normal atmosphere storage at 8°C. This may provide a new insight into controlling tissue softening.     

Intrinsic rate of population increase of Amitus fuscipennis MacGown and Nebeker (Hym., Platygasteridae) according to climatic conditions and bean cultivar

Data on adult longevity, reproduction and intrinsic rate of population increase of insect parasitoid Amitus fuscipennis MacGown and Nebeker (Hym., Platygasteridae) were obtained under different climatic conditions on two bean cultivars. The insect host was the greenhouse whitefly Trialeurodes vaporariorum (Westwood) (Hom., Aleyrodidae). On cv. Chocho, at 19°C, 75 ± 5% RH and at 22°C, 70 ± 5% RH, mean longevity of adult Amitus females on the host insect was 9.3 and 5.7 days; mean fertility was 64.1 eggs and 52.7 eggs; and r_m values were 0.102 day^–1 and 0.144 day^–1 for the respective conditions. On cv. ICA-Pijao, at 19°C, 75 ± 5% RH, mean longevity was 15.2 days; mean fertility was highest (162.8 eggs); and r_m value was 0.117 day^–1. The r_m values of Amitus were higher than those of its host under the same test conditions. This parasitoid may therefore be a suitable candidate for use in inoculative or seasonal inoculative control programs against T. vaporariorum .     

A study of water relations in neem (Azadirachta indica) seed that is characterized by complex storage behaviour

Neem (Azadirachta indica) seed is reputed to have limited tolerance to desiccation, to be sensitive to chilling and imbibitional stress, and to display intermediate storage behaviour. To understand this behaviour the properties of water in seed tissues were studied. Water sorption isotherms showed that at similar relative humidity (RH), the water content was consistently higher in axes than in cotyledons, mainly due to the elevated lipid content (51%) in the cotyledons. Using differential scanning calorimetry, melting transitions of water were observed at water contents higher than 0.14 g H2O g-1 DW in the cotyledons and 0.23 g H2O g-1 DW in the axes. Beside melting transitions of lipid, as verified by infrared spectroscopy, changes in heat capacity were observed which shifted with water content, indicative of glass-to-liquid transitions. State diagrams are given on the basis of the water content of seed tissues, and also on the basis of the RH at 20 degrees C. Longevity was considerably improved, and the sensitivity to chilling/subzero temperatures was reduced when axis and cotyledons were dehydrated to moisture contents < or = of approximately 0.05 g H2O g-1 DW. However, longevity during storage at very low water contents was limited. A possible mechanism for the loss of sensitivity to chilling/subzero temperatures at low water contents is discussed. The results suggest that dry neem seeds in the glassy state have great potential for extended storability, also at subzero temperatures.     

Purification and characterization of strongly chitin‐binding chitinases from salicylic acid‐treated leek (Allium porrum)

Six chitinases (EC 3.2.1.14) were purified from salicylate‐treated leek ( Allium porrum L.). They all strongly bind to chitin and can roughly be divided into two groups. One group has blocked N‐termini, is completely inhibited by 1 m M AgNO₃, has a relatively narrow pH optimum, a temperature optimum of 40°C and cannot degrade the tetramer of chitin. The other group has unblocked N‐termini showing homology to the chitin‐binding lectin WGA and is therefore considered as class I chitinases. This group is only moderately inhibited by 1 m M AgNO₃ (30%), has a relatively broad pH optimum, has a higher temperature optimum (50 to 60°C) and can degrade the tetramer of chitin to dimers. Furthermore, all isoforms have molecular masses around 34 kDa as estimated by SDS‐PAGE. They have isoelectric points ranging from 4 to 8 and no detectable lysozyme activity. Two isoforms investigated in more detail differ in their antifungal potential.     

The role of ABA in freezing tolerance and cold acclimation in barley

The role of ABA in freezing resistance in nonacclimated and cold‐acclimated barley ( Hordeum vulgare L.) was studied. Eleven nonacclimated cultivars differed in their LT₅₀, ranging from −10.8 to −4.8°C. Sugars, free proline, soluble proteins and ABA were analyzed in nonacclimated cultivars and during cold acclimation of one cultivar. There was an inverse correlation between LT₅₀ and both ABA and sucrose contents. Exogenous ABA caused a decrease in the freezing point of leaf tissue in the cultivar with the lowest level of endogenous ABA, but not in the cultivar with the highest level, suggesting that ABA in the latter may be near the optimum endogenous level to induce freezing tolerance. Plants of cv. Aramir treated with ABA or allowed to acclimate to cold temperature increased their soluble sugar content to a similar level. The LT₅₀ of leaves of cold‐acclimated cv. Aramir decreased from −5.8 to −11.4°C, with biphasic kinetics, accumulating proline and soluble sugars with similar kinetics. The biphasic profile observed during cold acclimation could be a direct consequence of cryoprotectant accumulation kinetics. ABA and soluble protein accumulation showed a single step profile, associated mainly with the second phase of the LT₅₀ decrease. Thus, a significant increase in endogenous ABA is part of the response of barley to low temperature and may be required as a signal for the second phase of cold acclimation. Endogenous ABA contents in the nonacclimated state may determine constitutive freezing tolerance.     

Sensitivity of a dynamic global vegetation model to climate and atmospheric CO2

The equilibrium carbon storage capacity of the terrestrial biosphere has been investigated by running the Lund–Potsdam–Jena Dynamic Global Vegetation Model to equilibrium for a range of CO₂ concentrations and idealized climate states. Local climate is defined by the combination of an observation-based climatology and perturbation patterns derived from a 4 × CO₂ warming simulations, which are linearly scaled to global mean temperature deviations, Δ T _glob. Global carbon storage remains close to its optimum for Δ T _glob in the range of ±3°C in simulations with constant atmospheric CO₂. The magnitude of the carbon loss to the atmosphere per unit change in global average surface temperature shows a pronounced nonlinear threshold behavior. About twice as much carbon is lost per degree warming for Δ T _glob above 3°C than for present climate. Tropical, temperate, and boreal trees spread poleward with global warming. Vegetation dynamics govern the distribution of soil carbon storage and turnover in the climate space. For cold climate conditions, the global average decomposition rate of litter and soil decreases with warming, despite local increases in turnover rates. This result is not compatible with the assumption, commonly made in global box models, that soil turnover increases exponentially with global average surface temperature, over a wide temperature range.     

Interaction of enriched CO2 and water stress on the physiology of and biomass production in sweet potato grown in open-top chambers

Abstract. The objective of this study was to investigate the effects of water stress in sweet potato ( Ipomoea batatas L. [Lam] 'Georgia Jet') on biomass production and plant-water relationships in an enriched CO₂ atmosphere. Plants were grown in pots containing sandy loam soil (Typic Paleudult) at two concentrations of elevated CO₂ and two water regimes in open-top field chambers. During the first 12 d of water stress, leaf xylem potentials were higher in plants grown in a CO₂ concentration of 438 and 666 μmol mol⁻¹ than in plants grown at 364 μmol mol⁻¹. The 364 μmol mol⁻¹ CO₂ grown plants had to be rewatered 2 d earlier than the high CO₂-grown plants in response to water stress. For plants grown under water stress, the yield of storage roots and root: shoot ratio were greater at high CO₂ than at 364 μmol mol⁻¹; the increase, however, was not linear with increasing CO₂ concentrations. In well-watered plants, biomass production and storage root yield increased at elevated CO₂, and these were greater as compared to water-stressed plants grown at the same CO₂ concentration.     

The effect of temperature and acclimation period on repeat swimming performance in cutthroat trout

Hatchery cutthroat trout Oncorhynchus clarki clarki were used to examine the effects of 48 h and 3 week temperature acclimation periods on critical swimming speed ( U _crit). The U _crit was determined for fish at acclimation temperatures of 7, 14 and 18° C using two consecutive ramp‐ U _crit tests in mobile Brett‐type swim tunnels. An additional group was tested at the stock's ambient rearing temperature of 10° C. The length of the temperature acclimation period had no significant effect on either the first or the second U _crit( U _crit‐1 and U _crit‐2, respectively) or on the recovery ratio (the quotient of U _crit‐2  U _crit‐1⁻¹). As anticipated, there was a significant positive relationship between U _crit‐1 and temperature ( P  < 0·01) for both acclimation periods, and an increasing, though non‐significant, trend between U _crit‐2 and temperature ( P  = 0·10). Acclimation temperature had no significant effect ( P  = 0·71) on the recovery ratio. These results indicate that a 48 h acclimation to experimental temperatures within the range of −3 to +8° C of the acclimation temperature may be sufficient in studies of swimming performance with this species. This ability to acclimate rapidly is probably adaptive for cutthroat trout and other species that occupy thermally variable environments.     

Sex differences in pectoral muscles but not in pectoral fins in the three‐spined stickleback Gasterosteus aculeatus

The pectoral muscle index ( I _PM)( I _PM = 100 M _PM M⁻¹, where M _PM and M are the pectoral muscle and body masses, respectively) fin‐area and fin ray length were studied over a year in male and female three‐spined stickleback Gasterosteus aculeatus from a marine population (Öresund, Sweden) kept under simulated natural light and temperature conditions. A castration‐replacement experiment was used to test androgen effects on the I _PM, fin‐area and fin ray length. Non‐breeding males were castrated or sham‐operated in winter ( i.e . the fish had low levels of androgens). Castrated control and sham‐operated fish were implanted with empty Silastic capsules and castrated groups with capsules containing the androgens testosterone or 11‐ketoandrostenedione into the abdominal cavity. The experiment was terminated after 41 days, when the controls had matured. No morphological differences were found in pectoral fins between sexes during the year, except during the peak breeding season (May), where females showed larger fin‐area and longer fin ray in length compared to males. No effects of androgens treatment or of castration on pectoral fin‐area or fin ray length was observed. Breeding and non‐breeding males showed higher I _PM compared to females. The lower I _PM in females than in males could not be explained by the larger gonads in the former alone, as a sex difference in I _PM was still present after deduction of the ovaries from the female body mass. The I _PM was higher in sham‐operated compared to castrated fish. No effects of androgens treatment on I _PM was observed.     

Combined effects of varying water content and CO2 concentration on photosynthesis in Spagnum fuscum

The photosynthesis of Spagnum fuscum (Schimp.) Klinggr. at different water contents and CO₂ concentrations was measured in the laboratory. The optimal water content for photosynthesis near the current atmospheric CO₂ concentration is 600–800% (percentage of dry weight). The decrease in photosynthesis is very steep towards lower water contents and less steep towards higher water contents. The optimal water content range moves higher and becomes wider with increasing CO₂ concentration. At 3000 ppm there is no longer any decrease in photosynthesis with increasing water content. The water content of S. fuscum has a considerable effect on the response of photosynthesis to CO₂ concentration. In a moss saturated with water, photosynthesis increases gradually until 8000 ppm CO₂, but this saturation concentration becomes lower with decreasing water content, being c. 1500 ppm at a water content of 700–800%. An increase in CO₂ concentration over 300 ppm will raise photosynthesis very little in dry moss with a water content of only 300–400%.     

Diurnal water storage in the stems of Picea sitchensis (Bong.) Carr.

Abstract. Two models of the relationship between diurnal variation in shoot water potential and transpiration in 14-year-old Picea sitchensis (Bong.) Carr. were compared. The first model was a physiologically based resistance-capacitance (R-C) analogue with its associated differential equations. The second was a non-physiological discrete-difference (D-D) or stochastic transfer function model. The RC model included only the effect of water storage in the phloem and bark while the D-D model implicity included all storage mechanisms. The R-C and D-D models explained similar fractions (62% and 68% respectively) of the variation in shoot water potential due to diurnal changes in transpiration rate. However, the D-D model had fewer parameters than the R-C model. The results from the D-D model showed that the resistance to flow from soil to shoots along the trunk, (RT), was 5 × 10³ MPa kg^-1s and the capacitance of the phloem and bark treated as a single store, (C_s), was 1.6 kg MPa^-1. It is suggested that the resistance to flow into storage (R_s) is much greater than R_T and can be disregarded. A non-linear version of the D-D model suggested [hat resistance to flow in the trunk increases with increasing transpiration rate.     

Protein kinase activities in ripening mango (Mangifera indica) fruit tissue. II. Purification and characterization of a casein kinase I

A protein kinase (PK‐II), phosphorylating casein, was purified from ripening mango, Mangifera indica L., fruit tissue. The purification procedure consisted of ammonium sulphate fractionation and sequential anion exchange‐, dye‐ligand, and gel filtration chromatography. The enzyme was purified over 500‐fold to near homogeneity with a recovery of 4%. The purified enzyme had a specific activity of ca 1 µmol mg⁻¹ protein min⁻¹ with ATP as phosphoryl donor. SDS‐PAGE results indicated a monomeric enzyme with molecular mass of 35 kDa. The protein kinase phosphorylated the acidic substrates casein and phosvitin, but had a very low activity with histones and protamine sulphate. The optimum pH and temperature for catalysis were determined to be 9.6 and 35°C, respectively. Mn²⁺ could not substitute for the Mg²⁺ needed for activity and Ca²⁺ had a slight stimulatory effect. Phospholipids, cAMP, calmodulin and the calmodulin inhibitor, calmidazolium, did not have any significant effect on activity, but the enzyme was inhibited by heparin and the specific inhibitor, CKI‐7, ( N ‐[2‐aminoethyl]‐5‐chloroisoquinoline‐8‐sulphonamide). Autoradiographic studies revealed the ability of the protein kinase to autophosphorylate as well as the presence of endogenous protein substrates in the crude extract. Initial velocity studies with casein as substrate and product inhibition studies with ADP indicated a K_m (ATP) and K_m (casein) of 14 µ M and 0.18 mg ml⁻¹, respectively, with a K_i (ADP) of 3.2 µM. The enzyme can be classified as a casein kinase I type of protein kinase (EC 2.7.10).     

Theoretical Basis of Protocols for Seed Storage III. Optimum Moisture Contents for Pea Seeds Stored at Different Temperatures

In previous work, we demonstrated that there was an optimummoisture level for seed storage at a given temperature (Vertucciand Roos, 1990), and suggested, using thermodynamic considerations,that the optimum moisture content increased as the storage temperaturedecreased (Vertucci and Roos, 1993b). In this paper, we presentdata from a two year study of aging rates in pea (Pisum sativum)seeds supporting the hypothesis that the optimum moisture contentfor storage varies with temperature. Seed viability and vigourwere monitored during storage under dark or lighted conditionsat relative humidities between 1 and 90%, and temperatures between-5 and 65°C. The optimum moisture content varied from 0·015g H₂O g^-1 d.wt at 65°C to 0·101 g H₂O g^-1 d.wt at15°C under dark conditions and from 0·057 at 35°Cto 0·092 g H₂O g^-1 d.wt at -5°C under lighted conditions.Our results suggest that optimum moisture contents cannot beconsidered independently of temperature. This conclusion hasimportant implications for 'ultra-dry' and cryopreservationtechnologies.Copyright 1994, 1999 Academic Press Seed storage, seed aging, seed longevity, water content, temperature, glass, desiccation damage, ultradry, Pisum sativum L., pea, cryopreservation     

Zeaxanthin and non‐photochemical quenching in sun and shade leaves of C3 and C4 plants

The relationships between non‐radiative energy dissipation and the carotenoid content, especially the xanthophyll cycle components, were studied in sun and shade leaves of several plants possessing C₃ ( Hedera helix and Laurus nobilis ) or C₄ ( Zea mays and Sorghum bicolor ) photosynthetic pathways. Sun‐shade acclimation caused marked changes in the organisation and function of photosynthetic apparatus, including significant variation in carotenoid content and composition. The contents of zanthophyll cycle pigments were higher in sun than in shade leaves in all species, but this difference was considerably greater in C₃ than in C₄ plants. The proportion of photoconvertible violaxanthin, that is the amount of violaxanthin (V) which can actually be de‐epoxidised to zeaxanthin, was much greater in sun than in shade leaves. The amount of photoconvertible V was always linearly dependent on the chlorophyll a/b ratio, although the slope of the relationship varied especially between C₃ and C₄ species. The leaf zeaxanthin and antheraxanthin contents were correlated with non‐radiative energy dissipation in all species under different light environments. These relationships were curvilinear and variable between sun and shade leaves and between C₃ and C₄ species. Hence, the dissipation of excess energy does not appear to be univocally dependent on zeaxanthin content and other photoprotective mechanisms may be involved under high irradiance stress. Such mechanisms appear largely variable between C₃ and C₄ species according to their photosynthetic characteristics.     

Shade‐tolerance as a predictor of responses to elevated CO2 in trees

Evidence from 10 studies comparing angiosperm trees and 5 studies comparing conifers of differing shade‐tolerance was analysed. The number of intraphyletic comparisons in which the more shade‐tolerant species showed the greater relative increase of biomass in elevated CO₂ was significantly higher than would be expected by chance alone. It is suggested that more shade‐tolerant species are inherently better disposed, in terms of plant architecture and partitioning of biomass and nitrogen, to utilise resources (light, water, nutrients) that are potentially limiting in elevated CO₂ and that these traits are responsible for the interaction between shade‐tolerance and CO₂ concentration. Compared with less shade‐tolerant angiosperm trees, more shade‐tolerant angiosperm species generally have a lower leaf area ratio in ambient CO₂ and show a smaller relative reduction in elevated CO₂. Furthermore, leaf nitrogen content is usually lower in more shade‐tolerant angiosperm species and tends to be more strongly reduced by elevated CO₂ in those species. Within angiosperm trees, more shade‐tolerant species showed a stronger stimulation of net leaf photosynthetic rate in most experiments, but this trend was not significant.     

Foliar antioxidant status of plants from naturally high‐CO2 sites

We compared the foliar antioxidant status of native Agrostis stolonifera L. communities growing at two distinct CO₂‐enriched sites of geothermal origin (E) and at a control field location with normal CO₂. Compared to the control, plants from both E‐sites showed an increased size of the GSH pool, essentially due to enhanced GSSG levels, and a consequent decrease in the ratio between reduced and oxidised glutathione forms. Such differences were maintained and even enhanced in the vegetatively‐propagated progenies of control and E‐plants, grown under both greenhouse conditions and normal CO₂ levels. The above results confirmed previous observations on native and crop plants exposed to elevated CO₂. It is therefore suggested that changes in the glutathione redox balance might be of adaptive significance under conditions of permanent exposure to high CO₂.     

H+/PPi stoichiometry of a membrane‐bound pyrophosphatase of plant mitochondria

The H⁺/PP_i stoichiometry of the mitochondrial H⁺‐PP_iase from pea ( Pisum sativum L.) stem was determined by two kinetic approaches, and compared with the H⁺/substrate stoichiometries of the mitochondrial H⁺‐ATPase, and the vacuolar H⁺‐PP_iase and H⁺‐ATPase. Using sub‐mitochondrial particles or preparations enriched in vacuolar membranes, the rates of substrate‐dependent H⁺‐transport were evaluated: by a mathematical model, describing the time‐course of H⁺‐gradient (ΔpH) formation; or by determining the rate of H⁺‐leakage following H⁺‐pumping inhibition by EDTA at the steady‐state ΔpH. When the H⁺‐transport rates were divided by those of PP_i or ATP hydrolysis, measured under identical conditions, apparent stoichiometries of ca 2 were determined for the mitochondrial H⁺‐PP_iase and H⁺‐ATPase, and for the vacuolar H⁺‐ATPase. The stoichiometry of the vacuolar H⁺‐PP_iase was found to be ca 1. From these results, it is suggested that the mitochondrial H⁺‐PP_iase may, in theory, function as a primary H⁺‐pump poised towards synthesis of PP_i and, therefore, acting in parallel with the main H⁺‐ATPase.     

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.