A broad variability for seed desiccation sensitivity, as quantified by the water content and the water activity at which half of the initial viability is lost, has been previously observed within nine African coffee species. In order to investigate if these different degrees of desiccation sensitivity correspond to an adaptive trait, additional data, such as the duration of seed development and seed water content at maturity, were measured for these species, and the relationships between these parameters and some climatic characteristics of their specific native environments were investigated. Since flowering in all coffee species occurs only a few days after the main rainfall marking the end of the dry season, simulations could be made, based on the continuous sequences of rainfall data compiled in databases of nine climatic stations, chosen for their appropriate location in the collecting areas. The simulations revealed a highly significant correlation between the duration of seed development and that of the wet season. Consequently, mature seeds are shed at the beginning of the following dry season. Moreover, the mean number of dry months that seeds have to withstand after shedding was significantly correlated with the parameters used to quantify seed desiccation sensitivity. By contrast, seed moisture content at maturity was not correlated with the level of seed desiccation tolerance. All these results are discussed on the basis of more detailed descriptions of the natural habitats of the coffee species studied.

There is a substantial literature on the basic physiology and response to desiccation of recalcitrant seeds, but little is known about their ecology and even less of their evolutionary status. It is difficult to assess the response of early land plants to dehydration, but it is likely that desiccation tolerance in vegetative tissue arose concomitantly with the invasion of the land. Similarly, from the fossil record it is not possible to assess the desiccation response of early seeds, and furthermore, it is difficult to see phylogenetic relationships among species producing recalcitrant seeds. A consideration of the available evidence, however, suggests that the first seeds were desiccation-sensitive, but tolerance evolved early and probably a number of times, independently. The desiccation sensitivity and short life span (generally shorter than the interval between flowering) of recalcitrant seeds have implications in terms of regeneration ecology. A long-term soil seed bank as such does not exist; rather the seeds germinate and form a seedling bank. However, there is a wide range in post-shedding physiology among recalcitrant seed species, and although species producing recalcitrant seeds are common in the humid tropics, they do occur in habitats with more marked seasonal variation. Here regeneration strategies may be more specialized.

Abstract An autoradiographic study was made of leucine and thymidine incorporation into the meristematic root primordia and hypocotyl tips of seeds of the recalcitrant mangrove species, Avicennia marina. The investigations show that although there is a temporary reduction of protein synthesis at shedding, root primordia and surrounding hypocotyl cells of the axis never wholly cease incorporation of [3H]leucine and regain pre-shedding levels of activity within a day. Precursor studies using methyl-[3H]thymidine show that, at shedding, there is a temporary cessation of incorporation into root meristem nuclei that lasts no longer than 48 h and, within a day, pre-shedding levels are regained in the meristem nuclei. Analysis of DNA fragmentation patterns in root tips at the time of shedding, and their ability to repair radiation-induced DNA damage, indicate that DNA repair processes are markedly compromised in these cells if water loss reaches 22%. Protein synthesis and DNA replication are reduced by more than half by a water loss of 18% and 16%, respectively. DNA replication does not fully recover on rehydration after only 8% water loss. DNA fragmentation to nucleosomes indicates a programme of cell death at a water loss of 10%. We suggest that the feature of continuous protein synthesis activity with only a temporary interruption in active cell cycling in A. marina root primordia helps to explain both the rapidity in seedling establishment and the extreme vulnerability to desiccation.

Recalcitrant seeds are shed moist from the plant and do not survive desiccation to the low moisture contents required for prolonged storage. It has been widely hypothesised that during desiccation of these seeds a stress induced metabolic imbalance develops that leads to free radical mediated damage and viability loss. We investigated this hypothesis in a comparison of two sympatric species of Acer during late seed development and post-harvest desiccation: A. platanoides (Norway maple) has orthodox seeds and A. pseudoplatanus (sycamore) has recalcitrant seeds. In both species, respiration rates declined to similar levels at shedding, and the extent of defences against free radicals appears no less in sycamore than that in Norway maple. During drying there was no evidence for the accumulation of a stable free radical, increased lipid peroxidation or decline in free radical scavenging enzymes in either species. In addition, there was a very similar, large increase in total tocopherol in both species. This increase in sycamore was largely of alpha-tocopherol, whereas in Norway maple the increase was largely from its precursor, gamma-tocopherol. Arguably this suggests a similar mechanism in both species, but increased oxidative stress in sycamore. In general, the results suggest that, although damage resulting in viability loss was clearly taking place, the limitation to desiccation tolerance did not result from inadequate free radical scavenging. Soluble carbohydrates and dehydrin-like proteins were also measured during late seed development and drying in sycamore and Norway maple. The greater concentrations of sucrose, raffinose and stachyose and amounts of dehydrins in the radicles and cotyledons of Norway maple compared with those in sycamore was consistent with greater desiccation tolerance in the former. Sycamore seeds are dormant and at the tolerant end of the continuum of desiccation sensitivity among recalcitrant species, and this may account for their different response to that of the seeds of other more sensitive recalcitrant species studied.

Germination and carbohydrate metabolism were studied in fresh developing pea (Pisum sativum L., cv Baccara) seeds and after artificial drying at 25°C and various relative humidities (20, 75 and 99% RH) to investigate whether the occurrence of desiccation tolerance was related to sucrose, raffinose and stachyose contents. Seeds became completely tolerant to fast drying at 25°C and 20% RH a few days after the end of reserve accumulation, i.e. when their moisture content dropped to approx. 50% (fresh weight basis). This acquisition of desiccation tolerance was associated with an accumulation of raffinose and stachyose, the latter being more abundant in the embryonic axis than in the cotyledons. The (raffinose+stachyose)/sucrose ratio increased during seed development and reached 1.1 in the axis and 0.2 in the cotyledons just before the onset of desiccation tolerance. When the natural acquisition of desiccation tolerance occurred on the mother plant, artificial drying of isolated seeds induced an increase in oligosaccharide content in the cotyledons. Immature seeds, the moisture content of which was higher than about 60% (fresh weight basis), did not tolerate fast drying (25°C and 20 or 75% RH). Such drying did not result in the synthesis of stachyose and induced an increase in electrolyte leakage, a decrease in the ability of seeds to convert 1-aminocyclopropane 1-carboxylic acid (ACC) to ethylene and an increase in ethane synthesis, thus indicating a deterioration of cell membrane properties and lipid peroxidation. In contrast, immature seeds tolerated drying either in the pods or at 25°C and 99% RH, and such drying induced a decrease in sucrose content, an increase in oligosaccharide content and a (raffinose+stachyose)/sucrose ratio higher than around 1. Soluble sugar contents of dried immature seeds depended on the conditions of dehydration. In cotyledons, the (raffinose+stachyose)/sucrose ratio reached 0.61 when seeds were dried at 25°C and 99% RH, whereas it was as low as 0.15 when drying was performed at 25°C and 20% RH. All the results obtained are consistent with the concept that oligosaccharides may well be involved in the protection of membranes during dehydration.

Three questions are considered in the context of the possible effects of seed-associated mycoflora, typified by Fusarium moniliforme, during hydrated storage of recalcitrant propagules of the tropical species, Avicennia marina. These pertain to storage lifespan, whether seed susceptibility to fungal attack changes and the possibility of discriminating ultrastructurally between inherent deteriorative changes and those that are fungallyinduced. The data indicate unequivocally that if fungal activity is curtailed, then the hydrated storage lifespan of A. marina seeds can be considerably extended. When inoculated immediately with F. moniliforme, newly harvested seeds were extremely susceptible to the adverse effects of the fungus, while seeds that had been wet-stored for 4 d showed a considerably heightened resilience to the effects of the fungus when inoculated at that stage. The enhanced resilience, although declining, persisted in seeds stored hydrated for up to 10 d prior to inoculation, being lost after 12 d. After 14 d of hydrated storage, seeds became more susceptible to the effects of the fungus than those in the newly harvested condition. The resilience of seeds that had been stored in the short-term was associated with ultrastructural changes indicative of enhanced metabolic activity associated with the onset of germination. However, with the sustained stress imposed by wet-storage conditions, the seeds became increasingly badly affected by the fungus. While it was not possible to discriminate with certainty among deteriorative subcellular events ascribable to inherent deterioration or the effects of the fungus, it is concluded that a comparison of the timing of the onset of degeneration may well be diagnostic of its cause.

Recalcitrant seeds of Trichilia dregeana were stored at 16 or 25°C, either at the water content at which they were shed or partially dried. Although having been exposed to a short period (approx. 6 h) at temperatures up to 30°C prior to storage, seeds at the original water content maintained viability for several weeks at 16°C. However, storage of undried seeds at 25°C was deleterious within 8 d, indicating a chemical basis for degeneration of hydrated recalcitrant seeds. Seeds that had been mildly dehydrated to the relatively high axis level of 1.68 g H2O g—1 dry mass, while maintaining full germinability immediately after drying, exhibited only 4% viability after 8 d in storage at 16°C and had completely lost viability after the same storage period at 25°C. Ultrastructural features characterizing hydrated seeds included indications of enhanced activity associated with initial exposure to the elevated temperature as well as some signs of stress. However, over an effective 15 d storage period at 16°C, ultrastructural features showed the cells to have retained little damage and to have been in an enhanced state of activity commensurate with ongoing development towards germination. After a longer storage period, however, signs of damage, including indirect evidence for disarray of the cytoskeleton in some axis cells, became apparent in line with the declining seed viability. Immediately following dehydration from an average axis water content of 1.97 to 1.68 H2O g g—1(the sub-imbibed condition), some ultrastructural abnormalities were apparent, but the seeds remained 100% germinable. However, within the 8 d storage period in this sub-imbibed condition, a spectrum of severe ultrastructural degeneration, including indirect evidence of the collapse of the nucleoskeleton and extensive cell lysis, accompanied viability decline of the seeds to 4%.

The fate of cells in the endosperm of developing wheat kernels was investigated under normal conditions and upon premature slow drying on the cut ear. To follow the changes in membrane integrity and cellular ultrastructure, an electron spin resonance (ESR) spin probe technique and low temperature scanning electron microscopy (LTSEM) were used. ESR data indicated that during development, the relative amount of cells with intact membranes decreased and became almost zero at the stage of mass maturity, i.e. several days before the onset of maturation drying. This suggests that the death of starchy endosperm cells is a developmental phenomenon rather than one induced by water loss. Even at 8 days after anthesis (daa), early in the differentiation phase, a considerable proportion of endosperm cells had already lost plasma membrane integrity. Comparison of ESR data with LTSEM micrographs revealed that the loss of plasma membrane integrity occurred early in the starch accumulation process, from which it was concluded that the programmes of starch accumulation and developmental death are simultaneously switched on. When the differentiation into starchy endosperm was arrested by premature slow drying of kernels on the cut ear, meristematic cells (aleurone initials) acquired desiccation tolerance as assessed by the combination of the spin probe technique and LTSEM. In contrast, fast drying caused immediate death of these cells. Thus, meristematic endosperm cells have the competence to acquire desiccation tolerance. This occurs upon premature slow drying or during normal differentiation into aleurone cells, but is lost when the cells differentiate into starchy endosperm.

Neem (Azadirachta indica) is a valuable multipurpose tree of tropical arid and semi-arid regions. The use of its seeds is hindered by their short storage longevity. The possible causes of rapid loss of viability were investigated on different seed lots during exposure to 32% and 75% RH at 20°C. Within 6 months the seeds almost lost germinability at 75% RH, whereas at 32% RH viability decreased only slightly. On rehydration, the axis cells from nongerminable seeds had lost turgor, whereas those from viable seeds were turgescent as visualized by low temperature scanning electron microscopy images of fractured axes. Glutathione oxidation status was used to estimate oxidative stress during storage. Oxidative stress was much higher at 75% RH storage than at 32% RH, mainly caused by the rapid loss of reduced glutathione at 75% RH. Oligosaccharides and phospholipids decreased, and free fatty acids increased during storage at the high RH but remained at a constant level at the low RH. However, the degree of fatty acid unsaturation between viable and nonviable seed lots was similar. During the (slow) dehydration of fresh seeds, total glutathione, oligosaccharides and phospholipids accumulated, particularly in the initially more hydrated seeds. We interpret this accumulation as a post-maturation process associated with acquisition of the capability for long-term survival in the dry state. The mass ratio of oligosaccharides to sucrose was 0.19 on average in dehydrated neem seeds. The data suggest that the storage behaviour of neem seeds has features that characterize it as orthodox.

Large differences in seed desiccation sensitivity have been observed previously among ten coffee species (Coffea arabica, C. brevipes, C. canephora, C. eugenioides, C. humilis, C. liberica, C. pocsii, C. pseudo-zanguebariae, C. sessiliflora and C.stenophylla). Of these species, C. liberica and C. humilis were the most sensitive to desiccation and C. pseudozanguebariae the most tolerant. A study was carried out using the same seed lots to investigate if these differences in desiccation tolerance could be correlated with differences in soluble sugar content. Soluble sugars were extracted from dry seeds and analysed using high performance liquid chromatography. The seed monosaccharide (glucose and fructose) content was very low (1.5 to 2 mg g-1dry weight [dw]) in all species studied. The sucrose content ranged from 33 mg g-1dw in C. liberica seeds to 89 mg g-1dw in seeds of C. pocsii. Raffinose was detected in the seeds of only five species (C.arabica, C.brevipes, C.humilis, C.sessiliflora, C.stenophylla), among which only three species (C.arabica, C.sessiliflora and C.brevipes) also contained stachyose. Both raffinose and stachyose were present in very low quantities (0.3–1.4 mg g-1dw and 0.1–0.7 mg g-1dw, respectively). Verbascose was never detected. No significant relationship was found between seed desiccation sensitivity and: (i) the sugar content; (ii) the presence/absence of oligosaccharides; and (iii) the oligosaccharide:sucrose ratio.

The plumules of newly-excised wampee embryos, which are more sensitive to dehydration than the roots, became more resistant to water loss when axes were allowed to sprout on woody plant medium [WPM; McCown and Lloyd (1981) Hortscience16, 453] before being dried. Pre-treatment of sprouting axes (seedlings) with sucrose incorporated in the WPM enhanced survival. Although the roots withered following further dehydration of seedlings cultured on WPM containing 60% sucrose, excised plumules were capable of generating adventitious roots when a combination of 10 mM α-napthaleneacetic acid and 10 mM indole-3-butyric acid was used during subsequent in vitro incubation.

The aim of the present study was to examine the micromorphology of the cork-oak (Quercus suber) acorn and to evaluate the efficiency of the pericarp as a barrier to water loss. When the desiccation curve obtained at 25 ± 1°C and 62% RH for physiologically mature acorns collected from the tree was compared with that corresponding to fully ripe acorns which had been shed, the latter showed a slightly lower rate of water loss. A marked reduction in the mean rate of water loss was recorded for intact acorns compared with those from which the pericarp had been removed. The external surface and transverse sections of the pericarp from both mature and fully ripe acorns were examined by scanning electron microscopy. Two zones presenting morphological and micromorphological differences were identified: an area including the point of attachment to the cupule and an apical zone covering the embryo. The microstructure of the pericarp showed an external thick cuticle and a single external palisade layer of closely packed cells with no intercellular spaces. Underlying this layer, there was a further parenchymatous layer of poorly differentiated, roughly isodiametric cells. The pericarp in the cupular zone consisted of only this undifferentiated layer between the two epidermal cell layers and contained vascular bundles with many xylem elements.