The sorption isotherm for excised embryonic axes of recalcitrant (i.e. desiccation-sensitive) Quercus robur L. acorns was determined to find the relation between moisture content and water potential. Subsequently, physiological studies on the effect of desiccating the axes to a range of water potentials were undertaken. The respiratory capacity declined steeply after short exposure to water potentials from −5 to −30 MPa. The leachate conductivity increased significantly after exposure to −5 MPa and rose steeply after exposure to between −12 and −40 MPa. Axes were equilibrated at different relative humidities and the proline content showed a 15-fold increase with a peak value at −10 MPa. It was concluded that the critical water potential for initiation of damage was −5 MPa, and that axes accumulated proline as a response to desiccation stress. The embryonic axes from Q. robur behave more like typical vegetative tissue of angiosperms than like orthodox seeds.

Mangosteen (Garcinia mangostana L.), rambai (Baccaurea motleyana Muell.- Arg.) and jelentik (Baccaurea polyneura Hook. f.) are tropical fruit species believed to have recalcitrant seeds. The seeds showed no dormancy; they germinated easily. At harvest, the mean moisture contents (fresh weight basis) were 53.54, 51.20 and 44.90% for G. mangostana, B. motleyana and B. polyneura, respectively. G. mangostana seeds lost viability when their moisture content fell to about 24% while B. motleyana seeds lost viability below 35.5% moisture content. However, for B. polyneura, the seeds could be dried to low moisture content with high survival. The viability was still high when the moisture content was reduced to 13.46%. At this moisture content, the percentage germination was 91.76% and it was found that the seeds survived cyopreservation with 8.3% viability. For B. motleyana axes, the loss of viability occurred when their moisture content fell to about 36% (15% viability with predominantly callus formation) while for B. polyneura axes, viability was reduced to 33-67% when the moisture was 27–30%. At various moisture contents, the seeds of the fruit species studied were exposed for 48 h to 7°C and −4°C. G. mangostana seeds did not survive either temperature. Baccaurea seeds survived 7°C but failed to survive −4°C. No axes from B. motleyana seeds at various moisture contents survived cryopreservation. However, some viability (20–30%) was observed in B. polyneura axes cryopreserved at a moisture content of about 27%. At this moisture content no normal growth was obtained; callus formation was observed. It appears that the seeds vary in the degree of desiccation sensitivity. They also vary in size. Seeds of G. mangostana are larger than Baccaurea seeds and thus, more sensitive to desiccation. Seed and embryonic axis structure may also play a role in desiccation sensitivity.

The desiccation tolerance of Zizania palustris seeds has been debatable, but recently it has emerged that survival to low moisture contents is a function of the temperature during dehydration. Survival of dehydration is greatest at 25–30°C and viability declines as the dehydration temperature is reduced. On the other hand, extended hydrated chilling (stratification) is required to break dormancy. The present contribution examines the ultrastructural condition of embryonic axes of Zizania palustris after dehydration at various temperatures, and following reimbibition and stratification. Axis cells sustained least damage when dehydration was carried out at 25°C and ultrastructural deterioration was more severe with lower temperatures during water loss. Damage sustained as a result of unfavourably low dehydration temperatures was visible when seeds were fixed from the dry state and was generally exacerbated during fully imbibed stratification. However, 25°C represented the optimum dehydration temperature; seeds that had been dried at 30°C also showed considerable ultrastructural disturbance when fixed from the dry state. This was largely reversed during fully imbibed stratification, although signs of the damage that had been sustained still persisted. These observations are in keeping with the germination behaviour of Z. palustris seeds dehydrated at temperatures above and below 25°C. The results are discussed in terms of the predominance of one of two mechanisms of membrane deterioration, depending on the dehydration temperature, which are probably not mutually exclusive. It is hypothesized that, at temperatures above 25°C, damage by free-radical-mediated events may predominate, whereas, at temperatures below the optimum, irreversible lipid-phase transitions may be the major factor resulting in membrane damage.

Seeds of wild rice (Zizania palustris var. interior) have been reported to show maximum survival when dehydrated at 25°C. It has also been reported that axis cells sustain least damage at this drying temperature. In the present study, a linear relationship between drying rate and dehydration temperature was established. Whereas the highest positive tetrazolium staining and lowest leakage were recorded for seeds dehydrated at 25°C, maximum germination was recorded for seeds dried at 20°C. A proportion of seeds showed glass formation irrespective of the dehydration temperatures used. Parameters of the glass to liquid transition, however, correlated with neither water content nor sugar profiles. The ratio of raffinose to sucrose was similar in all the treatments. A hydroperoxide test revealed a linear relationship between peroxide levels and drying temperature, although fatty acid amounts were not correlated with hydroperoxide amounts. Butanal amounts and total aldehydes evolved, on the other hand, showed a high negative correlation with peroxide amounts. Electron microscopy showed that the variability and relative abundance of peripheral membrane complexes (PMCs) was highest for embryonic axes dehydrated at 25°C and lowest for cells of embryonic axes of seeds dried at 10°C. Furthermore, intramembrane particles (IMPs) were evenly distributed in cells of axes dried at 25 or 37°C. In contrast, membranes of cells of axes dehydrated at 10°C showed large IMP-free areas. The relative abundance of IMPs was highest in cells of embryonic axes dried at 25°C and lowest in cells of axes dehydrated at 10°C. From these observations, it is suggested that membrane phase transitions, with the concomitant elimination of proteins, accompany dehydration of Z. palustris seeds at 10°C, whereas at 37°C peroxidation may predominate.

Embryos of Araucaria angustifolia seeds showed no dormancy; they germinated easily at temperatures ranging from 10° to 30°C, and the thermal optimum was about 25–30°C; they were recalcitrant. At harvest, their mean moisture content (dry weight basis) was about 120% and they completely lost viability when their moisture content fell to about 30%. The cotyledons were more sensitive to dehydration than the radicle. Dehydration induced deterioration of cell membranes as indicated by a high increase in leakage of solutes. It also resulted in damage in the nuclei, which was not repaired upon rehydration. During desiccation, respiratory activity decreased; however, O2 uptake was not an indication of germination ability, since it was significantly affected only when embryo moisture content reached the critical value of 30%. The decrease in the capacity to convert 1-aminocyclopropane 1-carboxylic acid to ethylene, which was observed at 30–60 min of dehydration, was a very early indicator of deterioration in embryos. Desiccation resulted also in a rapid decrease in the ability for protein synthesis as measured by [35S]methionine incorporation into total protein; 50% inhibition was observed after 30–60 min of desiccation for both axis and cotyledons.

Development of the highly desiccation-sensitive (recalcitrant) seeds of primarily one species, Avicennia marina, is reviewed and compared with the ontogeny of desiccation-tolerant (orthodox) seeds. A. marina seeds undergo no maturation drying and remain metabolically active throughout development, which grades almost imperceptibly into germination. While PGR control of histodifferentiation is essentially similar to that characterizing desiccation-tolerant seeds, the phase of growth and reserve deposition is characterized by exceedingly high cytokinin levels which, it is proposed, promote a sink for assimilate import. While some starch accumulation does occur, the predominant reserves are soluble sugars which are readily available for the immediate onset of seedling establishment upon shedding. ABA levels are negligible in the embryo tissues during seed maturation, but increase in the pericarp, which imposes a constraint upon germination until these outer coverings are sloughed or otherwise removed. The pattern of proteins synthesized remains qualitatively similar throughout seed development in A. marina, and no LEA proteins are produced. This suggests both that seedling establishment is independent of maturation proteins and that the absence of LEAs and desiccation sensitivity might be causally related. The study on A. marina reveals that for this recalcitrant seed-type, germination per se cannot be defined: rather, it is considered as the continuation of development temporarily constrained by the pericarp ABA levels. This leads to a reexamination of the role of rehydration as key event sensu stricto, in the germination processes in desiccation-tolerant (orthodox) seeds.

The present study assessed the desiccation sensitivity of the excised embryonic axes of the following Amaryllidaceae species: Scadoxus puniceus, Amaryllis belladonna, Nerine huttoniae, N. humulus, N. bowdenii, N. filifolia, Haemanthus humulus humulus, H. coccineus, H. deformis, H. bakerae, Brunsvigia gregaria, Brunsvigia orientalis, Boophane disticha, Strumaria discifera, Crinum macowanii and C. bulbispermum. Excised embryonic axes were rapidly dehydrated (flash-dried). Curves of water content (g g− 1) and viability (%) versus drying time (min) were used to interpolate the water content (WC) and drying time (DT) values corresponding to 80% viability (WC/DT, V = 80) for individual species, and used to compare desiccation sensitivity between developmental stages and among years, provenances, species and genera. The seeds of all 16 species were shed with high axis water contents (2.25 ± 1.34 to 6.95 ± 0.89 g g− 1) and were sensitive to desiccation, with WC V = 80 values ranging from 0.13 to 3.15 g g− 1. Germination in amaryllids is unusual in that part of the cotyledon, to which the embryonic axis is attached, grows right out of the seed. It is at this stage that the axes were found to be more desiccation-sensitive in six out of nine species investigated. Seed fresh mass (SFM), axis shedding water content (SWC) and the degree of desiccation sensitivity were species characteristic. SWC, SFM and drying time were not significantly related to desiccation sensitivity. Parent plants were not confined to any particular biome type, occurring in areas characterized by seasonal dry spells. Parent plant habitat and seed characteristics were not useful in predicting the degree of desiccation sensitivity across species belonging to the same family.

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.

This study examined the hypothesis that desiccation sensitivities of recalcitrant and intermediate seeds can be categorized into discrete levels of critical water potential. The equilibrium dehydration method was used to determine the critical water potential (CWP) below which desiccation damage started to occur. The CWP values of Bruguiera cylindrica, Lansium domesticum, Litchi chinensis and Lumitzera racemosa are approximately –4 MPa. The CWP values of Andira inermis, Avicennia alba, Castanea sinensis (from New Zealand), Citrus aurantifolia, Ginkgo biloba, Nephelium lappaceum and Theobroma cacao (immature axis) are approximately –8 MPa. The CWP values of Acer pseudoplatanus, Castanea sinensis (from China), Quercus rubra and Theobroma cacao(mature axis) are approximately –12 MPa. The CWP values of Artocarpus heterophyllus and Hevea brasiliensis are approximately -23 MPa, while the CWP values of Acer platanoides, Azadirachta indica, Carica papaya and Coffea arabica are approximately –73 MPa. Together with data available in earlier literature, these CWP values suggest that there are five discrete levels of critical water potential among desiccation-sensitive seed tissues. These data support the hypothesis that discrete levels of desiccation sensitivity occur among recalcitrant and intermediate seeds, and suggest that specific damaging and protective mechanisms exist at certain hydration levels.

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.

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.

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.