A suite of mechanisms or processes that together have been implicated in the acquisition and maintenance of desiccation tolerance in orthodox seeds is discussed in the context of the behaviour of desiccation-sensitive seeds, and where appropriate, parallels are drawn with the situation in vegetative plant tissues that tolerate dehydration. Factors included are: physical characteristics of cells and intracellular constituents; insoluble reserve accumulation; intracellular de-differentiation; metabolic ‘switching off’; presence, and efficient operation, of antioxidant systems; accumulation of putatively protective substances including LEAs, sucrose and other oligosaccharides, as well as amphipathic molecules; the presence and role of oleosins; and the presence and operation of repair systems during rehydration. The variable response to dehydration shown by desiccation-sensitive seeds is considered in terms of the absence or incomplete expression of this suite of mechanisms or processes.

Three categories of damage are envisaged: (i) reduction in cell volume which can lead to mechanical damage; (ii) aqueous-based degradative processes, probably consequent upon deranged metabolism at intermediate water contents. This is termed ‘metabolism-induced damage’ and its extent will depend upon the metabolic rate and the rate of dehydration; and (iii) the removal of water intimately associated with macromolecular surfaces leading to denaturation: this is referred to as desiccation damage sensu stricto. The effects of drying rate and the maturity status of seeds are considered in relation to the responses to dehydration, leading to the conclusion that the concept of critical water contents on a species basis is inappropriate. Viewing seed postharvest physiology in terms of a continuum of behaviour is considered to be more realistic than attempting precise categorization.

Rapid dehydration of excised embryonic axes (or other explants) from desiccation-sensitive seeds permits retention of viability (in the short term) to water contents approaching the level of non-freezable water. This opens up the possibility of long-term conservation, by cryopreservation techniques, of the genetic resources of species producing non-orthodox seeds.

We studied changes in dormancy (as imposed by the different structures surrounding the embryo, namely, endosperm, pericarp and glumellae) and its relationship with changes in embryonic ABA levels and sensitivity, in developing grains of two commercial barley cultivars: B 1215 and Quilmes Palomar, which have, respectively, a low and high dormancy level at harvest and, consequently, a contrasting sprouting behaviour in rainy years. Dormancy imposed by endosperm plus pericarp was gradually and similarly alleviated throughout development in both cultivars. The presence of the hull (glumellae), in contrast, completely inhibited germination of grains from both cultivars until physiological maturity (PM). From there on, hull-imposed dormancy was removed abruptly in B 1215 grains, while in Q. Palomar ones, it was removed at a much lower rate. This difference determined the contrasting sprouting behaviour of these two cultivars within the ‘time window’ going from PM to crop harvest. Embryonic ABA content and sensitivity were similar in the two cultivars throughout development until PM. From there on, ABA content and sensitivity in B 1215 embryos declined dramatically coinciding with the abrupt termination of hull-imposed dormancy observed in this cultivar. In contrast, ABA levels in Q. Palomar embryos remained high for longer and sensitivity to ABA declined at a much slower rate. This correlation suggests that hull-imposed dormancy in barley might be regulated by embryonic ABA levels and / or sensitivity. Inhibition of GA synthesis with paclobutrazol applied after anthesis lowered the germination capacity of grains from both cultivars without altering that of the naked caryopses, thus further suggesting that hull-imposed dormancy is under hormonal control.

Priming of seeds is generally intended to reduce time to germination, often leading to improved emergence. However, as a negative side effect, priming reduces longevity of seeds. We studied the possibilities to obtain primed seeds with reduced time to germination but with longevity similar to that of untreated seeds. For several species tested we found that the desired longevity could be obtained by keeping the seeds, after a priming treatment, under a mild water and / or temperature stress for a period of several hours to days. Time to germination did not increase again due to such a treatment. Optimal duration and degree of water stress were strongly temperature dependent. The methods applied to obtain primed seeds without loss of longevity are very similar to those used to induce desiccation tolerance in germinated seeds.

The cell cycle is regulated, at least partially, by protein phosphorylation, activity that is carried out by a series of calcium-independent protein kinases among which the p34cdc2 kinase plays a preponderant role. The behaviour of p34cdc2-like protein kinase(s) has been followed during germination of maize in the presence or absence of the synthetic cytokinin benzyladenine (BA). Whereas there is an increase in p34cdc2-like kinase(s) activity at 15 h of germination in the presence of BA, coinciding with a peak of Ca2+-independent kinase activity, the amount of either the transcript for p34cdc2 or of the corresponding protein(s) does not show much variation during the 0–24 h period of germination studied, whether the phytohormone is present or not. Benzyladenine stimulates the cell cycle and promotes an early mitosis during maize germination. We have found evidence that BA promotes the movement of the p34cdc2-like protein(s) to nuclei several hours before this takes place during germination of control seeds, and this may constitute part of the mechanism by which the phytohormone promotes cytokinesis in plants.

The prediction of the viability of stored seeds is important both for the management of germplasm collections and for the management of commercial seed production and storage. The Ellis and Roberts model for seed viability during storage is examined, and an inadequacy of the model highlighted. A modification is proposed, based on the ‘control mortality’ probit model developed for insecticide bioassays, to take proper account of variation in initial viability. This new ‘control viability’ model, relating seed viability to storage time, is fitted to data from a carrot seed storage experiment and found to fit well for a range of storage environments. A relationship, similar to that proposed by Ellis and Roberts for the effects of storage conditions on the rate of loss of viability, is fitted to the estimated rates from this new model. Data from a second carrot seed storage experiment are used to validate this relationship.

We have been studying a seed aspartic proteinase, termed AtAP, from Arabidopsis thaliana. In previous work, we purified the proteinase, analysed its activity and isolated the cDNA sequence. In this paper, the expression of the mRNA for the aspartic proteinase was analysed in seed tissues both by Northern blots for overall regulation and by in situ hybridization to follow cell-specific localization of message. We found a 1.9 kb aspartic proteinase message in dry seeds and seed pods. This message was expressed in many different cell types of the mature dry seed. The localization of the protein within these cells was also determined. Antibodies were raised against the AtAP and purified using affinity chromatography on an AtAP–immobilized-pepstatin A–agarose column. This purified antibody recognized several AtAP peptides in seeds. To localize the enzyme in cells, we isolated protein bodies from the dry seeds of Arabidopsis using a non-aqueous isolation method. The AtAP activity and antigenic peptides were found to be highest in the protein body fraction and co-localized with the seed storage protein 2S albumin and the vacuolar marker enzyme α-mannosidase. This protein body localization of the AtAP was confirmed with immunocytochemical localization by electron microscopy and shows that the protein is not secreted by these cells.

The correlation between physiological markers for seed maturity (acquisition of germinability, definitive morphological traits and desiccation tolerance) and timing of the shutdown of nuclear replication activity and β-tubulin synthesis has been investigated in developing pepper seeds. Nuclear replication activity was analysed by flow cytometry. Expression of β-tubulin, a cell-cycle related structural protein, was detected in western blots of total protein extracts. In pepper embryos, DNA synthesis stopped 42–49 days after anthesis (DAA), at which time seeds started to acquire desiccation tolerance. A clear β-tubulin signal was detected up to 63 DAA, only 1 week before the acquisition of complete desiccation tolerance. Both nuclear replication and β-tubulin synthesis can therefore be used as markers for the physiological stage reached by developing pepper seeds. Detection of β-tubulin signals in embryonic tissues can also be helpful in discriminating immature seeds within a commercial seed lot.

Cell cycle activity in dry and germinating untreated and treated (soaked in water and subsequently in fungicide) seeds of two sugarbeet cultivars, collected at commercial harvest time (late mature seeds) and about 2 weeks before this (immature seeds), was investigated by flow cytometry, and by immuno-detection of β-tubulin and the B-subunit of the 11 S globulin. Germination capacity and field emergence were tested. With dry seeds of both cultivars, higher G2 / G1 ratios were observed in the radicle tips of late mature seeds, as compared with those from immature seeds. The late mature seeds contained more partly degraded (soluble) B-subunit of 11 S globulin, typical of germinating or primed sugarbeet seeds. Thus events associated with the onset of germination had occurred in the seed lots collected at commercial harvest time. The cytoskeleton protein β-tubulin was not detectable in dry seeds from either harvest. Western blotting revealed an accumulation of β-tubulin during germination and this was faster in the late mature harvested seeds which was correlated with the onset of DNA replication. Soaking enhanced the rate of cell cycle activation during germination as well as vigour, germination capacity, and field emergence. There was positive correlation between the G2 / G1 ratio and the traits examined in laboratory and field tests. It is concluded that a combined analysis of proteins and cell-cycle-related events can be used in understanding and predicting sugarbeet seed quality.

The relationship between Calluna vulgaris seedbank density and climate was investigated using regression analysis and two related techniques (factor-ceiling analysis). The seedbank data originated from published and unpublished studies. Low seedbank densities were associated with sites in the relatively dry, sunny and warm south and east of Great Britain. However, as the climatic variables used in the study were highly intercorrelated, it was not possible to determine which climatic variable had the greatest influence on seedbank density. A hypothesis is described suggesting that the limits of seedbank density can be described for single environmental factors. The operation of other environmental and management factors may reduce the seedbank density to less than this maximum. This is illustrated for a range of climatic variables using factor-ceiling analysis. The processes following seed deposition and the longevity of seeds in the soil are highlighted as possible factors controlled by climate which determine seedbank density.