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.

Osmopriming of leek seeds (Allium porrum L.) in PEG 6000 solution (−10 bars) at 15°C for 14 days leads to reductions in both the spread of germination and mean time to germination, especially in low-vigour seed lots. In vivo methyl [3H]-thymidine pulse-labelling studies have demonstrated constant and low levels of DNA synthesis in leek embryo tissue during the osmopriming treatment. DNA synthesis during osmopriming was not inhibited by aphidicolin, an inhibitor of nuclear DNA replication. Replicative and repair-type DNA synthesis was investigated using BND-cellulose chromatography and these studies revealed that about 30% of the DNA synthesis after 1 day of priming was of a repair-type. DNA repair-type synthesis contributed to approximately 20% of the [3H]thymidine incorporated into DNA during the rest of the priming period in embryo tissue from both high-vigour and low-vigour seed lots. After 1 day of germination following priming, enhanced levels of both replicative and repair-type DNA synthesis were demonstrated. The replicative type of DNA synthesis detected in leek embyros during the priming period was not inhibited by aphidicolin and appears to represent a significant level of mitochondrial DNA synthesis. DNA synthesis could be detected in both nuclei and mitochondria of leek embryo tissue during the osmopriming treatment in the absence of any detectable cell division.

Five distinct groups of storage proteins are synthesized by embryos during development in the fruit at 35 days after pollination (35 DAP); none of them is synthesized in germinating embryos 48 h after isolation (48 HASI) of the seeds from the fruit. Endo-β-mannanase, a marker of germination and seedling growth, is produced in the isolated seeds, but not in the developing seed in situ 35 DAP. When seeds at this stage are removed from the fruit, but remain in intimate contact with the locular and sheath tissue, they continue to synthesize storage proteins for at least 48 HASI. Removal of the sheath from seeds isolated 35 DAP stops storage protein synthesis, and increases endo-β-mannanase activity. Both abscisic acid (ABA) and osmoticum at physiological concentrations maintain the synthesis of storage proteins in seeds isolated 35 DAP for at least 48 HASI, although osmoticum is more effective in preventing their eventual germination. Thus the effects of the sheath and the locular tissue are mimicked by both ABA and osmoticum in relation to the maintenance of storage protein synthesis in seeds 35 DAP and in the suppression of endo-β-mannanase activity for at least 48 HASI.

A protein named QP47 has been purified from quiescent nuclei of root meristems of Pisum sativum, and used to prepare a polyclonal antibody. Immunolocalization of this protein with fluorescent probes revealed a nuclear distribution of thread-like structures. However, the relationship between the distribution of QP47 immunofluorescence and the structural organization of the chromatin required further investigation. The decrease in content of this protein in the nuclei of embryo cells seems to be correlated with the transition from quiescence to proliferation. QP47 degradation seems to depend upon an increase in the state of its phosphorylation. This protein is not present in normally proliferating cells, or in cells whose cell proliferation has been arrested by starvation or differentiation. It is hypothesized that QP47 may be required specifically during the quiescent period for specific structural organization of the chromatin.

DNA synthesis following the addition of water to excised embryos of non-dormant rye and to embryos of non-dormant and dormant genetic lines of Avena fatua has been examined. All the samples exhibit an early unscheduled DNA synthesis, have similar responses to DNA polymerase inhibitors and a similar increase in DNA ligase function for the first 24 h of imbibition, irrespective of whether they germinate or remain dormant. A β-polymerase-mediated DNA repair activity is indicated immediately upon imbibition with the stable incorporation of 3H-methyl-thymidine into high molecular weight DNA. Following γ-irradiation of dry or imbibed embryos, inhibitor results suggest the appearance of an additional α- or δ-polymerase activity in the ensuing repair. Abscisic acid permits the early unirradiated repair synthesis, but like aphidicolin, it inhibits replicative DNA synthesis and partly inhibits the post-irradiation incorporation of thymidine. DNA synthesis takes place continuously throughout dormancy of imbibed embryos: precursor incorporation into DNA cannot be chased in the short term and occurs in the absence of an S-phase or endoreduplication of nuclear DNA. It is proposed that dormant imbibed embryos maintain the integrity of the genome by a continuous but slow replacement of DNA steered by a non-amplifying and modified form of replicative DNA synthesis and that abscisic acid may play a determining role in this process.

The contents of the cytokinins zeatin (Z), zeatin riboside (ZR), and isopentenyladenine (IPA), the combined contents of gibberellins1 + 3 (GA), and the contents of indoleacetic acid (IAA) and abscisic acid (ABA) were measured during the development of the desiccation-sensitive seeds of Avicennia marina (Forssk.) Vierh. During the stage of histodifferentiation the amounts of these plant growth regulators (PGRs) were measured on whole fruits. During the phase of seed growth and reserve accumulation measurements were made on the embryonic axis, cotyledons and pericarp separately. Patterns in the amounts of PGRs present during histodifferentiation were similar to those reported for desiccation-tolerant seeds and it suggested that this process is under similar ‘hormonal’ control in A. marina as in orthodox seeds. Very high contents of cytokinins, particularly ZR, were present in both axes and cotyledons during reserve accumulation. This is thought to be related to the nature of the reserves accumulated (soluble sugars), rather than to the phenomenon of desiccation sensitivity. With the exception of ABA, embryonic contents of PGRs were relatively high at seed shedding, consistent with the rapid germination of this highly recalcitrant seed. ABA contents in the embryo were low during reserve accumulation, but concentrations in the pericarp increased throughout this development stage. ABA in the perticap could act to prevent precocious germination. The low concentrations of ABA in the embryo could be related to the desiccation-sensitivity of the seeds of A. marina.

The development of (1→3,1→4)-β-glucan 4-glucanohydrolase (EC 3.2.1.73) has been examined in germinating wheat (Triticum aestivum cv. Millewa) grain, and in isolated aleurone layers and scutella. Activity is first detected in extracts of intact grain 2–3 days after the initiation of germination and thereafter increases until 6 days. In isolated aleurone layers, (1→3,1→4)-β-glucanase activity is secreted for up to 4 days. Treatment of aleurone layers with gibberellic acid (GA3) results in a 2-fold enhancement of secreted enzyme for the first 2 days, but activity decreases after 2 days. (1→3,1→4)-β-Glucanase secretion from GA3-treated aleurone layers clearly precedes the secretion of amylase and (1→4)-β-xylanase. Small but significant levels of cellulase are secreted from aleurone layers after GA3 treatment. Isolated scutella also secrete (1→3,1→4)-β-glucanase but GA3 has little apparent effect on secretion patterns from this tissue. Proteins secreted from excised aleurone layers and scutella, together with those in homogenates of intact, germinated grain, have been examined by Western blot analysis, using monoclonal antibodies against barley (1→3,1→4)-β-glucanases as probes. In each case the wheat (1→3,1→4)-β-glucanase is recognised only by the monoclonal antibody that is specific for barley isoenzyme El.