Seed development in relation to desiccation tolerance: A comparison between desiccation-sensitive (recalcitrant) seeds ofAvicennia marinaand desiccation-tolerant types

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Abstract
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. marinaseeds 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 inA. 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 onA. marinareveals that for this recalcitrant seed-type, germinationper secannot 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 eventsensu stricto, in the germination processes in desiccation-tolerant (orthodox) seeds.