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Turning on the gas for greater growth
Although many of the plant species we use as crops are, as a result of selection, very different from their nearest wild relatives, there are some for which this is not so. Thus, cultivated rice and red or weedy rice are regarded as conspecific. They are capable of interbreeding (although their flowering times tend to be different) and gene flow from one to the other certainly occurs [I have commented elsewhere on the problems that may be caused by gene flow from genetically modified cultivated rice to red rice: Bryant (2007). Rice, risk and regulations. Biological Sciences Review, in press]. Control of red rice amongst rice crops is a problem, one particular feature of which is the occurrence of banks of dormant seeds in the paddy fields. With this in mind, a joint Dutch–Italian team (Gianinetti et al., pp. 735–745) have investigated the physiology of dormancy and germination, focusing especially on the role of ethylene. From their very thorough paper we focus here on some of the key results. Treatment of after-ripened (non-dormant) caryopses with inhibitors of ethylene synthesis or action, or with promoters of ethylene synthesis and release, had no effect at all on germination (as indicated by splitting of the pericarp). Further, exposure to these agents or to ethylene itself had no effect on dormant seeds; neither was there any residual effect on seeds whose dormancy was subsequently broken by after-ripening. However, ethylene inhibitors did retard seedling growth after germination; this effect was negated if the ethylene precursor ACC was added at the same time. With very sensitive laser-based ethylene detectors (capable of detecting emission rates as low as 5 fmol per seed h21), the authors demonstrated that germinating seedlings began to produce ethylene at about the time of pericarp splitting but the latter was not dependent on ethylene production. Thus ethylene is involved not in germination per se but in seedling growth (and especially growth of the radicle). Further, the kinetics of ethylene emission in the presence of the inhibitors AVG and STS, and of the precursor ACC, suggest that the synthesis of ACC must be activated before ethylene emission can start.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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