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In the long dark night, fruit come first
When financial circumstances are difficult, a key question is ‘Which activities are expendable and which are essential?’ We can ask similar questions about the carbohydrate economy of plants, as shown by Gary et al. (INRA, Avignon and INRA, Villenave d’Ornon, France, pp. 429-438). In an investigation of source-sink relationships they exposed tomato plants to either ‘poverty’ or ‘wealth’ in respect of carbon assimilation, i.e. low light and low CO2 concentration or high light and high CO2 concentration. The plants were then transferred to total darkness and sampled at intervals. From a very thorough and wide-ranging data set, I focus here on aspects of respiration, carbohydrate metabolism and growth. As might be expected, the pre-treatments had a profound effect on the plants before transfer to darkness. With the exception of flower buds and in expanding fruit, concentrations of soluble carbohydrates and starch were very much lower throughout the plant after low-assimilation conditions than after high-assimilation conditions. Respiration in darkness reflected this. At the whole plant level, respiration rate in the low-carbon group declined to almost zero within 48 h, whereas in the high-carbon group, whole plant respiration was still detectable at 80 h. In both groups, carbohydrate content of all organs except for fruit declined as darkness continued and there was evidence for mobilization of proteins to provide emergency ‘respiratory’ substrates. However, in early fruit and in expanding fruit, in both in the low-carbon and high-carbon groups, carbohydrate content did not decline. Concomitant with this, fruit expansion rate actually increased significantly in the first 15 h of darkness and then declined (but fruit expansion was in fact maintained throughout the 80 h of the experiment). It is thus abundantly clear that in times of carbohydrate poverty, the next generation takes precedence.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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