Contrasted Responses to Root Hypoxia in Tomato Fruit at Two Stages of Development

The biochemical consequences of root hypoxia have been documented in many sink organs, but not extensively in fruit. Therefore, in the present study, the response to root hypoxia in tomato fruit (Solanum lycopersicum L.) was investigated at two developmental stages, during the cell division and the cell expansion phases. Our results showed that in dividing fruit, root hypoxia caused an exhaustion of carbon reserves and proteins. However, ammonium and major amino acids (glutamine, asparagine and γ-aminobutyric acid (GABA)) significantly accumulated. In expanding fruit, root hypoxia had no effect on soluble sugar, protein and glutamine contents, whereas starch content was significantly decreased, and asparagine and GABA contents slightly increased. Metabolite contents were well correlated with activities of the corresponding metabolising enzymes. Contrary to nitrogen metabolising enzymes (glutamine synthetase, asparagine synthetase and glutamate decraboxylase), the activities of enzymes involved in sugar metabolism (invertase, sucrose synthase, sucrose phosphate synthase and ADP glucose pyrophosphorylase) were significantly reduced by root hypoxia, in diving fruit. In expanding fruit, only a slight decrease in ADP glucose pyrophosphorylase and an increase in asparagine synthetase and glutamate decarboxylase activities were observed. Taken together, the present data revealed that the effects of root hypoxia are more pronounced in the youngest fruits as it is probably controlled by the relative sink strength of the fruit and by the global disturbance in plant functioning.