Time-course of tomato whole-plant respiration and fruit and stem growth during prolonged darkness in relation to carbohydrate reserves

To evaluate the relevance of a simple carbon balance model (Seginer et al., 1994, Scientia Horticulturae 60: 55-80) in source-limiting conditions, the dynamics of growth, respiration and carbohydrate reserves of tomato plants were observed in prolonged darkness. Four days prior to the experiments, plants were exposed to high or low light levels and CO(2) concentrations. The concentration of carbohydrates in vegetative organs was 30-50 % lower in plants that were exposed to low carbon assimilation conditions compared with those exposed to high carbon assimilation conditions. During prolonged darkness, plants with low carbohydrate reserves exhibited a lower whole-plant respiration rate, which decreased rapidly to almost zero after 24 h, and carbohydrate pools were almost exhausted in leaves, roots and flowers. In plants with high carbohydrate reserves, the whole-plant respiration rate was maintained for a longer period and carbohydrates remained available for at least 48 h in leaves and flowers. In contrast, fruits maintained fairly stable and identical concentrations of carbohydrates and the reduction in their rate of expansion was moderate irrespective of the pre-treatment carbon assimilation conditions. The time-course of asparagine and glutamine concentrations showed the occurrence of carbon stress in leaves and flowers. Estimation of source and sink activities indicated that even after low carbon assimilation, vegetative organs contained enough carbohydrates to support fruit growth provided their own growth stopped. The time of exhaustion of these carbohydrates corresponded grossly to the maintenance stage simulated by the model proposed by Seginer et al. (1994), thus validating the use of such a model for optimizing plant growth.