Increasing leaf export and grain import capacities in maize plants under water stress.

The export rate and the carbohydrate concentration were measured in maize plants submitted to water deprivation either at the fourth leaf stage or at pollination. Export rate was evaluated by a short pulse of labelling with CO followed by a 10-h chase. In stressed plants, C fixation was strongly reduced. When radioactivity was expressed relative to the initial value, the time course of label export from the labelled zone showed a faster decline in stressed plants than in well-watered plants. This was observed both under mild stress (fourth leaf stage) and severe stress (pollination stage). Another consequence of drought stress was an increase in fourth leaf vacuolar invertase activity and an increase in hexoses, which accumulated to the same content as sucrose. This occurred without a significant decrease in starch. At pollination stage, despite a large decrease in absolute quantity of C entering kernels, the proportion of leaf C export recovered in the kernel was not modified after a 4-d water deprivation, i.e. at day 0 after pollination (0 DAP), and was multiplied by a factor of 2-3 at 12 DAP. The major conclusion arising from these data appears to be an improvement of both leaf export and kernel import efficiency under water stress.