Environmental Effects on the Diurnal Accumulation of 45Ca by Young Fruit and Leaves of Tomato Plants

Abstract

Diurnal uptake and distribution of 45Ca in young fruiting tomato plants were assessed 12 or 24 h after 45Ca was applied to the nutrient solution at the beginning of either the light (12 h) or the dark (12 h) period. During the experiment, the salinity of the nutrient solution (measured as electrical conductivity, EC) was either 2·5 or 17 mS cm−1 and the relative humidity (measured as vapour pressure deficit, VPD) was either 0·2 or 0·6 kPa The uptake of 45Ca by a tomato plant over 12 h was higher in the light than in the dark but the difference was less at low humidity. More 45Ca was transported from the roots to the shoot in the light than in the dark. More than half of the 45Ca in the shoot was accumulated by the stem; the proportion of 45Ca in the stem was greater in the dark and was further enhanced by high humidity to more than 80% of the 45Ca in the shoot. The accumulation of 45Ca by the fruit truss in the dark was greater than in the light in all experimental conditions. Under low humidity the accumulation of 45Ca by young leaves was similar in both light and dark. In high humidity there was considerably less accumulation of 45Ca by the young leaves in the dark The uptake of 45Ca continued over 24 h but the transport of 45Ca to individual organs in the second 12 h period was affected by both light and humidity. Some of the 45Ca accumulated by young leaves and fruit in the second period appears to have been derived from 45Ca released from the xylem wall along the transport pathway in the stem The roles of root pressure and transpiration in the diurnal accumulation of calcium in young fruit and leaves are discussed