Diurnal changes in the flux of calcium toward meristems and transpiring leaves in tomato and maize plants.

Tomato (Lycopersicon esculentum Mill. cv. Moneymaker) and maize (Zea mays L. cv. spec.) plants were supplied with (45)Ca-labeled nutrient solutions for a period of 8 or 16 h in the dark, in the light, or in a light-dark régime. Plant parts were analyzed for (45)Ca content. The partitioning of (45)Ca between mature leaves and meristems was shown to be affected by the presence of light. The transport of (45)Ca to meristems was higher in a dark period than in a comparable light period. Experiments with excised tomato shoots yielded similar distribution patterns of (45)Ca over leaves and meristems, thus excluding root pressure as the main driving force for the enhanced import of (45)Ca into the meristems in the dark. Results are discussed in terms of cation-exchange transport and competition between the various calcium sinks.