Turnover of Tritiated Water in Calamondin Plants1

The turnover of water in small fruit-bearing plants of calamondin ( Citrus madurensis Loureiro) was studied by following the uptake of tritium-labelled nutrient solution. The turnover was much greater in vegetative tissue than in the fruit. Even after 3 days, the radioactivity of water in the fruit, relative to that in the solution, did not reach 20%, compared with about 60% in the leaf tissue. The radioactivity of water in the pedicel reached 60%, similar to that in leaf tissue. Decay of radioactivity in fruit was considerably slower than in other organs when the plant was transferred from a labelled to a non-labelled solution. The turnover of water in fruit was extremely slow even when transpiration was increased by exposing the entire plant or fruit alone to a stream of dry air. Defoliation of fruit-bearing branches decreased the entry of water into fruit tissues. The availability of water for fruit growth in view of the competition offered by the leaves is discussed.