Effect heat stress on subcellular localization of Ca2+ in tomato fruits

The aim of this paper was to compare the fruit cell ultrastructure and subcellular localization of Ca²⁺ after heat stress with the use of the potassium antimonate method (Slocum and Roux 1982, Tretyn et al. 1992). The tomato plants Robin cv., relatively tolerant to heat stress, were grown under uncontrolled greenhouse conditions to the stage of fruiting. The plants were placed for 20h in two temperature regimes: 23^oC (optimal temperature) or 40^oC (heat stress) in darkness, under water vapour saturated atmosphere. Immediately after heat stress the fruits were harvested to estimate water soluble and insoluble calcium contents and subcellular localization of Ca²⁺. After heating the concentration of calcium in tomato fruits increased about twice. In both temperature treatments the water soluble fractions were lower than insoluble ones at smaller differences between insoluble and soluble fractions after heat stress. The shapes and localization of Ca²⁺ detected with the use of potassium antimonate method show that in fruits of control plants the precipitates were numerous, small and of oval shape. They were dispersed in cytosol or adjoined to endoplasmic reticulum or to external membrane of chloroplast. In the fruit of heated plants the precipitates were irregular in shape, amorphous and singly dispersed in the cytosol. We observed also some cytological changes in the structure of membranes and organelles of the plants of both experimental treatments. The heat induced increase of calcium content and the changes in subcellular localization of Ca²⁺ under heat stress suggest that calcium ions may be involved in avoiding heat injury. The problem requires more detailed further investigations.