COMPARATIVE EFFECTS OF HEAT-STRESS ON PHOTOSYNTHESIS AND CHLOROPLAST ULTRASTRUCTURE IN TOMATO PLANTS WITH SOURCE-SINK MODULATED BY GROWTH-REGULATORS

Response to high temperature of rooted cuttings with modulated source-sink relations (flowers treated with, growth regulators: NOA +GA(3), denoted in text as NG), was studied. Net photosynthetic rate (P-N) was measured on the same attached leaves before and after high temperature 22-hour treatment, (40 degrees C during 14 h light and 25 degrees C during 8 h darkness). Heat stress strongly decreased CO2 assimilation, especially in NG-series. Lower rate of P-N caused by elevated temperature was attributed both to decreased stomatal conductance and to the destruction of chloroplast ultrastructure: the breakage of chloroplast envelope, increased number of plastoglobulin, disappearance of starch grains and in some chloroplasts disintegrations of grana. All described changes similar to natural senescence symptoms were more drastic in NG-plants with higher sink strength of fruits, presented hi our previous papers (Starck et al. 1987, 1989a), creating stronger competition about photosynthates between leaves and reproductive organs, In conclusion, leaf photosynthesis and chlolaplast ultrastructure of plants having their fruit growth stimulated by NG become more sensitive to heat stress.