Given future climate predictions of increased temperature and frequency and intensity of heatwaves in the tropics, suitable habitat to grow ecologically, economically, and socially valuable Coffea arabica is severely threatened. Therefore, comprehensive assessments of thermotolerance, photosynthetic performance, and physiological recovery from heat stress are critical for understanding C. arabica’s responses to increased temperature. The goals of this project are to: 1) evaluate C. arabica thermotolerance across leaf age classes and time after temperature exposure (15 min, 24 h) using chlorophyll fluorescence (ratio of variable to maximum fluorescence, FV/FM and minimum fluorescence, FO) and electrolyte leakage methods; and 2) monitor physiological aftereffects of heat stress in situ using chlorophyll fluorescence, leaf non-structural carbohydrate (NSC) dynamics, and gas exchange. Parameters derived from FV/FM and FO thermotolerance curves included the temperature at which a 50% reduction in initial FV/FM occurred (T50), and the critical temperature at which the temperature-induced FO rise began (Tcrit). To assess in situresponses to simulated heat waves, treated plants were heated in a growth chamber at 49°C for 45 or 90 min. Physiological recovery was monitored on mature and expanding leaves before and 2, 15, 25, and 50 days after treatment.
Results/Conclusions
FV/FM thermotolerance curves indicated that photosynthetic performance was significantly diminished above ~40˚C, while membrane damage occurred above ~50°C using the electrolyte leakage method. The 24 h post-treatment time more accurately estimated T50. Tcrit increased with leaf age while T50 did not differ among age classes. In the in situ experiment, regardless of leaf age class, the 90 min group exhibited a greater FV/FM decline 2 days after treatment and slower recovery than the 45 min group. Although rate of FV/FM recovery did not differ between age classes, photosynthesis of mature leaves recovered faster than expanding leaves. In both age classes, starch of the 45 min group significantly decreased 2 days after treatment for repair, then accumulated 15 and 25 days after treatment coinciding with recovery of photosynthesis and FV/FM. Starch of the 90 min group did not change with time, although sucrose accumulated at day 2 suggesting that phloem transport was inhibited. Both treatment group responses contrasted with control plant TNC, which declined with time due to subsequent flower and fruit production. No treated plants produced flowers or fruits. Our results show that leaf age and the extent of heat-induced damage influences recovery and carbohydrate dynamics.