Thermotolerance of Coffea arabica: Potential implications in a warming world

Background/Question/Methods

Models predict increasingly frequent and more intense high temperature events (heat waves) that may impact plant species distributions. Although high temperature stress is known to negatively affect photosynthetic performance, the legacy effects of heat waves are largely unknown. Shade-tolerant coffee (Coffea arabica) is the second most traded commodity in the world (after oil), but it is unknown how heat waves might impact its photosynthetic performance and productivity. We used chlorophyll fluorescence to evaluate thermotolerance in coffee. The ratio of maximum variable fluorescence (F_v) to maximum total fluorescence (F_m) reflects the potential quantum efficiency of photosystem (PS) II and is used as a sensitive indicator of photosynthetic performance. Minimum fluorescence (F_o) also indicates alterations in PSII. We constructed thermotolerance curves by measuring chlorophyll fluorescence after exposing dark-adapted leaf discs to a range of temperatures (between 25 and 55˚C). F_v/F_m and F_o were measured in >1-yr-old 'older' leaves and <1-yr-old 'younger' leaves 15 minutes and 24 hours after temperature exposure to evaluate PSII quantum efficiency. Second order polynomial regressions were used to estimate the temperature at which a 50% reduction in the F_v/F_m ratio occured (T₅₀), and the threshold temperature at which the slope of F_o (vs temperature) began to increase (T_crit).

Results/Conclusions

For both leaf age classes and recovery times, F_v/F_m declined from 0.75 to 0, indicating that increasing temperatures decreased photosynthetic performance. T₅₀ was 47.5˚C for the older leaves and 46.9˚C for younger leaves 15 minutes after heat exposure. T₅₀ for older leaves was 47.8˚C and 48.1˚C for younger leaves 24 hours after heat exposure. T_crit of older leaves was 46.2˚C and 44.6˚C for younger leaves 15 minutes after heat exposure. T_crit of older leaves was 46.7˚C and 44.5˚C for younger leaves 24 hours after heat exposure. These shifts in F_v/F_m and F_o suggest that the photosynthetic performance of coffee is significantly diminished by increasing temperatures above ~40˚C. The maintenance of F_m at higher temperatures 24 hours vs 15 minutes after heat exposure is consistent with previous work showing that measuring F_v/F_m 24 hours after heat exposure is a more reliable indicator of heat-induced leaf tissue damage. Future work will expand upon these results and assess F_v/F_m, F_o and leaf gas exchange properties in situ on coffee plants exposed to simulated heat waves in a growth chamber to assess the legacy effects that heat waves of varying intensity and duration may have on coffee leaves at different ontogenetic stages.