PS 81-214
Thermotolerance of Coffea arabica: Potential implications in a warming world

Friday, August 15, 2014
Exhibit Hall, Sacramento Convention Center
Danielle E. Marias, Forest Ecosystems and Society, Oregon State University, Corvallis, OR
Frederick Meinzer, PNW Research Station, USDA Forest Service, Corvallis, OR
Christopher J. Still, Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR
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 (Fv) to maximum total fluorescence (Fm) reflects the potential quantum efficiency of photosystem (PS) II and is used as a sensitive indicator of photosynthetic performance. Minimum fluorescence (Fo) 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). Fv/Fm and Fo 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 Fv/Fm ratio occured (T50), and the threshold temperature at which the slope of Fo (vs temperature) began to increase (Tcrit).

Results/Conclusions

For both leaf age classes and recovery times, Fv/Fm 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. Tcrit of older leaves was 46.2˚C and 44.6˚C for younger leaves 15 minutes after heat exposure. Tcrit of older leaves was 46.7˚C and 44.5˚C for younger leaves 24 hours after heat exposure. These shifts in Fv/Fm and Fo suggest that the photosynthetic performance of coffee is significantly diminished by increasing temperatures above ~40˚C. The maintenance of Fm at higher temperatures 24 hours vs 15 minutes after heat exposure is consistent with previous work showing that measuring Fv/Fm 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 Fv/Fm, Fo 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.