With rise in global temperature, terrestrial vegetation is increasingly witnessing unusual warming events. Our knowledge on genotypic variability and plasticity in photosynthetic thermotolerance (PT) is surprisingly limited. We don’t have enough empirical data quantifying PT response to short-term rise in air temperature (Tair). Citrus provides a useful model of acclimation and variation in PT because its evergreen habit and leaf thermal behavior make PT under a variety of environmental conditions crucial for leaf survival. We conducted two mesocosm experiments between Sep and Nov 2019 in the subtropical climatic conditions of central Florida. In experiment 1, 22 genotypes were studied for quantifying variability in intrinsic PT as measured by different chlorophyll fluorescence indices (Fo, Fm and Fv/Fm) of photosystem-II (PSII) performance. In experiment 2, within controlled greenhouse conditions, we simulated progressively increasing air temperature over 3 weeks (weekly day/night Tair: 26/20°C< 33/30 °C<36/32 °C) followed by a recovery period (26/20°C). By the end of each week, PT was examined to identify potential shifts in PT. Further, photochemical and non-photochemical quenching responses were studied over increasing light intensity to understand how elevated Tair affected PSII efficiency and photoprotection capacity.
Results/Conclusions
The genotypes differed moderately but significantly in intrinsic PT as determined by TC (critical Tleaf to rise in Fo) and T50 (Tleaf causing 50% reduction in Fv/Fm). T50 ranged from 47.05 to 50.62°C where T50 in the most thermotolerant genotype was ~3.5°C higher than the least thermotolerant one. The observed variation in T50 shows that citrus trees have high intrinsic PT. At Tleaf above ~45ºC, rapid increase in Fo was recorded with simultaneous decrease in Fm. The comprehensive relationship among these fluorescence variables was evaluated by fitting data into a path model which revealed that Fo had larger contribution to Fv/Fm loss than Fm. Tc, in the most thermotolerant genotype was only ~2.3°C higher than the least thermotolerant one. This informs that most genotypes have a close intrinsic threshold point for Fo rise. We did not find large shifts in PT over transient rise in Tair as evident by weekly measures of T50 and TC. Nevertheless, moderate positive increase in T20 (Tleaf causing 20% reduction in Fv/Fm) was recorded. This informs that genotypes with higher intrinsic PT, can still manifest a moderate rise in Tleaf for PSII threshold over transient elevation in Tair. We will discuss the contribution of acclimation of Tc on acquired PT and changes in quenching behavior to short-term acclimation.