Plants experience high temperature events at fine-scale time frames over the course of a single day and during heatwave events. When leaf stomata close for water conservation and the wind drops, leaf temperature can rise rapidly. Temperatures above a certain threshold for a given species (e.g. associated with 50% loss of function of photosystem II: T50), damage leaf photosystems and can lead to cell and leaf death. But what happens when short heat stress events occur multiple times throughout a day? If a leaf is given short periods of reprieve, could it handle repeated excursions to high temperatures? We addressed this question using an Australian arid zone shrub Myoporum montanum by measuring the health of photosystem II (FV/FM) and T50 in response to heat stress of varying intensity (48-52°C), duration (5-30 minutes), repetition (1-6 stress events, with and without 10 minute reprieves) and trialed three times to incorporate T50 changes with weather. Heat stress was imposed using temperature controlled water baths under subsaturating light. A linear mixed model with a varying co-variance matrix between groups was used to analyze the effect of short reprieves from heat stress on photosynthetic function under different stress temperatures, duration and trials.
Results/Conclusions
The influence of short reprieves was temperature dependent and provided minimal benefit for surviving extreme high temperatures. For the temperatures just below the innate T50 threshold for this species (48°C), reprieves did help to mitigate photosystem damage somewhat. This supports theory suggesting that if a heat wave close to a plant’s thermal threshold is accompanied by light wind gusts, leaves are more likely to survive it. However, for repeat stresses at 50 and 52°C, entirely possible for drought-stressed desert plants in the absence of air movement, the effects of repeated heat stress became additive. This pattern was mirrored in cooler measurement trials, when the plants’ T50 thresholds were lower. As duration of a stress increased, leaf FV/FM significantly decreased. Our findings suggest that there is not a single temperature that causes failure for a plant or species. Rather, the threshold of severe damage is dependent on the temperature and duration of the stress. Being able to link a range of high temperature events to physiological damage is imperative for predicting the potential survival of plants through future heat stress events under a warming climate.