Shan Kothari

The many biophysical factors that shape how plant species sort across environmental gradients may include photoinhibition, which I define broadly as oxidative damage that plants and other phototrophs risk incurring when they absorb excess light energy they cannot safely dissipate. Photoinhibition is seldom explicitly discussed as a potential driver of plant fitness and distributions. Here, I aim to show that it can be one by drawing on studies showing that natural gradients or experimental manipulations that increase the risk of photoinhibition thereby decrease plant fitness, or favor species with stronger photoprotective adaptations. A corollary is that alleviation of photoinhibition may be a common mechanism of facilitation. A large share of this research is set in stressful environments like alpine treelines or drylands, most likely because photoinhibition is most detrimental in the presence of other interacting stress factors. Nevertheless, knowing the specific role of photoinhibition may have practical value—for example, in understanding the uses of shading treatments in ecological restoration.

Much research assumes that photoinhibition alters fitness by reducing carbon assimilation. Nevertheless, a tension exists in ecological literature on photoinhibition: many kinds of stressful conditions that threaten oxidative damage to photosynthesis also directly constrain tissue expansion. If carbon sink strength declines more than source strength—as it often does—a carbon surplus may result and lead to feedback inhibition of photosynthesis. Under persistent carbon surplus, photoinhibition’s potential influence on carbon status may not matter much for fitness. This fact might be reconciled with photoinhibition’s known effects on plant fitness by considering its other consequences. Oxidative damage can have consequences far beyond photosynthesis—altering membranes, blocking phloem loading, and even triggering cell death. While such effects are harder to measure than photosynthesis, doing so may enrich our understanding of photoinhibition’s ecological consequences.