Wheat photosystem II heat tolerance responds dynamically to short- and long-term warming

Wheat photosynthetic heat tolerance can be characterized using minimal chlorophyll fluorescence to quantify the critical temperature (T-crit) above which incipient damage to the photosynthetic machinery occurs. We investigated intraspecies variation and plasticity of wheat T-crit under elevated temperature in field and controlled-environment experiments, and assessed whether intraspecies variation mirrored interspecific patterns of global heat tolerance. In the field, wheat T-crit varied diurnally-declining from noon through to sunrise-and increased with phenological development. Under controlled conditions, heat stress (36 degrees C) drove a rapid (within 2 h) rise in T-crit that peaked after 3-4 d. The peak in T-crit indicated an upper limit to PSII heat tolerance. A global dataset [comprising 183 Triticum and wild wheat (Aegilops) species] generated from the current study and a systematic literature review showed that wheat leaf T-crit varied by up to 20 degrees C (roughly two-thirds of reported global plant interspecies variation). However, unlike global patterns of interspecies T-crit variation that have been linked to latitude of genotype origin, intraspecific variation in wheat T-crit was unrelated to that. Overall, the observed genotypic variation and plasticity of wheat T-crit suggest that this trait could be useful in high-throughput phenotyping of wheat photosynthetic heat tolerance. Heat tolerance of wheat PSII was highly plastic in response to changing growth conditions. An upper limit to increasing heat tolerance was identified following a short-term heat stress.