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 °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 °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.