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.