Effect of heat hardening on allocation of excitation energy in Brassica oleracea seedling leaves

In this paper, Brassica oleracea was used to investigate the effect of heat hardening (HH) on its photosynthetic characteristics and chlorophyll fluorescence parameters. No distinct changes were observed in the variable fluorescence (Fv), photochemical quenching coefficient (qP), non-photochemical quenching coefficient (qN), and quantum yield of PS II electron transport (phiPS II) between HH treatment and control when the leaf temperature was 25-35 degrees C. When the leaf temperature was higher than 35 degrees C, there were obvious differences in the levels of Fv, qP, phiPS II and qN between HH treatment and control. For example, at 37 degrees C, the Fv, qP and phiPS II in HH treatment were 53%, 24%, and 86% higher than those in control, respectively, but the qN was 22% lower. The photoinhibitory quenching (qNs) in HH treatment was significantly decreased. The results showed that 'high energy' quenching (qNf) was an important photoprotection mechanism that effectively dissipated the excess excitation energy, protected the PS II reaction center complexes from photodamage, and increased the ability of resisting photoinhibition, which were consistent with high photosynthesis rates.