Enhanced thermal dissipation confers photoprotection in top leaves despite systemic regulation from lower leaves in cotton

To better understand the photosynthetic regulation between lower leaves and top leaves, leaf gas exchange and chlorophyll fluorescence were examined in field and climate chamber grown cotton (Gossypium hirsutum L. cv. Xinluzao 45). Two planting density treatments were used in the field: low planting density (LD) and high planting density (HD), and two artificial shade treatments were used in the climate chamber: no shade (NS) and lower leaves shaded (LS). Our results show that the maximum net photosynthetic rate (P_max), light saturation point (LSP) and light compensation point (LCP) of top leaves were decreased, but the apparent quantum efficiency of net carbon assimilation (AQE) of top leaves was increased in HD and LS, which had a similar trend to the lower leaves. Although top and lower leaves improved the utilization of light, the fractions of light absorbed by the PSII antenna that is utilized in PSII photochemistry Y(II) and photochemical quenching coefficient (qP) of top leaves and lower leaves were decreased in HD and LS. Furthermore, the fraction of absorbed light that is dissipated thermally via ΔpH and xanthophylls regulated processes Y(NPQ) and non-photochemical quenching (NPQ) of top leaves were increased in HD and LS. In summary, these results suggest that the light energy utilization and photochemical efficiency of the top leaves in cotton are regulated by the light environment of the lower leaves, and the activation of thermal dissipation confers photoprotection of top leaves despite systemic regulation from lower leaves in cotton. Therefore, considering that cotton is cultivated in high density in most cotton areas of the world, the systemic regulation of photosynthetic capacity in top leaves affected by light environment of lower leaves caused by high-density cultivation needs to be considered comprehensively in practical production. Then, the optimal photosynthetic capacity of the population can be obtained.