Energy dissipation in the leaves of Fe‐deficient pear trees grown in the field

Abstract

Chlorophyll fluorescence has been used to investigate the degree of energy dissipation in leaves of pear trees affected by iron deficiency in the field. The experiments were carried out in the presence of moderate water stress in 1996, and in the absence of water stress in 1997. In both cases, high photosynthetic photon flux densities were present at midday. Sustained damage to photosystem II (photoinhibition) was not observed. However, chlorotic leaves showed photosynthesis down‐regulation processes that were enhanced by moderate water stress. These protective mechanisms were associated with decreases in the intrinsic efficiency of photosystem II and increases in non‐photochemical quenching. As a consequence of these protective processes, the amount of light absorbed by photosystem II that was thermally dissipated was larger in iron‐chlorotic leaves than in controls, both early in the morning and at midday. These data indicate that iron‐chlorotic leaves are protected against light excess not only by a decreased light absorption but also by photosynthesis down‐regulation processes.