Photosystem II efficiency in low chlorophyll, iron-deficient leaves

Iron deficiency (iron chlorosis) is the major nutritional stress affecting fruit tree crops in calcareous soils in the Mediterranean area. This work reviews the changes in PS II efficiency in iron-deficient leaves. The iron deficiency-induced leaf yellowing is due to decreases in the leaf concentrations of photosynthetic pigments, chlorophylls and carotenoids. However, carotenoids, and more specifically lutein and the xanthophylls of the V+A+Z (Violaxanthin+ Antheraxanthin+Zeaxanthin) cycle are less affected than chlorophylls. Therefore, iron-chlorotic leaves grown in either growth chambers or field conditions have increases in the molar ratios lutein/chlorophyll a and (V+A+Z)/chlorophyll a. These pigment changes are associated to changes in leaf absorptance and reflectance. In the chlorotic leaves the amount of light absorbed per unit chlorophyll increases. The low chlorophyll, iron-deficient leaves showed no sustained decreases in PS II efficiency, measured after dark adaptation, except when the deficiency was very severe. This occurred when plants were grown in growth chambers or in field conditions. However, iron-deficient leaves showed decreases in the actual PS II efficiency at steady-state photosynthesis, due to decreases in photochemical quenching and intrinsic PS II efficiency. Iron-chlorotic leaves were protected not only by the decrease in leaf absorptance, but also by down-regulation mechanisms enhancing non-photochemical quenching and thermal dissipation of the light absorbed by PS II within the antenna pigment bed.