Photosynthetic dysfunction and in vivo changes in chlorophyll a fluorescence from manganese-deficient wheat leaves

Abstract

Leaves on wheat plants growing in manganese-deficient media showed much higher constant-yield fluorescence (Fo) but greatly reduced variable fluorescence (Fv). Higher Fo was symptomatic of a stronger overall emission which derived from functional changes within photosynthetic membranes and was not simply attributable to differences in leaf reflectivity of wavelengths relevant to fluorescence measurements (710-720 nm). Topical application of 4 mM MnCl, to such deficient material restored photosynthetic activity (O2 evolution by leaf slices) and also led to correlative changes in fluorescence: increased Fv, and lower Fo. Original symptoms were therefore attributable to Mn-deficiency per se. Quantitative relationships between leaf Mn concentration and certain fluorescence characteristics (the ratio Fo/Fv) were then established on flag levels excised from field plants. Critical levels of leaf Mn were discernible as an abrupt increase in the Fo/Fv ratio, once leaf Mn was below a threshold concentration, which was the minimum requirement of this element for dry matter productivity, viz. 10-12 ¦g Mn g-1 dry wt.