Objectives Precise diagnose of boron toxicity and the adverse impact on photosynthesis of leaf of Ponkan (Citrus reticulata Blanco) is necessary for the prevention and cure of the obstacle.

Methods Field investigation and Ponkan leaf nutrition determination was carried out in a Ponkan orchard in Anxi County, Fujian Province, which indicated Ponkan leaves yellowing and abnormal dropping was caused by B toxicity. The leaf samples were collected in normal, moderate yellowing and severe yellowing leaves respectively, and the photosynthesis, chlorophyll fluorescence characteristics and cell membrane permeability were determined.

Results The contents of K, Mg and Zn in normal, moderate yellowing and severe yellowing leaves all fell in the optimum range, while the B contents in moderate and severe yellowing leaves were 11.11 and 19.71 folds of that in normal leaves of Ponkan, indicating that the yellowing and abnormal dropping of leaves in the tested orchard were caused by B toxicity. There were two typical symptoms of boron toxicity in Ponkan leaves. One symptom was developed from leaf tip along midrib and yellow-bronze necrosis spots occurred on the yellowing area. The other one was developed from leaf edge along midrib and the green main veins surrounded by irregularly mottled yellow-green chlorosis. The content of photosynthetic pigments, the effective photochemical efficiency (Fv'/Fm'), electron transport efficiency (ETR), the effective quantum yield (ΦPS II) and photochemical quenching coefficient (qP) decreased with the aggravation of boron toxicity, whereas non-photochemical quenching coefficient (NPQ), excess excitation energy (E) and antenna heat dissipation (D) increased with the aggravation of B toxicity. Boron toxicity significantly decreased photosynthesis rate (Pn), starch and soluble sugar contents, whereas increased leaves cell membrane permeability.

Conclusions Excessive foliar boron application could cause the boron toxicity of Ponkan. Boron toxicity will cause the yellowing and abnormal drop of Ponkan leaves and increase the cell membrane permeability, decrease the photosynthetic rate and block the assimilation of photosynthetic products seriously.