Gas exchange and chlorophyll fluorescence in four citrus rootstocks under aluminium stress

A greenhouse experiment using nutritive solution was carried out to determine the effect of aluminium (Al) on several photosynthesis-related characteristics in Citrus limonia ('Cravo'), Citrus volkameriana ('Volkamer'), Citrus reshni ('Cleopatra'), and Citrus sunki ('Sunki') seedlings. The effects of five Al concentrations (0, 50, 100, 200, and 400 mu mol L-1) in the solution were determined. The lemon seedlings had the highest net photosynthesis in the absence of Al. The net photosynthesis decreased in the presence of Al in all the rootstocks, and the 'Cravo' lemon seedling was the most affected, with a decrease of 85 % at the highest Al concentration evaluated. Stomatal conductance of water vapor (g(s)) was not significantly affected in the lemon seedlings, but it was increased by the Al in the tangerine seedlings. The internal concentration of CO2 (C-i) increased significantly in all the rootstocks with the increase in the Al concentration, but this increase did not result in higher net photosynthesis values. The greatest water use efficiency (WUE) in the absence of Al, was observed in the lemon seedlings. The WUE decreased in all rootstocks with the increase in the Al concentration in the nutritive solution, due to the decrease in the net photosynthesis and increase in the transpiration rate. One of the probable reasons for the reduction in the net photosynthesis was the structural damage to the thylakoids, which affected the photosynthetic transport of electrons, as indicated by the reduction of the ratio between variable fluorescence and initial fluorescence (F-v/F-o).