Physiological and morphological responses of blueberry to manganese stress in soil

The effects of manganese (Mn) stress on photosynthetic pigment and malondialdehyde content, Mn accumulation, ascorbate (AsA), reduced glutathione (GSH), soluble sugar, sucrose, phytohormones metabolism, as well as some morphological characteristics of leaves were investigated in blueberry cultivar ‘Tifblue’ (Vaccinium ashei Reade) after 60 days treated by 1.0 mM, 2.5 mM, 5.0 mM, and 10.0 mM MnSO4 in soil. Mn accumulation increased in a concentration-dependent manner. A significant increase in MDA content was observed, except in the 1.0 mM and 2.5 mM Mn treatments. Levels of chlorophyll a (Chl a) and chlorophyll b (Chl b) first increased and then decreased, while carotenoid (Car) content decreased gradually. Mn stress induced a remarkable decrease in AsA content, but no significant change was observed in GSH. Soluble sugar content changed slightly, whereas sucrose content rose gradually with the increase in Mn concentration, reaching peak value at 5.0 mM. Jasmonic acid (JA) and salicylic acid (SA) contents increased alternately in response to Mn treatments, while 1-aminocylopropane-1-carboxylic acid content changed in a similar way to soluble sugar. Significant damage to palisade tissue cells and sponge tissue cells and stomatal closure imposed by high Mn concentration indicated a general disarray in the cellular functions. Our results suggested that Mn tolerance is regulated by mechanisms through multiple factors and is associated with elevated levels of photosynthetic pigments, soluble sugar, sucrose, JA, and SA. Besides, the critical value of Mn in soil on blueberry cultivar ‘Tifblue’ was 2.5 mM.