Effects of Salinity and Boron Excess on the Growth, Photosynthesis, Water Relation and Mineral Composition of Laurustinus Grown in Greenhouse

A greenhouse study was conducted to determine the interactive effects of NaCl salinity and boron on the growth, plant water status, gas exchange, chlorophyll fluorescence and concentrations of sodium (Na), chloride (Cl) and boron (B) in laurustinus (Viburnum tinus L.). Potted plants were grown in a factorial combination of salinity (2 and 6 dS m(-1)) and boron (1 and 6 mg L-1). Plant dry weight (DW) decreased with salinity and B excess, particularly as a result of the former. The salinity x B interaction on the plant DW was not significant (additive effects). Salinity increased Na and Cl concentrations in leaf (20 and 35 mg g(-1) DW, respectively) resulting in foliar injury. The application of 6 mg L-1 of B (B toxicity or B excess) produced injury symptoms in old leaves (leaf tip and edge burn). Salinity and B toxicity led to leaves dropping, especially the former. B toxicity led to higher B concentrations in insured leaves (1385 mg kg(-1) DW) and salinity reduced it to 425 (B x NaCl antagonistic effect). Boron excess did not alter Na and Cl concentrations in leaf. Salinity decreased stomatal conductance (g(s)) as a regulatory mechanism against osmotic stress, which resulted in a dropping photosynthesis (P-n). Leaf water parameters were only affected by salinity, which enhanced a process of osmotic adjustment and improving the plant water status. Salt-stressed plants showed an adaptive response to salinity, which decreased g(s), P-n and quantum yield of photosystem II (ePSII), and dissipated the excess radiant energy as heat (increased non-photochemical quenching [NPQ]). The combination of salinity and B excess maintained ePSII and decreased the effectiveness of stomatal regulation, NPQ and Pn. This caused the lowest plant DW and suggests disorders in electron transport (photorespiration). Our findings suggest that: (1) laurustinus is a B excess sensitive species, (2) salinity reduced the accumulation of B in leaves of the B excess stressed plants but was not enough to prevent injuries in PSII, and (3) B excess or/and salinity provide plants of poor commercial quality.