Salinity Effects of Leaf Water Relations and Gas Exchange of 'Valencia' Orange, Citrus sinensis (L.) Osbeck, on Rootstocks With Different Salt Exclusion Characteristics

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Abstract
Valencia orange [Citrus sinensis (L.) Osbeck] scions were grafted to rootstocks with different sodium and chloride excluding characteristics, viz. Trifoliata [Poncirus trifoliata (L.) Raf. and Cleopatra mandarin (Citrus reticulata Blanco). Grafted trees were grown at either 175 or 650 µmol m-2 s-1 photosynthetically active radiation and watered with nutrient solution with 0, 50 or 100 mol m-3 NaCl. Foliage of scions grafted to Trifoliata had higher levels of chloride but lower levels of sodium compared to foliage of scions grafted to Cleopatra mandarin. Although leaf water potential declined with salinisation, salt accumulation in leaves on both root- stocks contributed to more negative osmotic pressures and subsequent maintenance of turgor at or above levels in control leaves. Despite turgor maintenance, CO2 assimilation rate (A) was reduced by salinity. Inhibition occurred to a greater degree for leaves on Cleopatra mandarin than those on Trifoliata. Reductions in CO2 assimilation and stomatal conductance were usually in step with each other. This correlation was paralleled by little change in intercellular partial pressures of CO2 (pI) despite large changes in gas exchange. The response of A to pi was affected by salinity in such a way that impact of salt on CO2 assimilation was expressed to a greater degree at high pI. The extent of photosynthetic response to salt at high pI was independent of light level during growth. Room temperature chlorophyll a fluorescence induction kinetics remained unaltered. Scions grafted to Trifoliata showed slower onset of photosynthetic dysfunction following salinisation than corresponding foliage of scions on Cleopatra mandarin. Moreover, photosynthetic sensitivity to increased leaf chloride also appeared to diminish on Trifoliata. This rootstock effect on scion performance was attributed to a superior capacity in Trifoliata for exclusion of sodium ions from Valencia scions.