Water Relations and Ion Concentrations of Leaves on Salt-Stressed Citrus Plants

Grafted plants of Valencia orange scion [Citrus sinensis (L.) Osbeck] on six different rootstocks were grown under glasshouse conditions and supplied with dilute nutrient solution containing either 0 or 75 mM NaCl. Salt treatment was increased to 150 mM NaCl after 49 days. Leaf water relations and leaf chloride, sodium and potassium concentrations were followed throughout the period of salt treatment until day 105, when salt treatment ceased, and thereafter until day 140. Seedlings of Rangpur lime (C. reticulata var. austera hybrid?), Cleopatra mandarin (C. reticulata) and sweet orange (C.sinensis) were treated similarly and leaf water relations and chloride concentrations were followed until salt treatment ceased on day 77. All Valencia-rootstock combinations adjusted osmotically to the salt stress imposed and maintained turgor pressures at or above control values. Mature leaves on seedlings of sweet orange behaved similarly to Valencia orange leaves on sweet orange rootstocks by maintaining turgor pressures higher than control values. In contrast, mature leaves on seedlings of the genotypes Rangpur lime and Cleopatra mandarin tended to lose turgor during the period of treatment with 150 mM NaCl. Leaf chloride analyses indicated that Rangpur lime and Cleopatra mandarin rootstocks restricted the uptake and/or transport of chloride to shoots. However, comparatively high concentrations of sodium (>approx. 200 mM, tissue water basis) were accumulated in mature leaves on all rootstocks during salt treatment. Leaf potassium concentrations remained similar to control values. The reduction in osmotic potential in mature Valencia leaves on rough lemon (C. jambhiri), Trifoliata (Poncirus trifoliata), Camzo citrange (C. sinensis × P. trifoliata) and sweet orange rootstocks on day 77 could be accounted for largely by the increase in sodium and chloride, whereas chloride (as NaCl) accounted for only approximately 50% of the reduction in osmotic potential in Valencia leaves on Rangpur lime and Cleopatra mandarin rootstocks. Stomatal resistances in mature Valencia leaves on all rootstocks were increased by salt treatment and showed only partial recovery after the cessation of salt treatment. The incomplete recovery may have been associated with the retention in leaves of high concentrations of sodium.