Salt Tolerance, Ion Relations and the Effect of Root Medium on the Response of Citrus to Salinity

Abstract

This paper describes the ion relations of two citrus genotypes, Etrog citron (Citrus medica L.), a salt-sensitive genotype, and Rangpur lime (Citrus reticulata var. austera hybrid?), a salt-tolerant genotype, under conditions of high NaCl concentrations. Root morphology was modified by establishing seedlings for 6 months in solution or sand culture. When established in sand culture, both genotypes displayed a high efficiency, relative to plants established in solution culture, to exclude Na from leaves, but differed in their capacities to exclude Cl; this capacity was much lower in Etrog citron than Rangpur lime. The net Cl root to shoot transport rates over 7 weeks for Etrog citron and Rangpur lime, treated with 50 mol m^-3 NaCl, were 0.25 and 0.07 μmol g^-1 root FW h^-1, respectively. The net transport rates of Cl in Etrog citron were 10 times those in Rangpur lime between weeks 4 and 7. In Rangpur lime Cl reached pseudo steady-state levels in root and leaf tissue by week 4, thereafter, the net Cl root to shoot transport rate of Rangpur lime decreased while the rates in Etrog citron increased. The accumulation of Na and Cl in leaves of Rangpur lime, in contrast to Etrog citron, was not affected by transpiration. The high salt tolerance of Rangpur lime appears to be associated with low rates of net uptake and transport of Cl, feedback control of net root to shoot transport by Cl levels in the shoot, a low interaction between water flow and Cl and Na fluxes, and a reduction in the leafiroot ratio in response to salinity. When plants were established in solution culture the pioneer roots became elongated and the normal development of the laterals into a fibrous root system was suppressed. Plants of both genotypes, previously established in solution culture and then treated with 50 mol m^-3 NaCl, accumulated levels of Na and Cl in leaves up to 200-400 mol m^-3. Under these conditions the rates of net Cl root to shoot transport over the 7 weeks were 0.53 and 0.56 μmol g^-1 root FW h^-1 for Etrog citron and Rangpur lime, respectively. These high rates of Cl uptake and transport were attributed to changes in root permeability and increases in passive ion fluxes. Keywords: citrus, root morphology, root medium, transpiration, Cl, Na, salt accumulation, salinity.