SALT TOLERANCE, ION RELATIONS AND THE EFFECT OF ROOT MEDIUM ON THE RESPONSE OF CITRUS TO SALINITY

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, u nder conditions of high NaCl concentrations. Root morphology was modif ied by establishing seedlings for 6 months in solution or sand culture . When established in sand culture, both genotypes displayed a high ef ficiency, relative to plants established in solution culture, to exclu de Na from leaves, but differed in their capacities to exclude Cl; thi s capacity was much lower in Etrog citron than Rangpur lime. The net C l root to shoot transport rates over 7 weeks for Etrog citron and Rang pur lime, treated with 50 mol m(-3) NaCl, were 0.25 and 0.07 mu mol g( -1) root FW h(-1), respectively. The net transport rates of Cl in Etro g 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 t issue by week 4, thereafter, the net Cl root to shoot transport rate o f Rangpur lime decreased while the rates in Etrog citron increased. Th e accumulation of Na and Cl in leaves of Rangpur lime, in contrast to Etrog citron, was not affected by transpiration. The high salt toleran ce of Rangpur lime appears to be associated with low rates of net upta ke and transport of Cl, feedback control of net root to shoot transpor t by Cl levels in the shoot, a low interaction between water flow and Cl and Na fluxes, and a reduction in the leaf:root ratio in response t o salinity. When plants were established in solution culture the pione er roots became elongated and the normal development of the laterals i nto a fibrous root system was suppressed. Plants of both genotypes, pr eviously 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 mo l m(-3). Under these conditions the rates of net Cl root to shoot tran sport over the 7 weeks were 0.53 and 0.56 mu mol g(-1) root FW h(-1) f or 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.