Morphological factors determining salt tolerance in citrus seedlings: the shoot to root ratio modulates passive root uptake of chloride ions and their accumulation in leaves

The results presented in this work were obtained with two citrus genotypes, the chloride-tolerant Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and the chloride-sensitive Carrizo citrange [Citrus sinensis (L,) Osb, x Ponci rus trifoliata (L,) Raf.]. The data show that chloride uptake under saliniz ation is driven by passive forces. In both species, net rates of chloride r oot uptake increased linearly, without saturation, with the increase of ext ernal NaCl concentrations (30-240 mol m(-3)). Uptake rates, on a mu g g roo t dry weight(-1) h(-1) basis, in Cleopatra and Carrizo decreased (from 38 t o 21) and increased (from 21 to 35), respectively, with the increase (about three-fold) of the shoot to root ratio. With the appropriate shoot to root ratio in each genotype, it was demonstrated that at identical external dos es of NaCl, Cl- uptake rates and Cl- xylem concentrations in the two specie s were very similar. Root pruning and defoliation showed that the amount of chloride taken by the plant was a function of the size of the root system, whereas leaf chloride concentration, the parameter responsible for salt da mage, was dependent upon leaf biomass, Measurements of water transpiration suggested that chloride roof uptake and leaf accumulation might be linked t o water absorption and transpiration rates, respectively, The data indicate that plant morphology is a crucial factor determining salt-tolerance in ci trus.