Selecting for salt tolerance in white clover (Trifolium repens): Chloride ion exclusion and its heritability

The potential to improve the salt tolerance of white clover (Trifolium repens L.) was evaluated in populations developed by selecting plants within the cv. Haifa which had high and low concentrations of Cl- in the shoots. Under saline conditions (40 mM NaCl), the low-Cl populations consistently maintained lower concentrations of both Na and Cl in all plant parts and produced more dry matter than the original parent cultivar (Haifa), or the high-Cl populations. Under non-saline conditions, there were no differences between populations for these characters. In young seedlings, concentrations of Cl- in the shoots of the low-Cl and high-Cl populations started to diverge after 4 d exposure to saline conditions and there were significant differences between populations after 6 d growth at 40 mM NaCl. Results using Cl-36 suggested that the salt-tolerant low Cl population was better able to retain Cl-36 in the roots and to limit its translocation to the leaves and petioles than the high-Cl population. Realized heritability values for shoot Cl- concentration were found to be moderate in the first cycle of selection (0.24 and 0.37 for high and low Cl- concentrations respectively), but were substantially lower (0.10 and 0.09 respectively) in the second selection cycle. We propose that it is possible to increase levels of salt tolerance in white clover by selecting for low shoot Cl- concentrations under saline conditions and that this criterion could be used at very early stages of exposure to NaCl (i.e. day 4 or 6).