Tolerance mechanisms to salinity stress in Citrus and Poncirus

Salt stress responses of C. grandis (L.) Osb., P. trifoliata (L.) Raf. and their F-1 (17-40) were investigated. Different growth parameters, including shoot elongation, leaf, stem, structural root (> 2mm diameter), fine root (< 2mm diameter) and whole plant dry mass production as well as different tissue and whole plant accumulations of 11 macro and micro nutrients were determined for the three genotypes tested in 0, 40 and 80 mM NaCl environments for 20 weeks. In another experiment, the same parameters were examined for P. trifoliata plants treated with 0, 30, 60, 90, and 120 mM NaCl for 12 weeks. The latter experiment suggested that a 40-60 mM NaCl concentration is optimal to test P. trifoliata or its progeny for salt stress. In both experiments, root production of salinized P. trifoliata plants was found to be continuous and plants apparently used this process as an avoidance mechanism to remove excess ions and delay onset of ion accumulation. This phenomenon, named "Fine Root Turnover", is unique to P. trifoliata. It may be used as a genetic resource to improve Citrus for salinity tolerance through intergeneric hybridization since the F-1 plants displayed responses intermediate to its parents leading to increased salinity tolerance. While leaves of P. trifoliata were the tissues most sensitive to salinity, root tissues were the most sensitive in C. grandis. To avoid salt accumulation, C. grandis plants increased g/m leaf dry mass production as opposed to the increase in g/m root mass production in P. trifoliata. We suggest that not only ion content of leaf tissues, but ion content and mass production of all tissues should be considered when the salinity tolerance of Citrus and related genera is characterized.