Salinity Reduces Water Use and Nitrate-N-use Efficiency of Citrus

Five-month-old Cleopatra mandarin ( Citrus reticulata Blanco) (CM) and Volkamer lemon ( Citrus volkameriana Ten. and Pasq.) (VL) seedlings were grown in a glasshouse in 2·3-1 pots of Candler fine sand. Plants were irrigated with either non-saline (EC e = 0·23 dS m -1 ) or saline (6·13 dS m -1 ) water using 3:1 NaCl:CaCl 2 solution over a 4-week period. A single application of K 15 NO 3 (19·64 atom % excess 15 N) at 212 mg N1 -1 , was substituted for a normal weekly fertilization after 3 weeks and plants were harvested 7 d later. The transpiration rate, uptake of nitrogen, growth and nitrogen-use efficiency (NUE) on a dry weight basis (mg d. wt mg -1 N) of both species was reduced by salinity. Based on growth, water-use and chloride (Cl) accumulation in leaves, VL was more salt-sensitive than CM, but 15 N uptake was equally reduced by salinity in both species. Salinity reduced 15 N uptake relatively more than shoot growth over the 7-d period, such that the 15 NUE (mg d. wt µg -115 N) of new shoot growth of both species increased. There was no evidence of Cl antagonism of nitrate (NO 3 ) uptake but total plant 15 NO 3 uptake was positively correlated with whole plant transpiration in both species. Thus, it appears that reductions in NO 3 uptake are more strongly related to reduced water use than to Cl antagonism from salt stress.