Growth stage modulates salinity tolerance of New Zealand spinach (Tetragonia tetragonioides, Pall.) and red orach (Atriplex hortensis L.)

The response of two speciality vegetable crops, New Zealand spinach (Tetrag onia tetragonioides Pall.) and red orach (Atriplex hortensis L.), to salt a pplication at three growth stages was investigated. Plants were grown with a base nutrient solution in outdoor sand cultures and salinized at 13 (earl y), 26 (mid), and 42 (late) d after planting (DAP). For the treatment salt concentrations, we used a salinity composition that would occur in a typica l soil in the San Joaquin Valley of California using drainage waters for ir rigation. Salinity treatments measuring electrical conductivities (ECi) of 3, 7, 11, 15, 19 and 23 dS m(-1) were achieved by adding MgSO4, Na2SO4, NaC l and CaCl2 to the base nutrient solution. These salts were added to the ba se nutrient solution incrementally over a 5-d period to avoid osmotic shock to the seedlings. The base nutrient solution without added salts served as the non-saline control (3 dS m(-1)). Solution pH was uncontrolled and rang ed from 7.7 to 8.0. Both species were salt sensitive at the early seedling stage and became more salt tolerant as time to salinization increased. For New Zealand spinach, the salinity levels that gave maximal yields (C-max) w ere 0, 0 and 3.1 dS m(-1) and those resulting in a 50% reduction of biomass production (C-50) were 9.1, 11.1 and 17.4 dS m(-1) for early, mid and late salinization dates, respectively. Maximal yield of red orach increased fro m 4.2 to 10.9 to 13.7 dS m(-1) as the time of salinization increased from 1 3, to 26, to 42 DAP, respectively. The C-50 value for red orach was unaffec ted by time of salt imposition (25 dS m(-1)). Both species exhibited high N a+ accumulation even at low salinity levels. Examination of K-Na selectivit y data indicated that K+ selectivity increased in both species with increas ing salinity. However, increased K-Na selectivity did not explain the incre ased salt tolerance observed by later salinization. Higher Na-Ca selectivit y was determined at 3 dS m(-1) in New Zealand spinach plants treated with e arly- and mid-salinization plants relative to those exposed to late saliniz ation. This corresponded with lower C-max and C-50 values for those plants. Lower Ca uptake selectivity or lower Ca levels may have inhibited growth i n young seedlings. This conclusion is supported by similar results with red orach. High Na-Ca selectivity found only in the early-salinization plants of red orach corresponded to the lower C-max values measured for those plan ts.