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 ( Tetragonia tetragonioides Pall.) and red orach ( Atriplex hortensis L.), to salt application at three growth stages was investigated. Plants were grown with a base nutrient solution in outdoor sand cultures and salinized at 13 (early), 26 (mid), and 42 (late) d after planting (DAP). For the treatment salt concentrations, we used a salinity composition that would occur in a typical soil in the San Joaquin Valley of California using drainage waters for irrigation. Salinity treatments measuring electrical conductivities (EC i ) of 3, 7, 11, 15, 19 and 23 dS m −1 were achieved by adding MgSO 4 , Na 2 SO 4 , NaCl and CaCl 2 to the base nutrient solution. These salts were added to the base 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 ranged 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 ) were 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 from 4.2 to 10.9 to 13.7 dS m −1 as the time of salinization increased from 13, to 26, to 42 DAP, respectively. The C 50 value for red orach was unaffected by time of salt imposition (25 dS m −1 ). Both species exhibited high Na + accumulation even at low salinity levels. Examination of K-Na selectivity data indicated that K + selectivity increased in both species with increasing salinity. However, increased K-Na selectivity did not explain the increased salt tolerance observed by later salinization. Higher Na-Ca selectivity was determined at 3 dS m −1 in New Zealand spinach plants treated with early- and mid-salinization plants relative to those exposed to late salinization. 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 in 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 plants.