Physiological response of field-grown cabbage to salinity and drought stress

Soil salinization of arid and semiarid regions of the world is a major problem of irrigated agriculture. On a long-term salinized soil, we assessed growth response and water relations of cabbage plants irrigated with saline water. In addition, we compared plant performances under salt and drought stresses. Two NaCl concentrations of the irrigation water were considered, 0.25% (SW1) and 0.50% (SW2), corresponding to electrical conductivity (ECw) of 4.4 and 8.5 dS m−1 and osmotic potentials (OP) of −0.22 and −0.35 MPa, respectively. Furthermore, a non-salinized control (NSC, ECw = 0.5 dS m−1; OP = −0.02 MPa) and a non-irrigated control (NIC) were included. In salinized plants, midday leaf total and osmotic water potentials decreased during the growth season. NIC plants had the lowest leaf water potential (−1.5 MPa at harvest). The shoot/root ratio of NIC plants was 40% smaller than NSC plants. We did not find significant differences among the shoot/root ratios of plants under different salinity treatments. A 39% yield reduction was found upon salinization. Drought stress decreased the total head yield from 50.5 t ha−1 (NSC) to 17.5 t ha−1 (NIC). A modified Maas-Hoffman relationship of the relative growth, expressed as leaf area or aboveground dry mass per plant versus leaf total or osmotic water potentials of the combined drought and salt stressed plants revealed similar slope and threshold values for the two growth parameters. However, we did not find a significant correlation between leaf osmotic potential and leaf area per plant. Although it is known that plants share similar responses to salinity and drought stresses, our results indicate that morphological (shoot/root ratio) and metabolic (osmotic adjustment) adaptation mechanisms may significantly diverge in salt versus drought stressed plants. While the agronomic implications of these results suggest that the cultivation of cabbage in some Mediterranean environments is still feasible at moderate/low salinity (0.25% NaCl) they also highlight physiological responses, which certainly require further investigation.