Gas-exchange response and stomatal and non-stomatal limitations to carbon assimilation of sunflower under salinity

Sunflower (Helianthus annuus) was grown in both open-field and outdoor pott ed conditions in Southern Italy, and irrigated with water having electrical conductivity ranging between 0.9 and 15.6 dS m(-1) obtained by different N aCl concentrations. The aim of the work was to study the leaf area and phot osynthetic responses of sunflower to mild salt stress. The response curve ( A/c(i)) Of assimilation (A) to leaf internal CO2 concentration (c(i)) was u sed to determine leaf gas-exchange parameters, in order to evaluate stomata l and non-stomatal limitations to photosynthesis in relation to salt stress . In the field, a reduction of 19% in leaf area expansion occurred, while n o correlation was observed between psi (1) and stomatal conductance to wate r vapour (g(sw)) ranging between 0.76 and 1.35 mol m(-2) s(-1). This result was also evident at a higher salinity level reached in the pot experiment where leaf osmotic potential (psi (s)) varied from - 1.35 to - 2.67 MPa as compared with the field experiment, where psi (s) ranged from - 1.15 to - 1 .42 MPa. Considering the two experiments as a unique data set, the assimila tion rate, the stomatal conductance to CO2 (g(sc)) and the sensitivity of A to c(i) variation (g*) were not significantly influenced by salinity in th e whole range of psi (s). As a consequence, the stomatal and non-stomatal l imitations to photosynthesis were not affected by salt treatment, averaging around 20 and 80%, respectively. The variation in A (from 44 to 29 mu mol m(-2) s(-1)) was paralleled by the variation in g(sc) (from 0.47 to 0.84 mo l m(-2) s(-1)), with a remarkable constancy of both c(i) (200 +/- 12.5 mu m ol mol(-1)) and normalized water-use efficiency (5 +/- 0.7 mu mol mmol(-1) kPa), showing the optimal behaviour of the plant processes. These findings indicate that, under mild salt stress, the same as observed under water def icit, sunflower controls assimilation mainly by modulating leaf area rather than by stomatal closure, and that non-stomatal limitation of photosynthes is was not affected at all by the level of salinity reached in this study. (C) 2000 Elsevier Science B.V. All rights reserved.