Timing of salicylic acid application affects the response of maize (Zea mays L.) hybrids to salinity stress

The effect of genotype and timing of salicylic acid (SA) application on response of maize to salinity stress has been investigated. Single and triple hybrids (SH and TH, respectively) of maize were grown hydroponically and sprayed with 1 mM SA one week either before or after application of 150 mM NaCl. The effect of salinity on maize performance was stronger than that of SA regime or maize hybrid. The effect of treatments was most evident on root biomass but least evident on number of leaves. The genotypic difference in shoot biomass was vague in non-amended plants but emerged, in favor of the SH, in SA-amended plants. The more vigorous SH exhibited less pigment content (particularly the post-amended plants) and less salt resistance, with preferential native allocation of plant biomass to root compared with the TH. Salicylic acid, particularly the post amendment under salinity stress, was stressful to maize foliage but beneficial to roots. Salinity reduced root growth to a greater extent than shoot growth, with the production of wider and shorter blades; but SA led to the opposite effect. The effect of salinity on leaf chlorophyll a concentration was non-significant in the SH, versus an increase (in the post-amended plants) or a decrease (in the non-amended and pre-amended plants) of the TH. Sub-stomatal CO(2) concentration (C(i)) was higher in the SH than the TH, particularly in the post-amended plants. Both salinity and SA induced stomatal closure, reduced rates of transpiration and photosynthesis but increased C(i), with variable magnitudes in the two hybrids. Salinity increased concentrations of soluble sugars, proline and Na(+) in the leaves, decreased K(+) and phenolics concentrations but marginally affected protein concentration with limited effect of SA, which varied according to time of application. The further stressing effect of SA post-amendment to salt-stressed maize can be related to reduced stomatal conductance and concentrations of phenolics, proline and K(+) but increased Na(+) concentration of the shoot, particularly of the TH.