Salicylic acid (SA) and related phenolic compounds can excert control over stomatal opening and previous work in our laborabtory has shown that chronic injection of SA increases the photosynthetic rate of corn. The work reported in this paper investigated

The abscisic acid (ABA) stomatal closing signal might be transduced through different pathways, depending on the plant growth temperature (GT) and the applied ABA concentration. This was investigated in epidermal peels of Arabidopsis thaliana (L.) Columbia. Different Ca2+ buffers and guanosine-triphosphate-binding protein (G protein) modulators were tested on stomatal closing under light in response to 3 μmol/L ABA (ABA3μ) and 30 μmol/L ABA (ABA30 μ) at the 15-17 °C and 23-25 °C GT ranges. The Ca2+ buffer, 1,2-bis(0-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid, used as free acid (BAPTA) or acetoxymethyl ester (BAPTA-AM), similarly inhibited (up to approximately 70 percnt; inhibition) stomatal closing to ABA3μ and ABA30 μ, whereas ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid specifically inhibited (up to approximately 70 percnt; inhibition) the ABA3μ response at the 23-25 °C GT range. At the same GT range, the ABA3μ response was specifically affected by the phospholipase C (PLC) inhibitor 1-[6-[[17β-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122). Moreover, the ABA30 μ response was specifically inhibited by the G protein antagonist pGlu-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH2 (GP Ant-2) and by the inactive mastoparan analog, mas17. The inhibitory effects of GP Ant-2 and mas17 were additive. None of the tested pharmacological compounds were effective at the 15-17 °C GT range. Together, these results confirmed that, depending on GT and the exogenous ABA concentration, stomatal closing to ABA involves either one among two Ca2+ mobilisations or none of them.