To determine whether drought-induced root jasmonates (JA, jasmonic acid and JA-Ile, jasmonic acid iso-leucine) accumulation affected shoot responses to drying soil, near isogenic wild type (WT) tomato (Solanum lycopersicum cv. Castlemart) and the def-1 mutant (which fails to accumulate jasmonates during water deficit) were self- and reciprocally-grafted. Rootstock hydraulic conductance was entirely rootstock-dependent and significantly lower in def-1, yet def-1 scions maintained a higher leaf water potential as the soil dried due to their lower stomatal conductance (gs). Stomatal sensitivity to drying soil (the slope of gs versus soil water content) was low in def-1 self grafts but was normalised by grafting onto WT rootstocks. Although soil drying increased OPDA (a JA-precursor and putative antitranspirant) concentrations in def-1 scions, foliar JA accumulation was negligible and foliar ABA accumulation reduced compared to WT scions. A WT rootstock increased drought-induced ABA and JA accumulation in def-1 scions, but decreased OPDA accumulation.
Xylem-borne jasmonates were biologically active, since supplying exogenous JA via the transpiration stream to detached leaves decreased transpiration of WT seedlings but had the opposite effect in def-1. Thus foliar accumulation of both ABA and JA at WT levels is required for both maximum (well watered) gs and stomatal sensitivity to drying soil.