Gene Expression and Physiological Responses of Petunia At Specific Substrate Water Contents

Drought stress commonly limits plant growth. To understand plant responses to different severities of drought stress, we investigated the leaf physiology, ABA concentration, and expression of genes associated with ABA metabolism and signaling in Petunia ×hybrida.  Plants (Petunia ×hybrida ‘Apple Blossom’) were grown in a soilless substrate, which was allowed to dry out to a specific substrate water content (θ = 0.10, 0.20, 0.30, or 0.40 m³·m^-3), after which just enough water was applied to maintain θ at that level. Stomatal conductance (g_s) and photosynthesis (P_n) decreased after drought imposition, but g_s and P_n of plants under mild drought (θ at 0.20 and 0.30 m³·m^-3) partially recovered within a week after they reached their target θ. However, plants at θ of 0.10 m³·m^-3 did not acclimate and maintained low g_s (< 50 mmol·m^-2·s^-1) and P_n (< 5 μmol·m^-2·s^-1). Drought stress increased leaf ABA concentration, which was highly correlated with g_s (r² = 0.85). Plants at a θ of 0.10 m³·m^-3 increased leaf ABA concentration 6-fold compared to plants at 0.40 m³·m^-3. Despite the increase in leaf ABA concentration, we saw no significant effects on the relative expression of several ABA biosynthesis genes (NCED and AAO3) in leaves in response to drought stress. However, the CYP707A gene, which is regarded as the key gene in ABA catabolism, was down-regulated in leaves at a θ of 0.10 m³·m^-3, compared to plants in higher θ treatments, suggesting a decrease in ABA catabolism under severe drought. The relative expression of PLDα, a gene  involved in mediating stomatal responses to ABA, increased after drought imposition, but decreased again after the target θ was reached, irrespective of drought severity. Overall, plants showed physiological acclimation under mild drought (θ of 0.20 and 0.30 m³·m^-3), while plants at a θ of 0.10 m³·m^-3 showed severe drought stress responses and no acclimation. Our results suggest that combining gene expression with physiological measurements can provide a more integrated view of plant responses to environmental stress then either set of measurements by itself.