Abstract
Drought reduces leaf stomatal conductance (gs) and mesophyll conductance (gm). Both hydraulic signals and chemical signals (mainly abscisic acid, ABA) are involved in regulating gs. However, it remains unclear what role the endogenous ABA plays in gm under decreasing soil moisture. In this study, the responses of gs and gm to ABA were investigated under progressive soil drying conditions and their impacts on net photosynthesis (An) and intrinsic water use efficiency (WUEi) were also analyzed. Experimental tomato plants were cultivated in pots in an environment-controlled greenhouse. Reductions of gs and gm induced a 68–78% decline of An under drought conditions. While soil water potential (Ψsoil) was over −1.01 MPa, gs reduced as leaf water potential (Ψleaf) decreased, but ABA and gm kept unchanged, which indicating gs was more sensitive to drought than gm. During Ψsoil reduction from −1.01 to −1.44 MPa, Ψleaf still kept decreasing, and both gs and gm decreased concurrently following to the sustained increases of ABA content in shoot sap. The gm was positively correlated to gs during a drying process. Compared to gs or gm, WUEi was strongly correlated with gm/gs. WUEi improved within Ψsoil range between −0.83 and −1.15 MPa. In summary, gs showed a higher sensitivity to drought than gm. Under moderate and severe drought at Ψsoil ≤ −1.01 MPa, furthermore from hydraulic signals, ABA was also involved in this co-ordination reductions of gs and gm and thereby regulated An and WUEi.
Highlights
Soil water scarcity is one of the major environmental constraints to the plant physiological processes and yield (Easlon and Richards, 2009; Olsovska et al, 2016)
Relative soil water content (RSWC) and Ψsoil of the well-watered pots were maintained at an average of 75.13% and −0.43 MPa, indicating no water stress occurred during the experiment
We considered that the reduction in gm was not attributed solely to hydraulic regulation, abscisic acid (ABA) seemed to maintain the decrease in gm under moderate or severe soil drought, e.g., Ψsoil < −1.01 MPa in the present study
Summary
Soil water scarcity is one of the major environmental constraints to the plant physiological processes and yield (Easlon and Richards, 2009; Olsovska et al, 2016). For C3 plants, leaf photosynthesis is strongly limited by three factors, Photosynthesis i.e., stomatal conductance (gs), mesophyll diffusion conductance to CO2 (gm), and biochemical photosynthetic capacity (Grassi and Magnani, 2005; Cano et al, 2013). Revealing the mechanisms underlying the decreases of gs and gm in response to drought is necessary for enhancing our understanding of plant adaptation to water limitation. Different regulatory mechanisms such as chemical messengers like abscisic acid (ABA), electrical signals, and hydraulic signals have been identified in the control of stomatal movement (Dodd, 2005; Ache et al, 2010; Tombesi et al, 2015; Huber et al, 2019)
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