Aluminum-induced stomatal closure is related to low root hydraulic conductance and high ABA accumulation

Abstract

Many studies ask how aluminum (Al) reduces the root growth, but as Al is mostly retained in the root system, the physiological explanations for the also expected Al-induced decrease in stomatal conductance (gs) are unclear, mainly in well-watered conditions. We exposed tomato plants (Solanum lycopersicum) to 0, 25, 50 and 100 M Al in nutrient solution to investigate whether Al impairs root hydraulic conductance (Lpr), affecting leaf water potential (leaf) and possibly inducing abscisic acid (ABA) accumulation in roots and/or leaves. We also measured ABA delivery rate, xylem sap pH and the root/leaf area ratio in order to explain the low gs in plants exposed to Al. Declines in Lpr and gs were proportional to the increase in Al concentration, and all Al treatments similarly decreased leaf, indicating the plant's attempt to maintain leaf water status while accumulating more ABA. Despite Al-induced increases in root ABA, the root-to-shoot delivery of ABA did not enhance, but Al caused root xylem sap alkalization. Despite the stability of root/leaf area ratio across a range of Al concentrations (0, 25 and 50 M Al), the leaf hydration and stomatal opening was not conserved. Here we provide the first evidence that decreases in Lpr and increases in ABA might explain Al-induced stomatal closure.

NCED expression is related to increased ABA biosynthesis and stomatal closure under aluminum stress

“…2C), which would have greatly reduced the demand for water transport from the smaller root system. Similarly, tomato plants exposed to 0, 25 and 50 M Al showed similar root/leaf area ratio, reinforcing that the decrease in the root size is compensated by a low shoot growth (Gavassi et al, 2020).…”

“…However, this reduction in gs, preventing water loss, was not able to stop a decline in leaf water status, presumably caused by the impact of Al on root water transport function as suggested by Batista et al (2013), Banhos et al (2016), Silva et al (2019), Cavalheiro et al (2020) and Gavassi et al (2020), and also supported by the 80% lower KL in plants exposed to Al (Fig. 5B).…”

“…5B), the ability of the Al-treated plants to transport water to the leaves was dramatically impaired. Indeed, root hydraulic conductance (Lpr) of Solanum lycopersicum (tomato) also declined proportionally to the increase of Al in nutrient solution (Gavassi et al, 2020). Reductions in KL and low expression of aquaporins (PIP family) in C. limonia exposed to 1480 M Al found by Cavalheiro et al (2020) have been associated with fibrous xylem vessels (Banhos et al, 2016) and more lignin deposition in the vascular cylinder (Silva et al, 2019) of C. limonia grown under the same Al toxicity conditions.…”

“…For example, abscisic acid (ABA) signalling controls gs when roots are exposed to Al, but only few studies have examined the role of ABA in plants exposed to Al, and these studies have focused on the role of ABA in root Al resistance, without measuring gs (Shen et al, 2004;Hou et al, 2010;Reyna-Llorens et al, 2015;Kopittke, 2016). Recent evidence showed that decrease in root hydraulic conductance and increase in ABA could explain Al-induced stomatal closure in tomato plants (Gavassi et al, 2020). ABA strongly controls stomatal movement (Zhang and Davies, 1989;Merilo et al, 2015), and stomatal closure is one of the most studied roles of ABA in response to drought, high temperature and salt stress (Xiong and Zhu, 2003;Mehrotra et al, 2014).…”

Toxic Aluminum and Water Deficit Interaction in Plants: Physiological Aspects and Chemical Soil Management to Improve Root Environment in the Context of Global Climate Change

“…In addition, transpiration, root hydraulic conductivity, and leaf water potential are negatively affected by Al and these disorders in plant water relations coincide with increased levels of ABA in plants treated with toxic Al, indicating that this metal has a broad spectrum of action. Physiology in plants [31].…”