Aluminum-induced decrease in CO2 assimilation in ‘Rangpur’ lime is associated with low stomatal conductance rather than low photochemical performances

Abstract

Aluminum (Al) is toxic to most plants. Although inhibition of root elongation can occur even under 10 M Al, above 1000 M damage to photochemical performances has been reported, reducing the CO 2 assimilation rate (A). However, Al is retained in the roots of most plants, including Citrus limonia ('Rangpur' lime) with no direct explanation for the low A. In this study, 'Rangpur' lime plants were cultivated hydroponically in a nutrient solution with 1480 M Al, and we expected to determine the time range within which Al-induced decrease in A starts. To our surprise, low stomatal conductance (gs) rather than low photochemical performances was evident 45 days after exposing the roots to Al. Aluminum was mostly retained in the roots and histochemically observed in the stele, suggesting reduction in water uptake/transport in the xylem that became fibrous. We also found a 35% reduction in the midday leaf water potential in plants exposed to Al, indicating association between the fibrous xylem vessels and low gs, which could explain the Al-induced decrease in A in 'Rangpur' lime plants.

Aluminum prevents stomatal conductance from responding to vapor pressure deficit in Citrus limonia

“…The nutrient solution showed a chemical composition based on the solution proposed by Clark (1975), and it has been already used to test Al resistance in 'Rangpur' lime plants (Banhos et al, 2016 (Banhos et al, 2016), we observed that 1480 μM Al causes Al-induced decrease in gas exchange rates in 'Rangpur' lime plants after 45 days. Then, the solution contained 0 and 1480 μM Al provided through 1479.5 μM AlCl 3 6 H 2 O.…”

“…Then, the solution contained 0 and 1480 μM Al provided through 1479.5 μM AlCl 3 6 H 2 O. The pH of the solution was monitored daily and maintained at 4.0 ± 0.1 to keep Al as soluble as possible, and the solution was totally replaced every 15 days (Banhos et al, 2016). The nominal chemical composition of this solution was also tested on Geochem-EZ software (Shaff et al, 2010), resulting in more than 85% free Al 3+ available.…”

“…However, in these same studies gs was also significantly reduced in the presence of Al. In addition, for 'Rangpur' lime plants (Citrus limonia), Al-induced decrease in A is better explained by the low gs rather than low photochemical responses (Banhos et al, 2016). These authors have evidenced fibrous xylem vessels in the stele associated with low water uptake/transport causing low gs and leaf hydration.…”

pH exudate test as a method to estimate viability and vigor of Citrus limonia Osbeck seeds

“…The 'Rangpur Lime' (Citrus limonia Osbeck) is the most used rootstock in the Brazilian citrus orchards due to its compatibility to most scion cultivars, satisfactory yield, induction of early-bearing and adequate performance under water deficit condition, presenting substantial importance for the rain-fed Citrus growing area (Mourão Filho et al, 2007;Oliveira et al, 2015;Banhos et al, 2016).…”

“…Through the data obtained by pH-measuring devices or acid-base indicator solutions, it is possible to evaluate the physiological condition of each seed and, consequently, the physiological potential of lots (Araldi and Coelho, 2015;Alves et al, 2016).…”