Abstract:

This research explored the stress alleviation effect of exogenous auxin in Pb-stressed plants of Cynodon dactylon， a naturally lead tolerant species， and the underlying physiological mechanism. In a pot experiment， the foliar surface of C. dactylon under 324.4 mg·kg^-1 soil Pb stress was sprayed with different concentrations （0， 1， 10， and 100 mg·L^-1） of 3-indoleacetic acid （IAA） along with 5 and 10 mg·L^-1 of polar transport inhibitor of IAA （NPA）. Plant growth， physiological response， Pb accumulation in the roots， and its translocation to the shoots were systematically investigated. The findings demonstrated that exogenous IAA significantly increased the contents of IAA and cytokinin （CTK）， and lowered the abscisic acid （ABA） level and indoleacetic acid oxidase （IAAO） activity， which promoted an increase in plant height， root length， and above- and belowground dry weight of C. dactylon under Pb stress. Among the different IAA treatments， foliar spraying of 10 mg·L^-1 IAA exhibited the most pronounced stress alleviation effect. Compared to the control， foliar spraying of 10 mg·L^-1 IAA increased plant height， root length， aboveground and belowground dry weight by 32.4%， 30.1%， 32.2%， and 25.5%， respectively. Meanwhile， the contents of chlorophyll a， chlorophyll b， and carotenoids in plants treated with 10 mg·L^-1 IAA were 1.35， 1.36， and 1.21 times those in the control， and the contents of Ca²⁺ and soluble protein in leaves and roots were also significantly increased. Foliar spraying of 10 mg·L^-1 IAA also improved the antioxidant indexes in C. dactylon， which was demonstrated by the significantly decreased malondialdehyde （MDA） content and increased peroxidase （POD） activity as compared to the control treatment. More importantly， foliar spraying of 10 mg·L^-1 IAA promoted the adsorption of Pb by roots， and inhibited the Pb translocation from root to shoot. In contrast， foliar spraying of NPA aggravated the growth inhibition effect of Pb on C. dactylon. This effect exerted dose dependence on NPA， with 10 mg·L^-1 NPA exhibiting the strongest inhibitory effect， as well as reducing the chlorophyll fluorescence， and decreasing photosynthetic pigment contents， and increasing the shoot uptake of Pb， which led to a rise of MDA content. In summary， foliar spraying of 10 mg·L^-1 IAA exerted the best growth promotion and stress alleviation effects on C. dactylon. Exogenous IAA improved C. dactylon plant growth by stimulating hormone mediated stress response， enhancing photosynthetic capacity， and promoting anti-oxidative capacity. Exogenous IAA also increased cellular Ca²⁺ and soluble protein contents， which impeded the translocation of Pb from root to shoot， ultimately decreasing the level of membrane lipid peroxidation and oxidative damage induced by Pb stress. The results from the present study extend knowledge of mechanisms underlying Pb tolerance of C. dactylon and indicate a strategy for alleviating Pb poisoning in plants by applying exogenous IAA.

Key words: Pb stress, IAA, Cynodon dactylon, auxin polar transport, Pb tolerance