Response of apple dwarfing rootstocks M9-T337 to exogenous citric acid (CA) under saline and alkaline stresses

Abstract: 【Objective】As a typical rain-fed agricultural area, the Loess Plateau in Northwest China has become one of the most advantageous apple production areas in China due to its suitable conditions of altitude, temperature, and sunlight. Salinity stress has become an important influence factor limiting the development of high-quality and large-scale cultivation of apple in Northwest China. It is important to find substance that can alleviate salinity stress. Therefore, we investigated the effects of different concentrations of exogenous citric acid (CA) on the physiological characteristics of apple rootstock M9- T337 under saline and alkaline stresses in this study.【Methods】The experiment was conducted from April to August 2023 in the rain shelter of Gansu Agricultural University (E 103°34, N 36°10′ ). 80 one-year-old M9-T337 vegetatively propagated nursery trees with uniform growth were selected and transferred into 3.5 kg pots (25 cm inner diameter, 38 cm deep) containing substrate (20% vermiculite, 20% perlite, 60% peat) with a mass of 0.65 kg, one plant per pot. The plants were placed in the rain shelter at the College of Horticulture, Gansu Agricultural University for 30 d. The plants were weeded and watered regularly. Six treatments were set, including irrigation with fresh water (CK1), irrigation with200 mmol·L^-1 (100 mmol·L^-1 NaCl + 100 mmol·L^{- 1} NaHCO3) sodium salt solution (saline- alkali stress), saline-alkali stress +0.2 mmol·L^-1 CA (T1), saline-alkali stress +0.6 mmol·L^-1 CA (T2), salinealkali stress +1.0 mmol· L^-1 CA (T3), and saline-alkali stress +1.4 mmol·L^-1 CA (T4). The treatments were conducted with 3 biological replicates each consisting of 10 nursery trees , time of duration was 45 d. The functional leaves of the young potted M9-T337 trees were selected, the photosynthetic fluorescence parameters, chlorophyll content, root morphology, relative conductivity (REC), Na + , K+ content, antioxidant enzyme activities and osmoregulatory substances content of leaves of each treatment were determined, and the results were synthesized with principal component analysis.【Results】The saline concentrations of 100 mmol·L^{- 1}  and 150 mmol·L^{- 1}  had little effect on the M9-T337 trees, while 200 mmol·L-1 treatment led to obvious yellowing and starting og wilting, and 250 mmol·L-1 resulted in severe wilting and even death of the plants. Therefore, 200 mmol · L- 1 concentration of saline- alkali stress was selected for subsequent saline stress treatment of the young potted M9-T337 plants. The total root length, mean root diameter, total root surface area, total root volume, root tip number, root fresh weight ratio and root vigor of the young potted M9-T337 trees were significantly reduced by saline-alkali stress (CK2) compared with CK1. After exogenous CA was applied, all the 7 indexes of the young potted M9-T337 trees showed different degrees of enhancement, among which, T3 had the best effect, with an increase of 32.11%, 17.64%, 26.98%, 82.10%, 71.00%, 25.93%, and 71.68%, respectively, indicating that the addition of exogenous CA at appropriate concentration could promote the morphogenesis of the root system under saline and alkaline stress. The sprays of the different concentrations of citric acid effectively reduced the elevation of conductivity (REC), the REC of T1-T4 decreased by 11.56%, 25.71%, 47.42%, and 8.57% compared with CK2 respectively, reduced intercellular carbon dioxide (Ci) concentration, and Na+ content of the leaves of the young potted M9-T337 trees under saline and alkaline stress, and significantly reduced the relative water content (RWC), root vigor, root fresh weight ratio, chlorophyll a (Chl a), chlorophyll b (Chl b), and chlorophyll a+b (Chla+b) of the leaves, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), initial fluorescence (F0), maximum fluorescence (Fm), photochemical quenching coefficient (qP), unregulated energy dissipation Y (NO), superoxide dismutase (SOD) activity, decrease in catalase (CAT) and peroxidase (POD) activities, and significantly increased proline (Pro) content and peaked under T3 treatment; based on the ranking of the principal component scores. The ability of exogenous CA to alleviate the salinity stress to the young potted M9- T337 trees was from high to low: T3>T2>T4>T1>CK2.【Conclusion】The exogenous CA at 1.0 mmol·L^-1 could better improve the photosynthetic capacity of M9-T337 trees under saline stress, increase the activity of antioxidant enzymes, enhance the stability of biofilm and promote Na+ efflux, K+ compartmentalization, and inhibit the K+ efflux, being helpful to alleviate the saline-alkali stress to the young potted M9-T337 trees.