Effects of nitric oxide on drought tolerance in apple rootstocks Malus prunifolia

Abstract: 【Objective】Apple is one of the most important fruit crops in the world, and is also the most widely cultivated fruit tree in China. The Northwest Loess Plateau is one of the dominant apple producing areas for its super climate for apple production. However, drought seriously affects the growth and development of apple in this area, and is the important factor restricting the development of apple industry. Improving the drought resistance of apple with rootstock is imperative in this area. The grafting compatibility of most apple varieties with Malus prunifolia is very high, and this rootstock has high resistances to drought, waterlogging, and salt. Nitric oxide (NO) is an important bioactive molecule in plants, and is involved in regulating many plant growing and development processes, including promoting seed germination and lateral root formation, inhibiting the senescence and senescence of plant tissues, and participating in plant disease resistance and stress response. The study aimed to explore physiological effects of exogenous NO at different concentrations on apple rootstock Malus prunifolia under water stress, and select a proper concentration for treatment, in order to elucidate the mechanism of its effect in alleviating drought damage on the plant and improving yield and quality.【Methods】Ten-leafaged seedlings of Malus Prunifolia (Willd.) grown in pots were used as the experiment material and were subjected to water control treatment. Under a moderate water stress (water content was 45%-55%of the soil field capacity) , different concentrations (180 µmol· L-1 C1, 190 µmol· L-1 C2, 200 µmol· L-1 C3 and 210 µmol· L-1 C4) of exogenous NO (sodium nitroprusside, SNP) was sprayed. Spraying was carried out when replenishing water based on soil weight every two days. Solutions were sprayed on leaves till drip-off, and spraying water at the same amount was used as the control (CK) . The photosynthetic parameters [such as net photosynthetic rate (Pn) , stomatal conductance (Gs) and transpiration rate (Tr) and intercellular CO2 concentration (Ci) ], the activities of antioxidant enzymes such as superoxide dismutase (SOD) , catalase activities (CAT) and peroxidase avtivities (POD) , and the contents of osmotic substances such as proline (Pro) , relative electrical conductivity (REC) and malondialdehyde content (MDA) were examined every three days from the beginning of treatment. Finally, principal component analysis method was used to analyze drought resistance indexes of Malus prunifolia in order to obtain a more objective and reasonable result and conclusion.【Results】Pn, Gsand Trwere decreased under drought stress, and the SNP treatments delayed and alleviated the decrease in these indices. Pnin the control and in C1, C2, C3 and C4 s were decreased by 51.93%, 38.91%, 26.81%, 39.67%, and 37.70%respectively at 15 d of treatment; Gsdecreased by 57.83%, 43.20%, 32.55%, 40.24%, and 43.37%, respectively; and Trdecreased by 56.57%, 44.00%, 35.13%, 40.28%, and 42.46%, respectively. The degree of descent in Pn, Gsand Trat 190 µmol· L-1 was significantly lower than the control and the other treatments. Ciwas increased with the increase of SNP concentrations. The C2 treatment had the minimum increase. SOD and CAT activities were decreased under drought stress, and their decrease was relieved by spraying exogenous NO. At 15 d of treatment, the decline in SOD in control group and in C1, C2, C3 and C4 groups was 50.68%, 36.36%, 28.19%, 43.39%, and 47.10%, respectively, and that of the CAT was 40.80%, 29.77%, 19.85%, 33.34%, and 35.16%, respectively. The decrease of C2 group was significantly lower than that of control group and the treatment groups. The POD increased under drought stress, and at 15 d of treatment, the increase of POD in the control, C1, C2, C3 and C4 was 94.12%, 69.09%, 50.94%, 68.63% and 74.00% higher than at day 0, respectively. Pro, REC and MDA all increased under drought stress. Compared with the control, the increases of the three indices were slower and smaller in exogenous NO treatments. The increase of Pro in the control, C1, C2, C3 and C4 was 168.18%, 88.10%, 69.04%, 97.67% and 76.59%, respectively; that of REC was 80.65%, 75.86%, 47.46%, 62.07% and 60.01%, respectively; and that of MDA was 57.14%, 32.35%, 31.25%, 48.48% and 67.74% respectively compared with the values at 0 d treatment.【Conclusion】Exogenous NO treatment could lessen the reduction in Pn, Gs, Tr, SOD and CAT, meanwhile, and slow the increase in Ci, POD, Pro, REC and MDA. Principal component analysis (PCA) showed that the variance contribution rate of the 2 principal components with eigenvalue exceeding 1 reached 92.932%, and C2 treatment got the highest score. According to the principal component analysis, the effect of NO treatment exhibited a concentration-dependent manner and the 190 µmol· L-1 SNP treatment was the best to alleviate water stress for Malus prunifolia. In summary, the results indicate that appropriate concentrations of NO treatment could activate the antioxidant system in Malus prunifolia seedlings, protect the cell membrane system, enhance photosynthetic performance under water stress, and thus improve drought tolerance.