A study on the physiological responses of Malus hupenensis to exogenous silicon treatment under drought stress

Abstract:【Objective】Mauls domestica Mill. is the most important deciduous fruit tree in the world, which is well adapted to the climate of the loess plateau in Northwest China with sufficient sunlight and great diurnal temperature difference. The northwestern region is the best ecological area for apple culti- vation, but there are many adverse environmental factors, such as drought, have caused huge losses in agricultural production, and seriously threaten the sustainable development of the apple industry. It is of great significance to select suitable treatments to control the size of trees canopy, enhance resistance to diseases and abiostresses. Many studies have shown that spraying with SA, ABA or SE promotes the growth of apple seedlings under drought stress, improves the structure of trees and the photosynthetic characteristics of leaves, which can effectively alleviate the damage of drought stress on plants. With  seedlings of Malus hupenensis seedlings at 8 leaf ages as the experimental material, the effects of exoge- nous silicon (Si) on the physiological characteristics of apple rootstock under drought stress were stud- ied by spraying sodium silicate at different concentrations【. Methods】When the seedlings of M. hupe- nensis grew to 4-leaf age, they were moved into pots with the same size and the same weight of nutrient soil. Intact and uniform M. hupenensis with 8 true leaves were selected for the drought stress experi- ment. The soil moisture was controlled by weighing method, and the severity of drought stress in- creased with elongated drought. At the same time, Na2SiO3 · 9H2O solutions of 0, 1.25, 2.5, 3.75 and 5 mmol·L-1 were sprayed. When the field capacity was 50% and normal water (CK) soil maintained 75% of the field capacity, spraying was stopped. Treatments including normal irrigation (CK), drought stress (T1),droughtstress+1.25mmol·L-1 Na2SiO3·9H2O(T2),droughtstress+2.5mmol·L-1 Na2SiO3·9H2O (T3),droughtstress+3.75mmol·L-1 Na2SiO3·9H2O(T4),droughtstress+5.0mmol·L-1 Na2SiO3·9H2O (T5) were set for the experiment, which lasted for 20 days. The effects of these treatments on photosyn- thetic characteristic indices [such as net photosynthetic rate (Pn), stomatal conductance (Gs), transpira- tion rate (Tr) and intercellular CO2 concentration(Ci)], chlorophyll fluorescence parameters [such as Ini- tial fluorescence (F0), maximum fluorescence (Fm), the maximum light conversion rate of PSII (Fv/Fm) and photosynthetic electron transfer rate (ETR)], chlorophyll content (SPAD), proline content (Pro), su- peroxide dismutase (SOD), malondialdehyde (MDA) were determined every five days from the day of treatment. Finally, multiple indicators were analyzed using the correlation analysis method and princi- pal component analysis【. Results】With the prolongation of drought stress, Ci, MDA and Pro increased, while Pn, Gs, Tr, Fm, Fv/Fm, ETR and SPAD showed a decreased trend. Besides, as the drought stress ex- tended, Fo first decreased and then increased, while the activity of SOD increased first and then decreased. With the silicon treatments, the declines or increases in the above leaf indexes were significantly lower than that CK, and there was an obvious concentration effect. Under T4 (3.75 mmol · L- 1), the SPAD, Pn, Gs, Fv/Fm, ETR and SOD demonstrated minimum drop and were significantly higher than un- der T1, while MDA in T4 had the smallest increase and was significantly lower than in T1 and CK, while Pro had the largest increase and was significantly higher than that in T1. The correlation analysis of the 12 indexes displayed that Pn was positively correlated with SPAD, Gs, Tr, Fv/Fm, F0, Fm, ETR, Pro and SOD, but negatively correlated with Ci and MDA. Principal component analysis was performed on these 12 indicators, and three principal components with feature value >1 were extracted, whose eigen- values were 8.366, 2.370 and 1.060, respectively. The variance contribution rates of the first, second and third principal components were 69.719%, 19.754% and 8.834%, respectively, and the cumulative variance contribution rate reached 98.307%, which met the analysis requirements. The comprehensive analysis showed that the effects of exogenous silicon treatments on the physiological characteristics ofM. hupenensis were in the order of T4 > T3 > T5 > T2 > T1【. Conclusion】Exogenous silicon could alleviate drought stress by regulating osmotic substances, eliminating reactive oxygen species, activating antioxidant enzymes, and improving heat dissipation with improved light protection. The treatment of 3.75 mmol·L-1 sodium silicate could alleviate the drought stress by improving the photosynthetic capacity, antioxidant enzyme activity and the stability of the membrane.