Protective effects of foliar fertilizer on ‘Kyoho’ grape under photooxidation

Abstract: 【Objective】Photooxidative stress is one of the important factors that affect crop yield. Light oxi⁃dation or inhibition can cause up to 10% or more crop losses even without other stress factors. In the Tai’an area, the plastic film of protected culture is usually removed in the middle of July after the grape harvest. At this time light intensity in the natural field is 1 400 to 1 782 μmol·m^-2·s^-1 from 11:00 to 15:00,which far exceeds the light saturation point of grape leaves (576 to 972 μmol·m^-2·s^-1). Drastic increases oflight intensity after the plastic film removal can cause light inhibition symptoms in grape trees such as leaffunction decline, poor flower bud differentiation and photooxidative damage. Therefore, to explore effec⁃tive measures of reducing the high light injurys and yield losses, the effects of foliar fertilizer on protectinggrape leaves from photo oxidation damage were studied.【Methods】‘Kyoho’grapes under protected cultivation were chosen as the experimental material. The grapes were cultivated using a vertical trellis systemand with spacing of 1.5 m×3 m. The healthy mature leaves in the middle of the new shoots of three yearold‘Kyoho’grape trees were treated with water (control, T1), 600-fold calcium amino acid+200 mg·L^-1 NaHSO3 (T2), 600-fold calcium amino acid (T3), 0.3% CaCl2 (T4), 200 mg·L^-1 NaHSO3 (T5) respectivelyon July 3, 2015. The whole plant and both sides of the leaves were sprayed. Then methyl viologen (MV)was applied on leaves for all treatments except control to induce photooxidative stress at 8 am on July 5.The plastic film of the greenhouse was then removed. Two hours later, photosynthetic and chlorophyll fluorescence parameters were measured using a CIRAS-2 photosynthetic instrument and FMS-2 pulse modu⁃lated fluorometer respectively (PPSystems and Hansatech, England), and photosynthetic parameters whichincluded the net photosynthetic rate(Pn) and the stomatal conductance (Gs); fluorescence parameters whichincluded initial fluorescence (F0), maximum fluorescence (Fm), maximal photochemical efficiency (Fv/Fm)and actual photochemical efficiency(ΦPSⅡ). After determing the photosynthetic parameters, the leaves usedfor the determination of the photosynthetic parameters were quickly saved in liquid nitrogen and takenback to the laboratory; several light inhibition indexes, including the production rate of superoxide aninradical (O2^-·), H2O2 and malondialdehyde (MDA) contents were measured using the hydroxylamine oxida⁃tion method, spectrophotometric method and the thiobarbituric acid method respectively.【Results】Thephotosynthetic indexes and light inhibition indexes of all MV treatments were worse than the no MV control. However, when MV was applied, compared with only spraying water (T1), spraying foliar fertilizer(T2, T3, T4, T5) could alleviate the photooxidative stress damage of the grape leaves to a certain extent.Compared with the no MV control, the Pn of T1 declined 47.9% , and that of T2 and T3 declined13.1% and 14.2%, respectively. The difference between the two groups was significant, and Gs had thesame change trend, which illustrated that the application of foliar fertilizer could keep the grape leaves ata high level of photosynthesis. Chlorophyll fluorescence parameter F0 represented the fluorescence yieldwhen the PS II reaction center was completely opened, and the rise of F0 indicated that the PS II reactioncenter was damaged or reversible inactivation occured. F0 of all MV treatments increased, that of T1 in⁃creased the most, up to 22%, while that of spraying foliar fertilizer treatments increased much less thanT1, that of T2 increased 5.1%, with the smallest increase and 16.9 percentage points lower than that ofT1. Fm was the fluorescence level when the PS II reaction center completely closed and the decrease of Fmunder strong light was a characteristic of the photoinhibition. Fv /Fm represented the primary light energyconversion efficiency of the PS II reaction center, and the decline of Fv /Fm indicated that photoinhibitionoccurred in the plant leaves. ΦPSⅡ reflected the actual light energy capture efficiency. Fm, Fv /Fm and ΦPSⅡfor all MV treatments decreased, and that of T1 decreased by 15.7%, 22.7%, and 31.2%, respectively.And all spraying foliar fertilizer treatments decreased less than T1. Photooxidative damage could causethe accumulation of the reactive oxygen species (ROS) and MDA in the leaves, and when it reached a certain degree, it will cause the reduction of the photosynthetic efficiency. Compared to the no MV control,the light inhibition indexes (production rate of O2^-·, the content of H2O2 and MDA) of all treatments increased differently. The MV treatment significantly increased the production rate of O2^-·and the accumulation of H2O2, MDA, and the three indexes of T1 increased 56.0%, 83.0% and 34.8% respectively, with thebiggest increase; those of T2 increased 19.4%, 15.1% and 0.99% respectively, with the smallest increase.This indicated that spraying foliar fertilizer could obviously alleviate the light oxidative stress and the ac⁃cumulation of the reactive oxygen species (ROS) and MDA in the leaves.【Conclusion】In general, foliar fer⁃tilizer could effectively alleviate the damage of grape leaves caused by light oxidative stress, and keep thehigh level of photosynthesis and damage to the PS II system. Calcium amino acid+NaHSO3 has the best ef⁃fect and could be used by grape farmers.