Effects of ferrous sulfate applied by foliar spray on photosynthesis and photoproducts metabolism of the chlorotic pear leaves (Pyrus bretschneideri ‘Dangshan Suli’)

Abstract:【Objective】Dangshan Suli pear (Pyrus bretschneideri Rehd.) is extensively cultivated in the region of the Old Yellow River Valley, Anhui province, China. Currently, the iron-deficiency chlorosis has distributed widely and seriously restricted the development of pear industry in the region. It was shown in our previous work that FeSO4 applied by foliar spray could induce the regreening of chlorotic leaves in pears. However, little is known about the effects on the photosynthesis and the anabolism of its assimilated products.【Methods】The plants with normal and chlorotic leaves of Dangshan Suli pears (CN and CC) were taken as test materials in the region of the Old Yellow River Valley, Dangshan county, Anhui province, China in May of 2020. The water solution of 0.2% FeSO4 was sprayed to running off using a powered backpack sprayer during the growth period with three replicates, one non-sprayed tree was left as a buffer between the treated trees and distilled water was used as control. All the trees used in this study were relatively consistent in tree shape and vigor, spaced at 4 m in row and 6 m between rows, without crops between the rows, trained with the open center system, and the orchard was man- aged as usual beside FeSO4 spray. The leave samples of CN and CC were taken at 3, 6, 9, and 12 days af-ter FeSO₄ application (T3 d, T6 d, T9 d and T12 d), respectively. The samples were brought back to the labora- tory, and were quickly frozen in liquid nitrogen, and were stored in the refrigerator at -80 ℃ till use. The content of total chlorophyll in the leaves was determined by 80% acetone extraction, and the chloro-phyll fluorescence parameters were measured using portable pulse modulated fluorescence instrument (FMS-2). The photosynthetic characteristics of the leaves were detected using portable photosynthesis system (Li-6400, USA). The determination of sorbitol, sucrose, fructose and glucose contents in the leaves were conducted using High Performance Liquid Chromatoghraphy. The total RNA of the leave samples was extracted by the E •Z •N •A.®Plant RNA Kit (Omega Biochemical Technology Co. Ltd., Guangzhou, China), the cDNA was synthesized using PC18-TRUEscript 1st Strand cDNA Synthesis Kit (Adelab Biotechnology Co. Ltd., Dalian, China), and the relative expression of genes related to pho- tosynthesis and sugar anabolism were analyzed by qRT-PCR conducted on an ABI StepOne RT-PCR cycler (Applied Biosystems, Foster City, CA, USA).【Results】The FeSO4 application significantly induced the chlorophyll accumulation in the chlorotic leaves of Dangshan Suli pear, and notably enhanced its photosynthetic properties, such as leaf net photosynthetic rate (P_n), transpiration rate (T_r) and stomatal conductance (G_s). Meanwhile, FeSO4 treatment remarkably enhanced its chlorophyll fluorescence char-acteristics such as PSII maximum photochemical efficiency (F_v/F_m), PSⅡ potential activity ((F_v/F_o) and PSⅡelectron transport rate (ETR), resulting in a significant increase in the contents of sorbitol and glu-cose in the chlorotic leaves. The results of qRT-PCR showed that the significantly enhanced photosyn- thetic properties of the chlorotic leaves by FeSO4 application might be closely associated with the signif- icant up-regulation of the expression of the genes involved in photosynthesis, such as PSI subunit II/IV,PSII psbD, Rubisco activase VI, F-Type ATPase VI, and Chl a–b P4, Chl a–b CP24. The markedly in-creased sorbitol content in the chlorotic leaves by the FeSO4 application might be extremely related to the significant increase in the expression of the S6PDH1, a gene of 6–phosphate dehydrogenase in the sorbitol synthetic pathway. The significant up-regulation of the sorbitol dehydrogenase gene SDH2/3 could highly contribute to the accumulation of sucrose and fructose in the leaves after the application of FeSO₄. Besides, the prominently increased relative expression of the SOT8/14/15/26/30, a sorbitol trans- porter gene, might have facilitated the transport of sorbitol to other growth points during this period, thus promoting sorbitol synthesis. The increased expression of the phosphate sucrose synthase gene SPS1/3 in sucrose synthetic pathway reflected the enhanced sucrose-fructose metabolism in the chlorot-ic leaves after the FeSO4 application, and the accompanied decreased expression of the sucrose synthase gene SUS1/3/7/12 might catalyzed its degradation, while the significant increase in its transporter genes SUT1/2/3 and TMT2/4 might also have enhanced the sucrose transport to other growing points, thus maintaining sucrose dynamic equilibrium.【Conclusion】FeSO4 application promoted chlorophyll synthesis in the chlorotic leaves of Dangshansuli pear, and significantly enhanced leaf photosynthesis by regulating the relative expression of genes in photosynthesis and sugar anabolism, and sorbitol and su- crose biosynthesis-related genes promoting sorbitol and sucrose synthesis in the chlorotic leaves and stimulating their decomposition, transporting, thus resulting in the increase of the content of the fruc-tose and glucose in the leaves.