Relationship between leaf yellowing and magnesium deficiency and efficacy of foliar magnesium application in Guanximiyou pomelo

Abstract: 【Objective】Guanximiyou pomelo (Citrus grandis) is an important and popular Citrus spe- cies, and Pinghe county in Fujian province is the main Guanximiyou pomelo production region in China. Leaf yellowing could decrease photosynthetic physiology and products, resulting in the yield reduc- tion and quality deterioration. In the recent years, leaf yellowing has become widespread phenomenon in Guanximiyou pomelo orchards of Pinghe county. However, whether the yellowing of pomelo leaves is caused by magnesium (Mg) deficiency, insufficient information has been reported. Thereby, it is important to reveal the relationship between leaf yellowing and corresponding nutrient elements concentra-tions (especially Mg concentration) in Guanximiyou pomelo, and the effect of foliar Mg supplement on pomelo leaf yellowing.【Methods】Leaves of Guanximiyou pomelo with different yellowing degrees from 38 orchards were collected and divided into five grade levels including green leaf (control), about 25%, 50%, 75% and 100% leaf yellowing area. We also studied the effect of foliar Mg supplement (foliar spraying with 2% magnesium sulfate heptahydrate) on SPAD value, Mg, N and K concentrations in pomelo leaves. The control (CK) was foliar spraying with water. The spraying amount was 7 L per plant and the spraying time was April, May and June. The leaves on spring-growing shoots from the previous year wood were collected at the mature stage. Leaf SPAD value with five yellowing grades were deter- mined by the SPAD-502 chlorophyll meter. Leaf samples were digested with H2SO4–H2O2, then nitro- gen (N) concentrations were measured using flowage analyzer, phosphorus (P) concentrations were measured using inductively coupled plasma optical emission spectrometer, and the concentrations of po- tassium (K) were determined using flame photometer. The calcium (Ca), magnesium (Mg), iron (Fe), Boron (B), manganese (Mn), copper (Cu) and zinc (Zn) concentrations of samples were digested using HNO3-HClO4 and analyzed by the inductively coupled plasma optical emission spectrometer.【Results】Our results showed that the SPAD value of pomelo leaves decreased significantly with the increase of yellowing grade. Specifically, leaf SPAD value was 36.7 under yellowing grade 4 and only 43.3% of that under green leaf (yellowing grade 0). Meanwhile, the N, P, Ca and B concentrations in pomelo leaves decreased significantly with the increment of yellowing grade, and leaf N concentrations under yellowing grade 3-4 were lower than the appropriate range, whereas leaf P, Ca and B concentrations un- der yellowing grade 1-4 were in the appropriate range (except for leaf B concentrations under yellowing grade 4). The Mg concentrations in pomelo leaves decreased significantly with the increase of yellow- ing grade, leaf Mg concentration under yellowing grade 1 decreased by 44% as compared to green leaf (yellowing grade 0), and leaf Mg concentrations under yellowing grade 1-4 were lower than those with appropriate range. On the contrary, the concentrations of K, Fe and Mn in pomelo leaves increased with the increment of yellowing grade, leaf K concentrations in yellowing leaves (yellowing grades 1 to 4) were higher than those in green leaf, leaf Fe concentrations in yellowing leaves (yellowing grades 1 to 4) were higher than those of the appropriate range and leaf Mn concentration under yellowing grade 4 was significantly higher than that under green leaf. However, no significant differences in Cu and Zn concentrations among different yellowing grades were observed, whereas Zn concentration in leaves was lower than that of the appropriate range. The correlation analysis showed that SPAD of pomelo leaves was significantly and positively correlated with Mg (r=0.819), N (r=0.763), P (r=0.600), Ca (r= 0.476) and B concentrations (r=0.378), but negatively correlated with K (r=-0.570), Fe (r=-0.495) and Mn concentrations (r=-0.198). There were significantly positive correlations between leaf Mg concen- tration with N (r=0.776), P (r=0.677), Ca (r=0.404) and B concentrations (r=0.430). However, leaf Mg concentration was significantly and negatively correlated with K (r=-0.472), Fe (r=-0.421) and Mn con- centrations (r=-0.263). Path regression analysis and random forest analysis showed that Mg had the greatest effect on SPAD value, followed by nitrogen and potassium. When the Mg concentration in leaves was lower than 2.65 g · kg- 1, the SPAD value increased with the increase of Mg concentration in leaves, but when the leaf Mg concentration was higher than or equal to 2.65 g · kg- 1, the SPAD value reached the plateau stage. Moreover, the ratios of N/Mg, P/Mg, K/Mg, Ca/Mg, Fe/Mg and Zn/Mg in the leaves with the yellow grade 1 were significantly higher than those of green leaves. In addition, foliar spraying with magnesium sulfate increased significantly leaf Mg concentration and SPAD value, but de-creased significantly leaf K concentration compared with the control.【Conclusion】These results sug- gested that leaf yellowing was mainly related to Mg deficiency, foliar Mg supplementation can improve the nutrition in pomelo leaves and alleviate the symptoms of leaf yellowing. Therefore, it is important to supply timely Mg fertilizer in pomelo production. In addition, more attention should be paid to supplement other nutrients for sake of balanced fertilization. Our results may provide a scientific basis for correcting symptoms of pomelo leaf yellowing.