Identification of Photosynthesis Characteristics and Chlorophyll Metabolism in Leaves of Citrus Cultivar (Harumi) with Varying Degrees of Chlorosis

Abstract

In autumn and spring, citrus leaves with a Ponkan (Citrus reticulata Blanco cv. Ponkan) genetic background (Harumi, Daya, etc.) are prone to abnormal physiological chlorosis. The effects of different degrees of chlorosis (normal, mild, moderate and severe) on photosynthesis and the chlorophyll metabolism of leaves of Citrus cultivar (Harumi) were studied via field experiment. Compared with severe chlorotic leaves, the results showed that chlorosis could break leaf metabolism balance, including reduced chlorophyll content, photosynthetic parameters, antioxidant enzyme activity and enzyme activity related to chlorophyll synthesis, increased catalase and decreased enzyme activity. In addition, the content of chlorophyll synthesis precursors showed an overall downward trend expected for uroporphyrinogen III. Furthermore, the relative expression of genes for chlorophyll synthesis (HEMA1, HEME2, HEMG1 and CHLH) was down-regulated to some extent and chlorophyll degradation (CAO, CLH, PPH, PAO and SGR) showed the opposite trend with increased chlorosis. Changes in degradation were more significant. In general, the chlorosis of Harumi leaves might be related to the blocked transformation of uroporphyrinogen III (Urogen III) to coproporphyrinogen III (Coprogen III), the weakening of antioxidant enzyme system activity, the weakening of chlorophyll synthesis and the enhancement in degradation.

Keywords: Citrus; chlorophyll degradation; chlorophyll synthesis; chlorosis; photosynthetic characteristics.

Figure 1

Leaves were in different degrees of chlorosis (A) and the effect of different degrees of chlorosis on photosynthetic pigments (B) and photosynthetic parameters (C). Values are shown as mean ± SD (n = 3). Different letters above the bar chart indicate significant differences between different treatments (p < 0.05). FW, fresh weight. SD, standard deviation.

Figure 2

Effect of different degrees of chlorosis on the precursor of chlorophyll synthesis. ALA and PBG (A), Urogen III and coprogen III (B), Proto IX and Mg-Proto IX (C). Values are shown as mean ± SD (n = 3). Different letters above the bar chart indicate significant differences between different treatments (p < 0.05).

Figure 3

Effect of different degrees of chlorosis on GluTR, UROD and ChlM activity (A), chlorophyllase activity (B) and Mg-dechelatase activity (C). Values are shown as mean ± SD (n = 3). Different letters above the bar chart indicate significant differences between different treatments (p < 0.05). Chlase—chlorophyllase. MDCase—Mg-dechelatase. SD—standard deviation.

Figure 4

Effect of different degrees of chlorosis on the expression of HEMA1, HEME2, HEMG1, CHLH, CAO, CLH, PPH, PAO and SGR. Values are shown as mean ± SD (n = 3). Different letters above the bar chart indicate significant differences between different treatments (p < 0.05). SD, standard deviation.

Figure 5

Effect of different degrees of chlorosis on SOD activity (A), POD activity (B), CAT activity (C) and APX activity (D). Values are shown as mean ± SD (n = 3). Different letters above the bar chart indicate significant differences between different treatments (p < 0.05). SD, standard deviation.

Figure 6

Correlation between indicators and different degrees of leaf chlorosis (A), and correlations between photosynthesis and chlorophyll metabolism indicators in leaf chlorosis (B). The positive correlation between indicators is indicated by red, and the negative correlation between indicators is indicated by blue.

Figure 7

Leaf chlorosis regulatory network for chlorophyll synthesis and degradation, photosynthetic gas exchange parameters and antioxidant enzyme system in Harumi.