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 <i>Ponkan</i> (<i>Citrus reticulata</i> Blanco cv. Ponkan) genetic background (<i>Harumi</i>, <i>Daya</i>, 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 <i>Citrus</i> cultivar (<i>Harumi</i>) 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 (<i>HEMA1</i>, <i>HEME2</i>, <i>HEMG1</i> and <i>CHLH</i>) was down-regulated to some extent and chlorophyll degradation (<i>CAO</i>, <i>CLH</i>, <i>PPH</i>, <i>PAO</i> and <i>SGR</i>) showed the opposite trend with increased chlorosis. Changes in degradation were more significant. In general, the chlorosis of <i>Harumi</i> 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.