Activities of enzymes directly related with sucrose and citric acid metabolism in citrus fruit in response to soil plastic film mulch

• Soil plastic film mulch (SPFM) increased sugar and acid accumulation in fruit juice sacs.
• SS activities responded differently to SPFM in fruit segment membrane and juice sacs.
• AI activity in fruit segment membrane responded positively to SPFM.
• cyt-Aco and cyt-IDH activities in juice sacs were reduced by SPFM.
• An enzymatic model for sugar and acid accumulation in citrus is presented.

Soil plastic film mulch is commonly employed in citrus production regions of East Asia to improve fruit quality. In the present study, Ponkan tangerine (Citrus reticulata Blanco) was mulched under the tree canopy with silver-black reflective film during fruit development. At about 12 days after mulching, total soluble sugar and citric acid contents in the segment membrane and/or juice sacs of fruit from mulched trees increased significantly relative to the control. In the segment membrane, the activities of acid invertase (AI) and sucrose synthase (SS; cleavage direction) increased significantly following mulch treatment. However, the activities of other enzymes, including neutral invertase, SS (synthetic direction) and sucrose phosphate synthase did not respond significantly under mulch treatment. In the juice sacs, SS activity (cleavage direction) from mulched treatments was significantly lower than that from control trees while SS activity (synthetic direction) from mulched trees was significantly higher than that from control trees. Moreover, the activities of cytoplasm aconitase (cyt-Aco) and isocitrate dehydrogenase (cyt-IDH) were significantly lower than those in the control fruits after 36 days of mulching. In conclusion, the activities of SS (synthetic direction) and AI were significantly enhanced while those of cyt-Aco and cyt-IDH were significantly reduced by soil plastic film mulch. A schematic model is present indicating the possible important roles that these key enzymes play in sugar and acid accumulation in citrus fruits under soil plastic film mulch.