Physiological Studies On Carotenoid Formation And Regulation In Citrus Fruit

With enormous increase in yield of citrus in recent years, the possession per capita of citrus fruit in our courtry has approached 10 kg. Focus of our efforts in citrus production has changed from on yield to quality. After China's entrance into WTO, our domestic citrus production will face far more challenge from abroad. Therefore, fruit quality has become the essential factor that ensure the sound development of our citrus industry. Carotenoids are one of the important constituents which together with other factors constitute to citrus fruit quality, as carotenoid composition and its content in citrus fruit relates to fruit color and juice color, but also to health protection value of fruit.In order to elucidate the physiological mechanism of carotenoid formation in citrus fruit, wedetermined a-carotene, p-carotene, lycopene, P-cryptoxanthin, zeaxanthin and lutein content in 53varieties that belong to various citrus types using HPLC, analyzed the possible connectionbetween color formation and carotenoid accumulation, and studied the effects of light. GA3 andMPTA on carotenoid biosynthesis in peel of citrus fruit. The results are summarized below.l.In both peel and pulp of citrus fruit, the major carotenoids were lutein, zeaxanthin and P-cryptoxanthin. p-carotene content was lower than lutein, zeaxanthin and P-cryptoxanthincontent and a-carotene was less than P-carotene. Among the 53 varieties tested, lycopene wasdetected only in pulp of Cara Cara navel orange. Carotenoid content in both peel and pulp ofcitrus fruit was highest in Citrus reticulata Blanco and lowest in Citrus grandis Osbeck.Consequently, as far as the health protection value is considered, fruit of Citrus reticulataBlanco ranks probably higher than other citrus fruits. In fruit of most Citrus reticulata varities,P-cryptoxanthin was the main carotenoid component in pulp and its content approximated luteincontent in peel. Lutein, zeaxanthin and P-cryptoxanthin content in peel was about 2.5-15 timesas their respective content in pulp on the basis of fresh weight Hence the peel was the principallocation for the carotenoid stock in citrus fruit.2. With development and maturation of citrus fruit, chlorophyll content of citrus fruit peel decreased, being acompanied by a gradual decrease in a-carotene and P-carotene which areknown to be in the upstream of carotenoid biosynthesis pathway.The decrease continued up to complete vanishment of the carotenes. On the other hand, P-cryptoxanthin, P-citraurin and zeaxanthin that are in the downstream of carotenoid biosynthesis pathway increased steadily and became the principal carotenoid components. Hence, total carotenoid content was slightly reduced at the beginning, and then went up quickly, leading to change of fruit color from green to its color characteristic of mature fruit.3. Correlation between changes in color and carotenoid content in peel of citrus fruit was investigated using Mantouhong (C. erythrosa Hort ex Tanaka), Owari Satsuma mandarin( C. unshiu Marc), Huyou (Citrus hybrid) as representatives of red, orange and yellow cultivars, respectively. The results were as follows. The yellow color of the yellow cultivar fruit was resulted from relatively less acumulation of total carotenoids and p-citraurin, p-cryptoxanthin component in peel as compared with the orange and red cultivars. However, the difference between orange and red color of fruit was due not to difference in total carotenoid content. Rather, the composition and proportion of carotenoids in peel of citrus fruit were responsible. In peel of citrus fruit, red cultivar "Mantouhong" accumulated mainly P-citraurin component, whereas orange cultivar "Owari Satsuma mandarin" accumulated mainly P-cryptoxanthin component.4. Effects of shading fruit with opaque paper bag at the late stage of fruit enlargement on change in chlorophyll, carotenoid, sugar content and peel color in "Hongshigan" citrus (C.suavissima Hort.ex Tanaka X C.sinensis Osbeck) were examined. Foll...