| Open-field cultivation has been the main mode of citrus cultivation in China, withmajor blooming peak in spring and citrus fruits only shipped to markets in October throughDecember. Under protected cultivation, it is possible to force the flowering stage of citrusto an advanced date via temperature control for earlier shipments of fruits to markets. Innorth China, early autumn low temperatures in solar greenhouses have been used to induceearly citrus flower bud differentiation. Induction of flowers by the warming effect withplastic film covers or heating forced sprouting, in order that fruits ripening stage can beadvanced. Flowering time and fruit setting rate of citrus would directly affect the yield andbenefit of citrus cultivation. The examination used “Shatangju” tangerine (Citrus reticulata“Shatangju”) and “Nanfeng” tangerine （Citrus reticulata “Nanfeng”）seedlings withstock of trifoliate orange (Poncirus trifoliate Raf.) to study the biological characteristic andphysiological mechanism of flower induction by low temperature and fruitlet abscission ofcitrus under solar greenhouse, in order to offer the theoretical basis for citrus cultivation insolar greenhouse. The results were as follows:1. Under the condition of solar greenhouse, the chlorophyll, nitrate nitrogen andamino nitrogen contents of spring leaves were the lowest. In addition, fruit setting rate ofleafy fruits was higher than leafless fruits.2. Floral ratio of axillary buds was the highest treated with low temperature10/5℃(15℃at day for12h,10℃at night for12h), while no flower buds formed under lowtemperature4℃(at constant temperature for24h). Axillary buds was not only inducted toflower, but also was promoted germination treated with low temperature15/10℃(15℃atday for12h,10℃at night for12h). Floral ratio apparently decreased while germinationratio sharply increased. Under solar greenhouse conditions, floral ratio of “Shatangju”tangerine and “Nanfeng” tangerine apparently increased during the whole of winter, whichshowed order of average floral ratio: February> January> December. A significantlynegative correlation was noted between leafy to leafless flower ratio and accumulatedhours of <5℃. When accumulated time of low temperature5~20℃reached1309hours,floral ratio of “Shatangju” tangerine stabilized at40.51%. Floral ratio of “Nanfeng”tangerine stabilized at39.22%with low temperature5~20℃reaching1456hours. Theresults indicated there were differences in the response of different citrus variety flower to low temperature.3. Under low temperature4℃, C/N ratio was gradually decreased, mainly owing tothe increased total C content. The response of the change in C/N ratio and proline contentof citrus was positive to flower induction. ZRs content was related with flower formationand beneficial to flower formation for citrus with high content. The results revealed thatfloral ratio of citrus was related to ZRs/GA, ABA/GA,(ABA+ZRs)/(IAA+GA) of shoots.4. Within60days after withering of flowers was physiological fruit-drop period ofcitrus under solar greenhouse, Fruitlets abscission was the most serious during20~40daysafter withering of flowers. The number of dropping fruits significantly reduced on the50day and fruit drop tended to the end. During fruitlets largely dropped, sucrose metabolismin citrus fruitlets was mainly in catabolism, that acid invertase played a major role. Solublesugar and starch content decreaced in fruitlets. While sucrose synthase activity reduced inadjacent leaves, resulting in sucrose decreasing. At the same time, amino nitrogen contentcould be reduced by the lower activity nitrate reductase of leafy and leafless fruits, whichlimited reduction and assimilation of nitrate nitrogen. Glutamine synthetase and nitratereductase activity reduced in adjacent leaves, which affected nitrogen metabolism of leaves.Meanwhile, polyamines catabolism enhanced and ABA content accumulated in fruitlets.The results suggest that sugar metabolism (especially in sucrose and starch) and nitrogenmetabolism in fruits and leaves, polyamine metabolism and the change of abscisic acidcontent in fruits had important influence on the fruitlets abscission.0~20days after withering of flowers, fruitlets abscission ratio of leafless fruits washigher than leafy fruits. The reason was that sucrose anabolism of the adjacent leaves ofleafless fruits was effected, which led sucrose content transferred from leaves to fruitsdecreasing. The starch content of leafless fruitlets was significantly lower than leafyfruitlets. While the capacity of sucrose anabolism of leafy bearing shoots was graduallyincreased. The supply capacity of leafy bearing shoots that photosynthate with sucrosetransported from leaf to fruit enhanced. Sucrose was converted to starch and stored in fruit,which was favorable to leafy fruitlets set. At the same time, compared with leafy fruits,nitrate reductase activity was lower, polyamine catabolism was stronger and ABA contentwas more in leafless fruits.5. Results showed that soil pH value decreased obviously after organic fertilizer andsulfur application. Under mixed application, soil organic matter, total nitrogen andavailable nitrogen content decreased at first but later increased with the increase of amountof sulfur application，while soil available sulfur content increased gradually. Under organic fertilizer and sulfur application treatment, soluble protein content of “Nanfeng” tangerineincreased and nitrate reductase activity was promoted. When sulfur application was lessthan100mg/kg, the branch diameter and the chlorophyll content both increased undermixed application. This could effectively alleviate the problem of leaf chorosis. Undermixed application(sulfur application was100mg/kg), starch accumulation in leafy fruitswas promoted, which was beneficial to fruit set. |
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