Studies On Mechanism Of Fruit Growth And Development Of Different Ripening-Season Of Pears

The pear is a plant which belongs to Rosaceae, Pomoideae, Pyrus. Pears are by far one of the most important deciduous tree fruits, and are widely grown in China. Fruit growth and development is one of the foremost life activities for fruit trees and is regulated by complex interactions. The early researches on it focused mainly on fruit quality, or on one endogenous hormone or enzyme only. The systematic studies are not enough. Especially, pear fruits grow and develop more differently on the condition of high temperature and humidity, rich light, caloric and water in south of China than in north. On the other hand, different procedures of growth and development of pear fruits exist among the different cultivars. The characteristics of heredity are looked as one of the important factors to elucidate this physiological phenomenon. But in fact there are so many other factors influencing fruit growth and development. Qualitative and quantitative researches are extremely necessarily combined to clarify the relevance of these factors and ripening-season of pear fruits. The past research indicated that fructose, glucose, sucrose and endogenous IAA, ABA, GAs and CTK contents in pear fruits didn't change in same regularity during fruit growth and development for different experiments. And there have no reports about the changing regularity of endogenous polyamines contents, activiety of cell wall enzymes and LOX during fruit growth and development among the different pear cultivars. Only one cultivar or factor was selected to study the mechanism of fruit growth and development in kiwifruit, strawberry, persimmon, grape, apple and some other frutis. Therefore, researches on chemical substances, such as carbohydrates, proteins, endogenous hormones and polyamines are very crucial to find the difference of growth regularity and disclose mechanism of fruit growth and development of pear fruits among the pear cultivars in different ripening-seasons. To some extent it could provide advice for fruit physiologist and field production.Both Zaomi and WhangKeumbae pear in 5-year-old trees with uniform growing vigor were selected for this experiment. Trees were grafted on Pyrus pashia Buch-Ham rootstocks and planted in Chengdu plain. Cultural management such as fertilization and pest control were the same as those used in a commercial orchard, without fruit thinning and bagging. Characteristics of trees, photosynthetic characteristics of leaves and fruit quality were analyzed for these two pear cultivars. At the same time carbohydrates, proteins and nucleic acid contents, activities of pectinesterase (PE), polygalacturonase (PG), cellulase (Cx) and lipoxygenase (LOX) were determined along with endogenous indol-3-acetic acid (IAA), gibberellins (GA_s), zeatin (ZT), abscisin acid (ABA) and putrescine(Put), spermidine(Spd), spermine(Spm) contents during the period of fruit growth and development. The followings were the results:1,Zaomi and Whangkeumbae pear had similar characteristics of trees in Chengdu plain. The trees grew strongly and had earlier phenophase than that in origin region. The fruits of these two cultivars were in similar shape and good in fruit quality. However, Zaomi pear fruits were sweeter than Whangkeumbae. Photosynthesis rate, chlorophyll a and the total chlorophyll contents of leaves of Zaomi pear were little higher than that of Whangkeumbae pear in the same season. Chlorophyll especially chlorophyll a was the key factor influencing photosynthesis rate for this two pear cultivars. And Zaomi pear trees had stronger tolerance of shading.2,There were obvious differences of developmental stages of fruits between these two pear cultivars. The growth curve of fruit showed as single sigmoid and was divided into three phases. The fruit growth and development lasted 100days and 130days for Zaomi and Whangkeumbae pear, respectively. For Zaomi pear fruits, period from fruit setting to 30 days after full bloom (DAFB) was looked as the early stage, period from 30 to 70 DAFB as middle stage, and period from 70 to 100 DAFB as late stage. For Whangkeumbae pear fruits, period from fruit setting to 50DAFB was divided as the early stage, period from 50 to 100DAFB as the middle stage, period from 100 to 130DAFB as the late stage. The early stage of Whangkeumbae pear held longer for 20days than that of Zaomi pear, and the middle stage kept longer for 10days. The growth of longitudinal length and transverse diameter during fruit growth and development accorded with logistic curve for these two cultivars.3,The contents of main chemical substances changed in different regularities during fruit growth and development for the experimental pear cultivars.(1) Content of soluble sugar, fructose, glucose and sucrose rose gradually along with fruit growth and development for both Zaomi and Whangkeumbae pear. The fructose content was the highest at any given stage of fruit development. Glucose content was at second and higher than sucrose. The contents of three sugar components of Zaomi pear fruits were obviously higher than that of Whangkeumbae pear. Significant difference of soluble sugar contents was produced between these two cultivars at the ripening stage of fruit (p<0.01). Sugar contents increased very fast in the early stage of ovary developing and the late stage of fruit development for the two cultivars. And the latter stage was significant for accumulating sugar. Among the three sugar components fructose transformed and accumulated fastest followed with glucose and sucrose. The accumulation of fructose and glucose of Zaomi pear, and that of fructose, glucose, sucrose and soluble sugar of Whangkeumbae pear accorded with logistic curve during fruit growth and development.(2) Protein contents kept at high level at the initial stage and decreased along with fruit development for Zaomi and Whangkeumbae pear. There were no obvious differences during the most stages between them. Zaomi pear fruits showed two peaks of content, but Whangkeumbae pear fruits changed in slow rate.(3) For free amino acid content, it was higher in Zaomi pear fruits than that in Whangkeumbae pear fruits both at initial and late stages, and was significantly higher at fruit maturation (p<0.01). However, it was lower at the middle stage.(4) Nucleic acid contents kept at high level at initial stage and decreased gradually along with fruit development for the two cultivars. It peaked at 10, 50 and 90 DAFB for Zaomi pear fruits and was significantly higher than that of Whangkuembae pear at fruit maturation (p<0.01). It peaked at 20DAFB for Whangkeumbae pear fruits.4,The activity of enzymes changed in similar trend during fruit growth and development for these two pear cultivars.(1) For the two cultivars, polygalacturonase (PG) activity increased gradually along with fruit development. It peaked at 50DAFB for Zaomi pear fruits, and peaked at 30, 90and 130DAFB for Whangkeumbae pear fruits. Zaomi pear fruits had higher activity of PG at the middle stage but had no obvious difference with Whangkeumbae pear fruits.(2) The activity of pectinesterase (PE) kept at high level at the early stage and decreased gradually along with fruit development, but rose in slightly range before maturation for the two cultivars. It peaked at 10, 70 and 100DAFB for Zaomi pear fruits, and at 10, 80 and 130DAFB for Whangkeumbae pear fruits which had higher activity at early stage than Zaomi pear fruits, and had lower activity at the middle and late stage.(3) The activity of cellulase (Cx) was a little low at the early stage and increased along with fruit development up to maturation followed with slightly decrease for the two cultivars. It rose sharply from 30 to 70DAFB and from 80 to 90DAFB for Zaomi pear fruits, from 20 to 30DAFB, from 50 to 70DAFB and from 80 to 120DAFB for Whangkeumbae pear fruits.(4) The activity of lipoxygenase (LOX) was a little low at initial stage and increased along with fruit development for the two cultivars. It rose sharply from 20 to 40DAFB and from 50 to 90DAFB, but increased in slow rate for Whangkeumbae pear fruits during fruit development.5,The difference of endogenous hormones contents both in pulp and seeds was the key factor resulting in different fruit growth and development of Zaomi and Whangkeumbae pear.(1) In the same habitat IAA and ZT contents in pulp of Whangkeumbae pear which matured later were higher than that of Zaomi pear which matured earlier at initial stage and held in longer time. GA3 contents were higher than that of Zaomi pear fruits at the most stages except of 20DAFB. Cell division in pulp of Whangkeumbae pear was stronger and held for longer time. The three endogenous hormones IAA, GA₃ and ZT contents in pulp were higher than that of Zaomi pear at the most stages of fruit growth and development. However, peaks of contents for Whangkeumbae pear appeared late for 10 to 20days.(2) An increase for ABA contents appeared in pulp of the two cultivars at 30DAFB. It might result in dropping of young fruits at this stage. ABA contents in pulp of Zaomi pear peaked earlier for 20days before fruit maturation. Thus Zaomi pear fruits might shift to maturate much earlier, and had shorter period of fruit growth and development.(3) IAA and ZT contents in seeds of Whangkeumbae pear were obviously lower than that of Zaomi pear at the same stage. It might result in shifting into next developing stage for seeds, and was advantageous of being transferred into pulp for cell development. At the most stages GA₃ contents in seeds of Whankeumbae pear were higher than that of Zaomi pear at the same time. And it lasted for longer time with high contents and got to the first peak earlier for 10 days. This could indicate that Whangkeumbae pear fruits had vigorous production of GA₃ in seeds, which could be transferred into pulp to promote cell expansion. On the other hand, ABA contents in seeds of Zaomi pear peaked earlier for 30days. Therefore it could go into maturing earlier than Whangkeumbae pear.(4) The ratio of IAA contents between pulp and seeds was less than 1 at the most stages for Zaomi pear. Therefore IAA might be mainly synthesized in seeds, especially at initial stage of fruit growth and development. For the ratio of ZT contents this two pear cultivars had similar changing trend. But Zaomi pear, which had vigorous synthesis of ZT in seeds, showed lower ratio value than Whangkeumbae pear. Few of ZT could be synthesized in pulp at the late stage for the two cultivars, but in seeds some ZT could be synthesized before maturation. The ratio of GA₃ contents was less than 1 for Zaomi pear during fruit development, so endosperm was the important place to produce endogenous GA₃.6,Endogenous polyamines were very important for development of young fruits.(1) This two pear cultivars contained high level of Put, Spd, Spm and free polyamines contents both in pulp and seeds at initial stage. It could stimulate vigorous cell division. Put, Spd and Spm contents in pulp of Zaomi pear were significantly higher than that of Whangkeumbae pear at 10DAFB (p<0.01). In seeds, Spd and Spm contents were significantly higher than that of Whangkeumbae pear at 20DAFB (p<0.01). It could speculate that young fruits of Zaomi pear developed more vigorous. However, the Spm contents in pulp of Whangkeumbae pear were higher at the most stages, especially it rose sharply from 10 to 20DAFB and was significantly higher than that of Zaomi pear at the same stage. So Spm contents might be very crucial for the first fast growth of young fruits of Whangkeumbae pear.(2) Put contents both in pulp and seeds got the peak value for two times at the late stage for the two cultivars. And these two peaking values were approximate, but it appeared late for 30 to 40 days for Whangkeumbae pear. The fruit maturated late for 30days, too. This could be correlated with releasing of ethylene and starting to maturate at the late stage.