Effects Of CPPU On Development And Physiological And Biochemical Characteristics Of Pear Fruits

To improve the quality of fruits, promoting accumulation of sugar is very important in the research field of fruit science. Many of exogenous hormones such as GA, IAA, ABA were reported to regulation sugar accumulation, because they increased the accumulation of fructose, glucose and sucrose; however, effects of N-(2-chloro-4-pyridy)-N'- phenylurea (CPPU) on sucrose metabolism in fruits were seldom reported. CPPU, as a cytokinin-like regulator with high activity, could promote fruits expansion and assimilatory product accumulation, however, its negetive effect is induction of malformed fruits. In this study, firstly, effects of CPPU with adjuvants on growth and quality in 'Cuiguan' (Pyrus pyrifolia Nakai cv. 'Cuiguan') pear fruits were studied; secondly, regulation of CPPU on sucrose metabolism and physiological mechanism on source-sink relationship of 'Akemizu' pear fruits were studied. Lastly, the interactive effects of CPPU and bagging were studied. The result showed as follows:1. It has been reported that CPPU induces malformation fruits when it was used to promote fruit enlargement. In order to eliminate the by-effect of CPPU, various adjuvants were added to CPPU solution to test their effects on fruit growth and quality of pear cultivar (Pyrus pyrifolia Nakai cv. 'Cuiguan'). The results showed that fruit growth was promoted with heavier weight and later maturation by 20 mg L^-1 CPPU treatment 2 weeks after full bloom. It suggested that CPPU treatment prolonged fruit expansion period, inhibited chlorophyll degradation, delayed starch content decrease, reduced the respiration rate of fruits, increased the activity of antioxydase, which resulted in maturation delayed. CPPU treatment increased malformed fruit percentage, but, if suitable adjuvants such as 0.1% Triton or PVAC added the malformed effect was dramatically decreased and the promotion on fruit growth became further significant, implying that the role of adjuvants in CPPU application should be paid more attention to.2. The developing 'Akemizu' pear fruits were used to analyze the levels of sucrose, glucose, fructose and starch, and the activities of sugar-metabolizing enzymes, including acid invertase (AI), neutral invertase (NI), sucrose synthase (SS), sucrose phosphate synthase (SPS) and amylase in the work. The results showed that the starch accumulated in the early stage and reduced in the last stage. The glucose content increased quickly in the early stage, and the fructose and sucrose content increased slowly in the early stage but accelerated in the last stage. The content of sorbitol was low and had little change during the whole development. The activities of invertase and SS cleavage direction failed in full stage, however, the activities of amlyases and SS synthetic direction rose, and SPS with lower activities in the early stage and higher in the last stage. A positive correlation was found between sucrose content and the activities of SS synthetic direction, thus, the key enzyme affecting sucrose accumulation was probably SS. The total change tendency of carbohydrate accumulation and the enzymes activities on sugar metabolization could not be affected, but it enhanced the content of glucose, fructose and sorbitol, reduced sucrose content; at the same time, the activities of amylases, invertase and SS (cleavage direction and synthetic direction) rose, and SPS activity depressed of fruits with CPPU treatment. It enhanced sink strength and promotoed carbohydrate accumulation in fruits.3. The antioxidant enzyme activities were increased in pear fruits by CPPU treatment after full bloom a week. During the development of fruits, the activities of enzymatic protective systems such as SOD, APX, CAT were rose, the downtrend of AsA content in non-enzymatic protective systems was delayed, lower production rate of superoxide anion and higher soluble protein were kept, at the same time, the rising trend of PPO activity and the downtrend of soluble phenol were restrained, abilities of antioxidant defense system advanced in pear fruits, it will help to delay the riping process of pear fruits.4. CPPU played a role in regulation of source-sink relationship, compared with control. CPPU increased the fruit setting and fruit weight. At the same time, the content of chlorophyll a, chlorophyll b and carotenoids increased remarkably in leaves near fruits with CPPU, but CPPU didn't affect the ratio of chlorophyll a/b. CPPU reduced the minimal fluorescence(F_o), enhanced the maximum fluorescence (Fm), the variable fluorescence (Fv), PSⅡmaximal photochemical efficiency (Fv/Fm), PSⅡpotential photochemical efficiency (FvlF_o) and ability of PSⅡreaction center to obtain energy from antenna (1/F_o-1/Fm), advanced PSⅡactual photochemical efficiency (Φ_PSⅡ), photochemical quench (qP), electron transfer rate (ETR), photochemical rate (PCR), increased photochemistry (P), reduced PSⅡreaction center (E) and antenna heat dissipation (D), suggesting CPPU treatment promoted the photosynthesis of pear leaves. It showed that the sink strength enhancement induced the increasing photosynthesis rate of leaves. 5. CPPU treatment delayed maturation and bagging hastened maturation. CPPU treatment delayed the degradation of chlorophyll, inhibited starch transforming into soluble sugar, postponed fruits respiration rate rise, kept steady at a stronger rigidity and a higher activities of antioxidant enzymes. Bagging promoted degradation of chlorophyll and starch, enhances fruits respiration rate, decreased firmness of fruits; At the same time, CPPU could restrain early ripening effects induced by bagging.