| Purple flesh sweet potato has great marketing potential in medicine and food industries, however, very little information is available on the formation of quality among different genotypes under various cultivation conditions. The present study was carried out in 2005 and 2006 cropping seasons with three purple flesh sweet potato cultivars Jishu18, Ayamuraska and Yamagawamuraska. Those three genotypes varied in yield potential, starch and anthocyanin content in tuberous root. Field trials were subject to shading treatment in root thicking stage with different shadingt treatment (0, 40% and 70%). The second field trial was subject to cover the ridge with plastic film. Recorded traits included yield potential, dry matter distribution, developmental dynamics of source and sink organs, physiological activities of leaves, hormone physiology of tuberous root formation and enlargement, dynamic changes of starch synthase in the process of tuberous root enlargement and starch accumulation, and the effect of weak light stress and plastic mulch growing on the physiological mechanism of tuberous root yield and quality formation of different sweet potato genotypes. The main results were present below.1. There were significant differences in root yield and root quality among different sweet potato cultivars. The root yield of Jishu 18 was the highest, which was 1.91 times of Aya. and 3.92 times of Yama. The dry matter content and starch content of Aya. were 6.6% and 2.9% higher than that of Yama., and 11.0% and 6.8% higher than that of Jishu18. There were no significant differences in terms of amylopectin content among three genotypes. However, the amylose content of Aya. was 5.1 % and 8.2% higher than that of Yama. and Jish18, respectively. The anthocyanin contents were significant different among three genotypes. The anthocyanin content of Aya. was 60.3mg/100g in fresh sample, which was 1.5 times that of Yama., and 4.3 times that of Jishu18. There were no significant differences for the soluable sugar and protein content among different cultivars.The weak light condition (shading) caused the root yield decreased significantly, and root processing quality decreased as well. When field plots were shaded by 40% and 70% during root thicking stage, the tuberous root yield was decreased by 29.8%-69.3%, and the processing quality decreased as well. The starch content and anthocyanin content were decreased significantly because of shading. The amylose content of sticky Jishu 18 increased and the amylopectin content of it decreased. However, Aya. exhibited a reverse trend for amylose and amylopectin content. Root soluble sugar and protein content increased to different extent.The culture practice with plastic mulch could increase the root yield significantly. The tuberous root yield of Jishu18 and Aya. increased by 18.85% and 26.96% on the average in two years, respectively. The contents of dry matter, starch, amylose, soluble sugar and anthocyanin of Jixu 18 were increased significantly, however, its soluble protein content, amylase/ amylopectin ratio and RVA value decreased. The performance of Aya. was quite different from Jishu 18. The dry matter content, soluble protein content, anthocyanin content, amylopectin content, amylase/ amylopectin ratio and RVA value of Aya. were increased significantly, but its soluble sugar concent, amylase content and starch content decreased to different extent.2. The differences of sweet potato leaf physiological activities were the main factors influencing root yield and qualities. During the whole growing period, the leaf soluable sugar content, protein content, chlorophyll content, net photosynthetic rate, phosphoenolpyrubate carboxylase(RuBPCase) and ribulose-1,5-bisphosphate carboxylase(PEPCase) activities of Jishu 18 were significantly higher than that of Yama, SOD and CAT activities of Jishu18 were higher than that of Aya. and Yama either, but the POD activity and MDA content of Jishu18 were lower than that of Aya. and Yama.. The leaf IAA content of Jishu 18 was significantly higher than that of Aya. and Yama. in the early and fast thicking stages of root, but in the late growth stage, which was much lower than that of Aya.and Yama. ZR changing trend was similar as IAA. ABA content of Jishu18 was always higher than that of Aya.and Yama during the whole growing period. For the above characters, Yama always performed the lowest among three of them.Under field shading, the leaf chlorophyll, chla and chlb content increased in different level, but the chla/chlb decreased significantly, the soluble sugar content, RuBPCase activity, ATP content and ATPase activity were all decreased significantly, which resulted in the net photosynthesis rate, the transpiration rate and leaf Gs decreased significantly. However, the leaf CO2 concentration among cells increased markedly. The leaf SOD and CAT activities decreased, MDA content increased under shorter shading condition, whereas which performed the contrast results under longer shading condition. Under shorter shading condition, the leaf IAA content decreased, ABA content increased rapidly, which would inhabit the leaf growth. Under long term shading condition, the leaf IAA content increased rapidly, which was markedly higher than that of CK. ABA content decreased significantly, and the contents of GAs and ZR were significantly lower than that of CK according to the process and strength of the weak light stress. The results from cultivars told that the variation scopes of every trait measured in Jishu 18 were much higher than that of Aya., which indicated that Jishu 18 was more sensitive to weak light stress.Under plastic mulch condition, the leaf soluble sugar and protein content increased significantly for two cultivars. The chlorophyll content of Jishu 18 was higher than that of CK during all growing stages. However, that of Aya. was higher than CK in the early growing stage and lower than CK in the middle and late growing stage. The leaf RuBPcase activity, content of ATP and ATPase activity and leaf photosynthesis rate were significantly increased. The metabolic ability of active oxygen markedly improved in early and middle growing stage. The leaf activity of SOD, CAT and POD were significantly higher than that of CK, and the accumulation of MDA was obviously decreased in early and middle growing stage. However, the performance of those traits was no obvious difference between the treatment of plastic mulch and CK in the late growing stage. The culture practice of plastic mulch could significantly increase the content of IAA, GAs and ZR, and decrease the content of ABA, in the leaf during early and middle growing stage. However, there was no obvious difference between the treatment and CK in the late growing stage 3. The dynamic changes of endogenous hormone concentrations in sweet potato root with different genotypes in different root enlarging stage and different shading treatment were investigated. The results indicated that the formation and enlargement of sweet potato tuberous root were caused in coordination with many hormones like ZR, ABA and IAA and so on. And the concentrations of ZR and ABA played very important roles in the rate of tuberous root enlarging, which were positively correlated with root yield. The photosynthetic assimilate produced from green biomass above ground allocated to different'sink'under the ground in different root development stage. The bigger roots were the key point for root development in early root enlargement stage, because the concentrations of endogenous hormones were highest in bigger roots at the stage. Hormone concentrations in middle sized roots were highest in peak stage of root enlargement, whereas the key development part was little roots in late root enlargement stage. Endogenous hormones distribution in root demonstrated apical advantage. The hormone concentrations in top, middle parts of a root were always higher than that of end part of a root in different development stages. The active development of secondary cadium probably closely related with the concentrations of IAA and ZR. The concentrations of IAA, ZR and DHZR were all highest in the middle and inner part in roots at peak enlargement stage, apart form ABA.Long term of weak light stress greatly affected the endogenous hormones balance in sweet potato root. The concentrations of ABA, ZR and IAA were significantly lower than that of CK, and the decreasing range was positively correlated with stress strength and stress time. However, the concentrations of GAs demonstrated the reverse trend. The weak light stress caused the increase of GAs in root with different extent. The results showed that Jishu 18 was more sensitive to weak light than Aya. The decrease values of ABA, ZR and IAA concentrations of Jishu 18 roots at conditions of 40% shading and 70% shading were much higher than that of Aya.. However, the increase value of GAs concentration of Jishu 18 was lower than that of Aya. The results above further indicated that ABA, ZR and IAA played very important roles in the enlargement of sweet potato tuberous root.Under plastic mulch condition, the hormone content in the root changed as the following: The IAA content of Jishu 18 decreased significantly in the early and middle growing stage, and increased in the late growing stage. The GAs content of Jishu 18 increased in the early and late growing stage, and decreased in the middle growing stage. However, the IAA and GAs contents of Aya were lower than that of CK in the whole growing stage. The ABA and ZR contents increased significantly, especially, they were extremely higher than that of CK in the root fast thickening stage. Those result showed once again that ABA and ZR play key roles in the tuberous roots thickening of sweetpotato.4. The root starch content was closely correlated with relative enzymes associated with starch synthesis. The root starch contents in purple sweet potato were positively correlated with the activities of ADPG PPase and UDPG PPase significantly. The activities of ADPG PPase probably had a bigger influence on root starch content than that of UDPG PPase. The amylose content in tuberous root increased with the increase of the GBSS activity. However, there was no significantly positive correlation between those two traits. The GBSS activity was not positively correlated with the root amylopetin content as well.The weak light stress with different level caused the activities of ADPGase and UDPGase decreased significantly. The differences of those enzymes among treatments were extremely significant in the period of 10-30 days after shading. The differences of enzymes were relatively small in late shading stage. And enzymes in Jishu 18 decreased much than that in Aya.. The root activities of SSS in Jishu 18 significantly decreased under weak light stress, and the decrease range were positively correlated with the strength of weak light stress. However, the activities of GBSS were higher than CK in different level. For Aya., the activities of SSS in root was higher than CK, whereas, the activities of GBSS were decreased with lowering of weak light stress strength. The activities changes of enzymes related to starch synthesis resulted in the decrease of starch accumulation rate and significant reduction of starch accumulation, and starch components and processing quality changed as well.Under plastic mulch condition, the ADPGPPase activity of Jish18 increased by 19.5%, but it decreased by 7.4% in Aya. The UDPGPPase activity in two cultivars increased to certain. The increase of Jishu 18 was higher than that of Aya, and the increase in the root fast thickening stage higher than that in the early and late root thickening stage. The SSS activity of Aya increased, whereas the GBSS activity decreased. The SSS activity of Jishu 18 was slightly lower than that of CK, and the GBSS activity was significantly higher than that of CK. The culture practice with plastic mulch made the enzymes activities changed greatly, which caused the increase of starch accumulation rate and starch final accumulation amount, the starch composition and processing quality changed as well.5. The activity of PAL was closely associated with anthocyanin accumulation in tuberous root of purple sweet potato. The anthocyanin content was significantly positively correlated with the activity of PAL in the beginning of root enlargement (35-50 days after transplant). As long with the enlargement of root gradually, although the accumulation of anthocyanin increased, the PAL activity decreased, and the correlation between those two traits significantly reduced. There was almost no correlation in the peak and late stages of root enlargement (65-120 days after transplant).The activities of PAL were not lowered under weak light stress conditions during root enlargement period. The PAL activities of different treatments at the beginning of shading were slightly lowered than CK, and the difference was not significant. The PAL activities in late shading stage were slightly higher than that of CK, which indicated that PAL participated in anthocyanin synthesis, and probably played key role in the synthesis initiation stage. However, PAL was not key enzyme for the synthesis of anthocyanin in other stages.
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