The Preliminary Studies On Starch Morphology And Enzymic Activities Relating To Starch Accumulation In Cassava Storage Root

The accumulation, transportation, and conversion of sucrose affect the development of cassava "storage root" and starch accumulation from" source", and starch synthesis, which directly relates to the yield of starch. The period from 100 d to 200 d after planting, three high yield and far related cassava cultivars (SC124, SC8, and Arg7) were used for researches,’ which contributed in the following aspects:the changes of root starch and its component accumulation characteristics; the changes of leaf sucrose content, reducing sugar content and enzyme activity involved in sucrose metabolism; the relation between root starch content and leaf growth, sucrose accumulation as well as key enzymes activity involved in sucrose metabolism; the changes of root sucrose content, reducing sugar content and root enzyme activity including in sucrose metabolism and starch synthesis; the relation between root starch content and sugar content along with enzymes activity including in sucrose metabolism and starch synthesis.1. The characteristics of starch accumulation in cassava storage roots have been studied in the three cassava cultivars (SC124、SC8、Arg7) during root development from 100 d to 200 d after planting. The quantity of total starch per plant was continuously increased, but the starch content (%) was increased at intervals. The starch content (%) was significant higher in Arg7, among which it was Arg7>SC124>SC8. The content and rate of starch accumulation among the three cultivars was changed in the pattern of a single-peak curve during root mature processing, in which SC124 and SC8 was much earlier to reach the peak in content, and in the peak of rate was SC124>SC8>Arg7. Thus, the starch accumulation in SC124 and SC8 was faster than Arg7, but it lasted longer period in Arg7.The total quantity, content, accumulation rate of amylopectin was similar as those of total starch accumulation in the three cultivars. Between the cultivars, the average value of amylopectin content (%)in SC8 was much lower than other two cultivars, but the peak of accumulation rate and quantity of amylopectin was much slowly appeared in Arg7 than SC124 and SC8. At the level of P<0.05, amylose content(%) was not significantly different, but it was significantly different in amylopectin and total starch content (%) among SC124、SC8 and Arg7. 2. The enzymes involved in sucrose metabolism and starch synthesis have been investigated from "source" to" storage roots", and the relationship between enzymes and starch content of storage roots have been discussed. The results are as follows:AI activity was declined during root mature processing, and the activity of SPS and NI was changed in the pattern of a single-peak curve; The AI activity in Arg7 was much lower than that in SC8 or Arg7, and SPS and NI activity in SCI 24 was much higher than that in SC8 or Arg7; The activity of SPS was significantly correlated with the activity of NI, showing the coordinate reduation in controlling sucrose content of leaf; The activity of AI was significantly correlated with the activity of the reducing sugar content in leaves, but was negatively significantly correlated with the root starch content (%), it can be deduced that the lower activity of AI could produce less reducing sugar, which was propitious to sucrose transportation and root development.The changes of AI and NI activity in storage roots are almost similar among three cassava cultivars, and both of them showed high activity in early developing stage. The peak of SS was much slowly appeared, but kept high level in the later developing stage. The activity of NI was significantly correlated with the AI activity in the storage roots, but was negatively significantly correlated with the root starch content, it can be deduced that the reducing sugar deprived from sucrose not only was used for substrate in starch synthase, but also play an important role in the root growth regulation, so that the starch content may declined slightly as a result of rapid swelling of root. The higher activity of AGPase was identified at two periods, and the first peak of AGPase activity arrived much earlier in SC8 than that in SC124 and Arg7, but the second peak arrived at the same period. The activity of soluble starch synthase (SSS), granule-bound starch synthase (GBSS) was changed in the pattern of a single-peak curve during root mature processing. The activity of AGPase and SBE in Arg7 cultivar with higher starch content was higher than that in SC124 and SC8. The content of total starch and amylopectin was significantly correlated with the activity of AGPase, but was not correlated with the activity of other enzymes (GBSS, SSS and SBE). The activity of AGPase may be used as standard to estimate the capability of starch accumulation in cassava storage roots.3. The three cassava cultivars (SC124、SC8、Arg7) with far relationship and high-yielding were used for analysis of the isozyme electrophoresis. The correlation between isozyme locus, enzyme activities of SBE and AGPase, and starch content was investigated. The results showed that there were at least six isozyme loci in AGPase (AGPa、AGPb、AGPc、AGPd、AGPe、AGPf), which manifested genetic diversity among three cultivars. The locus of AGPe only appeared in Arg7, and it may closely relate with higher AGPase activity and starch content. SBE I manifested genetic diversity among three cultivars, while SBEⅡdid not exhibit genetic polymorphism, but showed different in activity among different varieties and root developing stages. The loci of SBEIc and SBEⅡmay provide much great contribution to SBE activity and amylopectin content.4. By studying the starch development of the induced cassava storage roots in vitro of SC124, it indicated that in the early stage of starch grain development, a number of starch grains were developing in a closed amyloplast with membrane. With the maturation of amyloplast, the starch granules were released and many single granules were observed in induces cassava storage roots.The storage roots of the three cassava cultivars (SC124、SC8、Arg7) in 120 and 160 d after planting have been scanned under SEM and TEM. Our observations by TEM confirm that the images sanned by SEM at 120 d were the amyloplasts with membrance envelope, and the morphology of starch such as momultidimensional were the starch granules at one amyloplast. On the 160 days after planting, the membrane envelop of mature amyloplast was disaggregated, and mature starch granules have been released from the amyloplasts. Both amyloplasts and starch granules can be observed in parenchyma. The starch granules in cassava mainly consist of many simple starch grains, few of them are compound starch granule. The frequency of the starch multigrain varies in different cultivars may be interrelated to starch quanlity. Our observations by SEM show that the size (the average diameters), pattern and arrangement are diversity among the three cassava cultivars:The average size of amyloplasts in SC124 are smallest, but their coefficient of variation and interspaces are larger; the granule size of amyloplasts in SC8 are relative uniform with smaller coefficient of variation and tight arrangement; however in Arg7, the granule size and coefficient of variation are smaller, while the interspaces are larger. SEM image showed that of the amyloplasts are stratiformly arranged in zylem parenchyma in SC124, sarciniformly arranged in SC8; and granularly arranged in Arg7. On the 160 days after planting, the starch granules were increased remarkablely.Under TEM, two patterns of proliferation were observed:1, envelope dilates and invaginates to form new amyloplasts; 2, the amyloplast envelope buds to form the double membrane vesicle. The size, arrangement, the pattern and efficient of proliferation may maybe the immanent cause of character difference.5. Two cassava cultivars SC124 and SC8 were used for the study of starch synthesis and accumulation in storage roots in response to shading light to reduce light intensity from the early stage (70 d after planting)to storage root filling stages (160 d after planting). The starch accumulation, starch structures and activities of the enzymes involved in starch synthesis have been examined at the developing stages of cassava storage roots. The results showed that the shading treatment (ST) has changed the distribution of the product from photosynthesis between the "source" and "storage" organs. The content of total starch and amylpectin was remarkably reduced, but the amylose content was significantly increased in comparison to the control (CK, no shading light). The ratio of amylose to amylopectin in ST conditions was significantly higher than that of CK. The activity of adenosine diphosphate glucose pyrophosphorylase (AGPase) in shading conditions was significantly lower than that . in control plants. The activity of GBSS was increased in SC8, but no obviously changed in SC124. The activity of SSS was increased in SC124, but was decreased in SC8, which was contrast to SBE; It was increased in SC8, but was decreased in SC124. It suggests that the low activity of AGPase resulted from shading light may relate to the lower total starch and amylopectin content. The GBSS activity was in response to the change of amylose content; and the any reduced activity of SSS, or SBE may result in lower amylose content. When the cassava was under shading conditions, the organization of starch structure became looser, the size of starch granule was significantly smaller, but the number of the single starch granule was obviously increased.