Grain size,sucrose level and starch accumulation in developing rice grain

Five rices (Oryza sativa L.) differing in final grain size were studied at the midmilky stage to determine if any factor could be identified which might limit rate of starch accumulation. Only UDP glucose pyrophosphorylase activity increased with increasing grain size. Detached rice panicles incubated in liquid medium containing 1% sucrose and 0.1% glutamine, in addition to minerals and vitamins, produced grains similar to those on intact plants. Sucrose level (0–1.5%) in the medium determined the extent of dry matter and starch accumulation and influenced physiological development of the ripening grains. Chemical and enzymic composition of the grain were similar to previously reported levels in grains of intact panicles analysed at regular intervals after anthesis. Addition of 3-P glycerate or K⁺ to the medium did not improve dry matter accumulation in the developing grain.     

Effects of treatments potentially influencing the supply of assimilate on its partitioning in sugarcane.

Two pot experiments and one field experiment were conducted on sugarcane to assess the effects of treatments expected to change total carbon assimilation on the partitioning of assimilate. In the first experiment pots of cultivars CP and N14 were arranged to simulate normal field spacing. At 5 months, plants were partially defoliated or left intact. In the subsequent four months, defoliation resulted in a small (not significant) decrease in total dry mass increment; it increased the proportional partitioning of assimilates to leaves in N14, whilst in CP it increased the proportional partitioning to stems. In both cultivars defoliation increased proportional allocation to non-structural dry matter, and thus sucrose, in the stem. In the second experiment pots of cv. CP were grown at normal spacing for 4 months, then left untreated, shaded, or placed further apart. During the subsequent four months shading decreased total dry matter increment, but increased proportional partitioning to the stems, and within stems to non-structural dry matter, and so sucrose. Widened spacing increased total assimilation, but decreased proportional allocation to stems; partitioning within the stems was not affected. In the field experiment plants of both cultivars were partially defoliated at 6 months, or left intact. Defoliation resulted in only a very small decrease in stem dry mass increment during the subsequent four months (leaves were not measured). Within the stem partial defoliation caused proportionally increased partitioning to non-structural dry matter, hence to sucrose. The results suggest that sucrose storage receives priority in the allocation of assimilate, rather than representing the accumulation of assimilate not required for vegetative growth.     

Sucrose synthase activity in developing wheat endosperms differing in maximum weight

Past research on kernel growth in wheat (Triticum aestivum) has shown that the kernel itself largely regulates the influx of sucrose for consequent starch synthesis in the endosperm of the grain. The first step in the conversion of sucrose to starch is catalyzed by sucrose synthase (EC 2.4.13). Sucrose synthase activity was assayed in developing endosperms from kernels differing in growth rate and in maximum dry weight accumulation. From 10 to 22 days after anthesis, sucrose synthase activity per wheat endosperm remained constant with respect to time in all grains. However, kernels which had higher rates of kernel growth and which achieved greatest maximum weight had consistently and significantly higher sucrose synthase activities at any point in time than did kernels with slower rates of dry matter accumulation and lower maximum weight. In addition, larger kernels had a significantly greater amount of water in which this activity could be expressed. Although the results do not implicate sucrose synthase as the “rate limiting” enzyme in wheat kernel growth, they do emphasize the importance of sucrose synthase activity in larger or more rapidly growing kernels, as compared to smaller slower growing kernels.     

Calcium-mediated conversion of sucrose to starch in relation to the activities of amylases and sucrose-metabolizing enzymes in sorghum grains raised through liquid culture

Detached ears of sorghum (Sorghum vulgare) were cultured in complete liquid medium containing Ca2+(0, 3, 10 and 30 mM) and effect of this ion on the conversion of sucrose to starch with respect to the activities of amylases, sucrose synthase, sucrose phosphate synthase and soluble invertases were studied in developing grains. Presence of 3 mM Ca2+ in culture medium enhanced both accumulation of starch and activity of alpha-amylase in grain but without having any influence on the activity of beta-amylase. However, with 10 and 30 mM Ca2+, the accumulation of starch and activities of both amylases decreased and with advancement in culturing period, starch accumulation was further decreased. Irrespective of its concentration, Ca2+ enhanced the activities of sucrose synthase (synthesis), sucrose-phosphate synthase, soluble acid invertase and soluble-neutral invertase. Increase in the concentration of Ca2+ in culture medium was concomitant with an elevation in relative proportion of sucrose in the grain reflecting a net balance in per cent increase with Ca2+ in the activities of sucrose-synthesizing enzymes over sucrose-hydrolysing ones. Based on the results, it is suggested that assimilation of Ca2+ by grain is essential for maintaining high activity of alpha-amylase to generate starch primers required for the conversion of sucrose to starch during grain filling in sorghum.     

Nitrogen fertiliser rate and post-anthesis waterlogging effects on carbohydrate and nitrogen dynamics in wheat

Waterlogging is predicted to increase in both magnitude and frequency along with global warming, and will become one of the most severe adversities for crop production in many regions. Nitrogen is considered to be an effective up-regulatory nutrient for crops grown under stress and non-stress conditions. In this study, we try to evaluate N fertiliser effects on contents of carbohydrate and N dynamics, dry matter accumulation in shoot, yield under post-anthesis waterlogging. Waterlogging after anthesis significantly reduced grain yield due to decrease in thousand-kernel-weight and in grain number per spike. High N fertiliser application aggravated grain yield loss due to post-anthesis waterlogging. These yield losses were related to the decreases in dry matter accumulation, redistribution of stored photosynthate to the grain, and the conversion capacity from carbohydrate to starch in grain. The decrease in dry matter accumulation could be attributed to the reduced activities of Pn (photosynthesis) and SPS (sucrose phosphate synthase) in the flag leaf, while the low capacity in starch synthesis could be explained by the reduced activities of sucrose synthase (SS) and soluble starch synthase (SSS) in grain. Total N uptake in shoot was also reduced, which could contribute to the losses in biomass and yield by waterlogging. The decrease in Pn was inconsistent with the increase in N content in the flag leaf at high N fertiliser application under post-anthesis waterlogging.     

Effect of Defoliation During the Early Vegetative Phase and at Silking on Growth of Maize (Zea mays L.)

Two experiments were conducted in the glasshouse and in thefield to evaluate the effect of leaf loss on development, drymatter accumulation and yield of maize. In the glasshouse, defoliationtreatments were imposed on maize after 3 weeks of planting.Removing 2 or 3 leaves every 2 weeks affected plant height,days to tassel, root and shoot yield. The plant diameter androot:shoot ratio were not affected by defoliation. In the field experiment, six defoliation treatments were imposed7 days after 50 per cent silking. Defoliation of all leaveswas the most severe treatment on cob d. wt, dry matter accumulationin grains, weight of 100 test grains and yield. The effect ofremoving all leaves above the ear was not significantly differentfrom that of removing all leaves below the ear. The effect ofremoving half of the leaves above the ear was not differentfrom the control. Zea mays, vegetative growth, dry matter accumulation, yield, defoliation     

Effect of increased temperature in apical regions of maize ears on starch-synthesis enzymes and accumulation of sugars and starch

Apical florets of maize (Zea mays L.) ears differentiate later than basal florets and form kernels which have lower dry matter accumulation rates. The purpose of this study was to determine whether increasing the temperature of apical kernels during the dry matter accumulation period would alter the difference in growth rate between apical and basal kernels. Apical regions of field-grown maize (cultivar Cornell 175) ears were heated to 25 ± 3°C from 7 days after pollination to maturity (tip-heated ears) and compared with unheated ears (control). In controls, apical-kernel endosperm had 24% smaller dry weight at maturity, lower concentration of sucrose, and lower activity of ADP-Glc starch synthase than basal-kernel endosperm, whereas ADP-Glc-pyrophosphorylase (ADPG-PPase) activities were similar. In tip-heated ears apical-kernel endosperm had the same growth rate and final weight as basal-kernel endosperm and apical kernels had higher sucrose concentrations, higher ADP-Glc starch synthase activity, and similar ADPG-PPase activity. Total grain weight per ear was not increased by tip-heating because the increase in size of apical kernels was partially offset by a slight decrease in size of the basal- and middle-position kernels. Tip-heating hastened some of the developmental events in apical kernels. ADPG-PPase and ADP-Glc starch synthase activities reached peak levels and starch concentration began rising earlier in apical kernels. However, tip-heating did not shorten the period of starch accumulation in apical kernels. The results indicate that the lower growth rate and smaller size of apical kernels are not solely determined by differences in prepollination floret development.     

Grain sink strength may be related to the poor grain filling of indica-japonica rice (Oryza sativa) hybrids

The physiological and biochemical factors contributing to poor grain filling of indica-japonica rice ( Oryza sativa L.) hybrids were studied by analyzing the role of grain sink strength in dry matter accumulation of grains of two types of rice cultivars, Yayou 2 (an indica-japonica hybrid) and Yanjing 2 (a japonica cultivar). Carbon dioxide enrichment and plant hormone application were imposed at anthesis and the number of endosperm cells, dry matter accumulation and the activities of some sugar-metabolizing enzymes of grains were measured during grain filling. In Yayou 2, strong-potential grains (SPGs) accumulated dry weight much earlier than weak-potential grains (WPGs), but this difference was not obvious for Yanjing 2. Carbon dioxide enrichment imposed after heading significantly stimulated dry matter accumulation of WPGs of Yayou 2, but had little influence on WPGs of Yanjing 2 and SPGs of both cultivars. Leaf sheath dry matter decreased steadily in both cultivars during early stages of grain filling and accumulated during the later stages. Carbon dioxide enrichment increased leaf sheath dry matter. Dry matter accumulated by grains was linearly related to the increases in endosperm cell numbers and the activities of sucrose synthase (SS) and ADP-glucose pyrophosphorylase (AGPase) in SPGs and WPGs of both cultivars. Application of either 6-benzyladenine or abscisic acid had no significant influences on both endosperm cell number and grain dry matter accumulation. These results suggest that grain sink strength, determined by both cell numbers, SS and AGPase activities in the endosperm control the dry matter accumulation of grains.     

The effect of assimilate supply on fruit growth and hormone levels in tomato plants

Rates of dry matter accumulation and contents of starch, sugars and abscisic acid (ABA) of tomato fruits differed significantly during development at three positions (proximal, middle and distal) on a truss. Proximal fruits, which accumulated dry matter most rapidly during early development, generally had least ABA (per g DW).Partial defoliation reduced carbon accumulation by all fruits but increased ABA, especially in distal fruit, and indoleacetic acid (IAA), particularly in proximal fruits. The ABA content of leaves in partially defoliated plants was similar to that of leaves on non-defoliated plants.Removal of distal fruits on a truss enhanced carbon movement to the remaining proximal fruits and also increased their ABA content early in development but did not affect their IAA content. On the other hand, when proximal fruit were removed there was no large or lasting increased accumulation of carbon by the remaining distal fruits and they contained less ABA and IAA than fruits on plants without fruit thinning. Leaf carbon and ABA levels showed no marked trend in response to fruit thinning.The amount of carbon in the stems was increased by fruit thinning but decreased by partial defoliation.The possible roles of ABA and IAA in regulating fruit growth are discussed.Part of this work has been presented to a Symposium on Phloem loading and related processes at Bad Grund/Oberhar, W. Germany, July 1979.     

Conversion of sucrose to starch in the developing Pennisetum typhoides grain

Developing grains of pearl millet ( Pennisetum typhoides Burm. S & H cv. PIB 155) were sampled and analyzed for starch and its free-sugar precursors. The activities of invertase, sucrose-ADP (UDP) glucosyl transferase and of α-amylase and β-amylase in relation to the rate of starch accumulation in the developing grain were assayed. By culturing detached ears, the incorporation of ¹⁴C from free sugar precursors to starch was studied. The starch content gradually increased until grain maturity. The rate of starch accumulation was maximum around 12 days after anthesis. Around this period, the activities of sucrose-ADP(UDP) glucosyl transferase and α-amylase, β-amylase were also at a peak. Invertase activity was high during the early period of grain development but gradually declined as the grains matured. In the most actively metabolising milky grains, incorporation of ¹⁴C from [¹⁴C]-sugars to starch was maximum in the mid mid-milky grains. Addition of 20 m M K⁺ to the culture solution did not affect the incorporation of ¹⁴C from supplied sucrose to the free sugar pool and to the starch of the grain, but Mg²⁺ supply at 20 m M concentration lowered ¹⁴C incorporation from exogenous sucrose to grain free sugars, although the utilization of the latter for starch synthesis was enhanced.     

Manipulation Through Liquid Culture in the Contents of Sucrose and Glutamine and Their Transformation to Starch and Protein in Wheat Grain

Detached ears of wheat were cultured in liquid medium manipulated for sucrose and glutamine contents, and the accumulation of starch and protein in relation to the activities of sucrose cleaving—, ammonia assimilating—, and transaminating enzymes was studied in the grain. With an increase in the concentrations of sucrose from 44 to 176 mM and glutamine from 6.4 to 25.7 mM (keeping their ratio at a constant value of 7:1), the contents of starch and protein increased in the grains. However, when the grains were cultured in the medium containing 8.5 to 34 mM glutamine and a fixed concentration of 117 mM sucrose, there was a gradual increase in protein and decrease in starch content in the grain. By such manipulation in the liquid medium, the content of free amino acids also increased in the grain up to 12 days culturing. Amongst sucrose cleaving enzymes, the activities of sucrose-UDP glucosyl transferase and soluble alkaline invertase were much lower than the activity of soluble acid invertase. At high concentration (34 mM) of glutamine in the medium, containing 117 mM sucrose, there was drastic decrease in the activities of soluble acid invertase and UDPG pyrophosphorylase but the activities of ADPG pyrophosphorylase, alkaline inorganic pyrophosphatase, glutamate dehydrogenase, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase increased in the grain with increase in glutamine concentration in the culture medium. Evidently, an increase in the level of amino nitrogen, coupled with an optimum sucrose concentration in the grain raised through liquid culturing enhances the conversion of sucrose to protein at the cost of starch accumulation in wheat.     

Carbohydrate metabolism enzymes in CO2-enriched developing rice grains of cultivars varying in grain size

The increased supply of photosynthate from maternal tissue is known to promote grain growth in several crop species. However, the effect of increasing photosynthate supply on grain growth receives little attention in rice. This study was aimed at evaluating the effect of increasing photosynthate supply through CO₂ enrichment (650 μl I_-1) on grain growth in three rice cultivars differing in grain size. CO₂ enrichment was applied to the pot-grown plants between anthesis and final harvest. The results indicated that high CO₂ treatment enhanced the CO₂ exchange rate of leaf tissue, and subsequently increased the sucrose level of peduncle exudate, but it did not promote starch accumulation in the developing grains. This phenomenon was linked to the poor CO₂ responses for the grain activities of sucrose synthase, UDP-glucose pyrophosphorylase. ADP-glucose pyrophosphorylase, and starch synthases involved in the conversion of sucrose to starch. Significant cultivar differences also existed for the activities of sucrose to starch conversion enzymes with larger grain size cultivars tending to have higher enzymes activities (expressed on a grain basis), resulting in a greater carbohydrate accumulation.     

Effect of N-source on soybean leaf sucrose phosphate synthase, starch formation, and whole plant growth

Soybeans (Glycine max L. Merr. cv Tracy and Ransom) were grown under N₂-dependent or NO₃⁻-supplied conditions, and the partitioning of photosynthate and dry matter was characterized. Although no treatment effects on photosynthetic rates were observed, NO₃⁻-supplied plants in both cultivars had lower starch accumulation rates than N₂-dependent plants. Leaf extracts of NO₃⁻-supplied plants had higher activities of sucrose phosphate synthase (SPS) and cytoplasmic fructose-1,6-bisphosphatase (FBPase) than N₂-dependent plants. The variation in starch accumulation was correlated negatively with the activity of SPS, but not the activity of FBPase, UDP-glucose pyrophosphorylase, or ADP-glucose pyrophosphorylase. These results suggested that starch accumulation is biochemically controlled, in part, by the activity of SPS. Leaf starch content at the beginning of the photoperiod was lower in NO₃⁻-supplied plants than N₂-dependent plants in both cultivars which suggested that net starch utilization as well as accumulation was affected by N source.

Total dry matter accumulation and dry matter distribution was affected by N source in both cultivars, but the cultivars differed in how dry matter was partitioned between the shoot and root as well as within the shoot. The activity of SPS was correlated positively with total dry matter accumulation which suggested that SPS activity is related to plant growth rate. The results suggested that photosynthate partitioning is an important but not an exclusive factor which determines whole plant dry matter distribution.

     

Changes in Starch Formation and Activities of Sucrose Phosphate Synthase and Cytoplasmic Fructose-1,6-bisphosphatase in Response to Source-Sink Alterations

Short term experiments were conducted with vegetative soybean plants (Glycine max L. Merr. `Ransom' or `Arksoy') to determine whether sourcesink manipulations, which rapidly changed the `demand' for sucrose and partitioning of photosynthetically fixed carbon into starch, were associated with alterations in activities of sucrose-P synthase and/or cytoplasmic fructose-1,6-bisphosphatase in leaf extracts. When demand for sucrose from a particular source leaf was increased by defoliation of other source leaves, starch accumulation was restricted and activities of both enzymes were markedly enhanced. When demand for sucrose from source leaves was limited by excision, starch accumulation in the detached leaves was increased while activity of sucrose-P synthase declined sharply. The consistent responsiveness of sucrose-P synthase activity to changes in demand for sucrose supports the contention that regulation of sucrose-P synthase is an integral component of the system which controls sucrose biosynthesis and partitioning of carbon between starch and sucrose biosynthesis in the light.     

Effect of Carbohydrate Demand on the Remobilization of Starch in Stolons and Roots of White Clover (Trifolium repens L.) after Defoliation

White clover plants were grown from stolon tips in growth cabinetsand then defoliated. Thereafter, changes in the contents ofnon-structural carbohydrates such as starch, sucrose, glucose,fructose, maltose, and pinitol in stolons and roots were monitored.Initial contents of carbohydrate reserves, photosynthetic supplyof new carbohydrates and carbohydrate demand after defoliationwere varied by growing the plants at various CO₂ partial pressures,by varying the extent of defoliation and by removing eitherroots or stolon tips at the time of defoliation. Remobilization of carbohydrate reserves in stolons increasedproportionally to their initial contents and was greater whenplants had been severely defoliated, suggesting that carbohydrateswere remobilized according to availability and demand. Starchwas the predominant reserve carbohydrate. Starch degradationwas associated with decreased contents of sucrose, glucose andfructose in young stolon parts and roots but not in old stolonparts suggesting that starch degradation was not strictly controlledby the contents of these sugars. A decrease in the demand forcarbohydrates by removal of roots did not decrease starch degradationbut increased the contents of sucrose, glucose, and fructose.Removal of stolon tips decreased starch degradation and contentsof sucrose, glucose, and fructose. The results suggest thatstarch degradation was controlled by a factor other than sucrose,glucose, and fructose which was exported from stolon tips, e.g.gibberellin. Key words: White clover, storage carbohydrates, remobilization, regrowth     

Metabolism of free sugars in relation to starch synthesis in the developing Sorghum vulgare grain

Accumulation of starch at expense of its free-sugar precursors was studied in the developing grains of the ‘SL-44’variety of Sorghum vulgare Pers. The content of starch gradually increased with the maturation of the grain and this increase was relatively fast until 18 days after anthesis. The daily rate of starch accumulation was at a maximum 15 days after anthesis. The content of total free sugars, reducing sugars, non-reducing sugars other than sucrose, total and non-sucrosyl fructose, and glucose also increased, reaching maximum values at 18 days after anthesis. Sucrose content gradually increased with a concomitant decrease in the activity of invertase, and sucrose was the major non-reducing sugar in the matured grains. Detached heads incubated in labelled sugars indicated that, compared to sucrose and fructose. ¹⁴C was more efficiently incorporated from glucose into grain starch, which was maximally synthesized at the mid-milky stage of grain development. Exogenous supply of NAD⁺ plus ATP stimulated the in vivo incorporation of ¹⁴C from sucrose to starch. The decline in the rate of starch accumulation did not synchronise with that of protein synthesis.     

Photosynthate supply and utilization in alfalfa : a developmental shift from a source to a sink limitation of photosynthesis

Long-term carbon dioxide enrichment, ¹⁴CO₂ feeding, and partial defoliation were employed as probes to investigate source/sink limitations of photosynthesis during the development of symbiotically grown alfalfa. In the mature crop, long-term CO₂ enrichment does not affect the rates of net photosynthesis, relative growth, ¹⁴C export to nonphotosynthetic organs, or the rates of ¹⁴C label incorporation into leaf sucrose, starch, or malate. The rate of glycolate labeling is, however, substantially reduced under these conditions. When the mature crop was partially defoliated, a considerable increase in net photosynthesis occurred in the remaining leaves. In the seedling crop, long-term CO₂ enrichment increased dry matter accumulation, primarily as a result of increases in leaf starch content. Although the higher rates of starch synthesis are not maintained, the growth enhancement of the enriched plants persisted throughout the experimental period. These results imply a source limitation of seedling photosynthesis and a sink limitation of photosynthesis in more mature plants. Consequently, both the supply and the utilization of photosynthate may limit seasonal photosynthesis in alfalfa.     

Evidence against sink limitation by the sucrose-to-starch route in potato plants expressing fructosyltransferases

To investigate whether the route from sucrose to starch limits sink strength of potato tubers, we established an additional storage carbohydrate pool and analyzed allocation of imported assimilates to the different pools. Tuber specific expression of the fructan biosynthetic enzymes of globe artichoke resulted in accumulation of fructans to about 5% of the starch level, but did not increase tuber dry weight per plant. While partial repression of starch synthesis caused yield reduction in wild-type plants, it stimulated fructan accumulation, and yield losses were ameliorated in tubers expressing fructosyltransferases. However, a nearly complete block of the starch pathway by inhibition of sucrose synthase could not be compensated by the fructan pathway. Although fructan concentrations rose, yield reduction was even enhanced, probably because of a futile cycle of fructan synthesis and degradation by invertase, which is induced when sucrose synthase is knocked out. The data do not support a limitation of sink strength by enzyme activities of the starch pathway but point to an energy limitation of storage carbohydrate formation in potato tubers.     

Regulation of starch and protein accumulation in alfalfa (Medicago sativa L.) somatic embryos

Compared to seeds, somatic embryos accumulated relatively low levels and different types of storage carbohydrates. The regulation of starch accumulation was studied to determine its effects on desiccation tolerance and vigor of dry somatic embryos. Somatic embryos of Medicago sativa are routinely matured through three phases: 7 days of development; 10 days of phase I maturation, a rapid growth phase; and 10 days of phase II maturation, a phase leading to the acquisition of desiccation tolerance. The control of starch deposition was investigated in alfalfa somatic embryos by manipulating the composition of the phase I maturation medium with different levels of sucrose, abscisic acid, glutamine and different types of carbohydrates and amino acids. After phase II maturation, mature somatic embryos were collected for desiccation and subsequent conversion, or for biochemical analyses. Starch deposition occurred primarily during phase I maturation, and variations in the composition of this medium influenced embryo quality, storage protein and starch accumulation. A factorial experiment with two levels of glutamine × three levels of sucrose showed that increasing the sucrose concentration from 30 to 80 g/l increased embryo size and starch content, but had minimal effect on accumulation of storage proteins; glutamine also increased embryo size, but decreased starch content and increased accumulation of the high salt soluble S-2 (medicagin) storage proteins. ABA did not influence any of the parameters tested when included in phase I maturation at concentration up to 10 μM. Replicating sucrose with maltose, glucose, or glucose and fructose did not alter embryo size or starch accumulation (mg/g fresh weight), but replacement with fructose alone reduced embryo size, and replacement with glucose alone reduced germination. Suplementation with the amino acids, asparagine, aspartic acid and glutamine increased seedling vigor, but decreased the starch content of embryos. The data indicate that starch accumulation in somatic embryos is regulated by the relative availability of carbon versus nitrogen nutrients in the maturation medium. The quality of mature somatic embryos, determined by the rate of seedling development (conversion and vigor), correlated with embryo size, storage protein and free amino acid but not with starch. Therefore, further improvements in the quality of somatic embryo may be achieved through manipulation of the maturation medium in order to increase storage protein, but not starch deposition.     

Activities of enzymes involved in starch synthesis in wheat grains differing in starch content

This work was carried out to characterize starch accumulation and activities of key enzymes during grain filling in two wheat cultivars differing in starch content. The results showed that the starch accumulation rate (SAR) and activities of sucrose synthase, ADP-glucose pyrophosphorylase, soluble starch synthase, granule-bound starch synthase, and starch branching enzyme in the cultivar with a high starch content were significantly higher than those in the cultivar with a low starch content. The simulation with Richards’ equation showed that it was average starch accumulation rate but not active starch accumulation duration that determined starch accumulation. As compared with the cultivar with a low starch content, plants of the cultivar with a high starch content maintained the higher SAR and greater activities of related enzymes during mid and late grain filling stages. Consequently, the cultivar with a high starch content had advantages over that with a low starch content in terms of the amount of starch accumulation at mid and late grain filling stages.