Carbon partitioning in leaves and tubers of transgenic potato plants with reduced activity of fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase

The role of fructose-2,6-bisphosphate (Fru-2,6-P₂) in regulation of carbon metabolism was investigated in transgenic potato plants ( Solanum tuberosum L. cv Dianella) transformed with a vector containing a cDNA-sequence encoding fructose-6-phosphate,2-kinase (F6P,2-K, EC 2.7.1.105)/fructose-2,6-bisphosphatase (F26BPase, EC 3.1.3.46) in sense or antisense direction behind a CaMV 35S promoter. The activity of F6P,2-K in leaves was reduced to 5% of wild-type (WT) activity, and the level of Fru-2,6-P₂ was reduced both in leaves (10% of the WT level) and in tubers (40% of the WT level). Analysis of photosynthetic ¹⁴CO₂ metabolism, showed that in plant lines with reduced Fru-2,6-P₂ level the carbon partitioning in the leaves was changed in favour of sucrose biosynthesis, and the soluble sugars-to-starch labelling ratio was doubled. The levels of soluble sugars and hexose phosphates also increased in leaves of the transgenic plants. Most notably, the levels of hexoses were four- to six-fold increased in the transgenic plants. In tubers with reduced levels of Fru-2,6-P₂ only minor effects on carbohydrate levels were observed. Furthermore, carbon assimilation in tuber discs supplied with [U-¹⁴C]-sucrose showed only a moderate increase in labelling of hexoses and a decreased labelling of starch. Similar results were obtained using [U-¹⁴C]-glucose. No differences in growth of the transgenic lines and the WT were observed. Our data provide evidences that Fru-2,6-P₂ is an important factor in the regulation of photosynthetic carbon metabolism in potato leaves, whereas the direct influence of Fru-2,6-P₂ on tuber metabolism was limited.     

Application of transgenic plants in understanding responses to atmospheric change

Acclimation of plants to an increase in atmospheric carbon dioxide concentration is a well described phenomenon. It is characterized by an increase in leaf carbohydrates and a degradation of ribulose 1, 5-bisphosphate carboxylase protein (Rubisco) leading in the long term to a lower rate of CO₂ assimilation than expected from the kinetic constants of Rubisco. This article summarizes studies with transgenic plants grown in elevated pCO₂ which are modified in their capacity of CO₂ fixation, of sucrose and starch synthesis, of triosephosphate and sucrose transport and of sink metabolism of sucrose. These studies show that a feedback accumulation of carbohydrates in leaves play only a minor role in acclimation, because leaf starch synthesis functions as an efficient buffer for photoassimilates. There is some evidence that in elevated pCO₂, plants grow faster and senescence is induced earlier.     

The contribution of fructose 2,6-bisphosphate to the regulation of sucrose synthesis during photosynthesis

This review discusses (a) how the concentration of fructose 2,6-bisphosphate is controlled in spinach leaves, (b) how fructose 2,6-bisphosphate and cytosolic metabolites control the cytosolic fructose-1,6-bisphosphatase (EC 3.1.3.11), and (c) how the activities of the fructose-1,6-bisphosphatase and of sucrose phosphate synthase (EC 2.3.1.14) are coordinated. These features provide the elements of a fine control network that regulates sucrose synthesis during photosynthesis. The rate of sucrose synthesis is coordinated with the supply of photosynthate, so that concentrations of metabolites and phosphate are maintained at a level in the chloroplast which allows rapid CO₂ fixation. The rate of sucrose synthesis can also be modified to alter the amount of photosynthate that remains in the chloroplast for conversion to starch.     

The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration

Abstract. Herbaceous C₃ plants grown in elevated CO₂ show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO₂-enriched herbaceous plants and decrease for CO₂-enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO₂ show no alteration in the whole-plant partitioning of biomass. Little information is available for interactions between temperature and CO₂. Cold-adapted plants show little response to elevated levels of CO₂, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO₂-enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.     

Photosynthesis, carbohydrate levels and chlorophyll fluorescence-estimated intercellular CO2 in water-stressed Casuarina equisetifolia Forst. & Forst.

The effect of long-term water stress on photosynthetic carbon metabolism in Casuarina equisetifolia Forst. & Forst. was analysed by measuring CO₂ assimilation, stomatal conductance, the quantum yield of photosystem II ( Φ _PSII), enzyme activities, and the levels of photosynthetic intermediates and carbohydrates. CO₂ assimilation decreased under water stress while the intercellular CO₂ concentration ( C _i) as estimated by gas exchange measurements remained high. However, the estimates of C _i from measurements of Φ _PSII suggest that the decrease in photosynthesis can be explained in terms of stomatal closure. Water stress decreased total stromal fructose-1,6-bisphosphatase activity and did not alter the activities and activation states of ribulose bisphosphate carboxylase oxygenase and NADP-dependent malate dehydrogenase (NADP-MDH). The concentration of photosynthetic metabolites, glucose, fructose and sucrose decreased, whereas starch concentrations increased under drought conditions.     

Gas exchange parameters, water relations and carbohydrate partitioning in leaves of field-grown Prunus domestica following fruit removal

The effect of fruit removal on gas exchange, water relations, chlorophyll and non-structural carbohydrate content of leaves from mature, field-grown plum trees ( Prunus domestica L. cv. Stanley) was determined over 2 consecutive growing seasons. Removal of fruits during stage II of fruit development decreased CO₂ assimilation rate within 24 h from 12.6 to 8.5 μmol m^-2 s^-1 in 1986, and from 12.1 to 10.2 μmol m^-2 s^-1 in 1987. Depression of net photosynthesis persisted for at least 5 days and was greatest in the early afternoon. Recovery of the CO₂ assimilation rate to pretreatment levels coincided in defruited trees with vegetative growth that was more than 5-fold that of fruiting trees in the first 6 weeks after fruit removal in 1986. Estimated photorespiration was similar in both fruiting and defruited trees. The stomatal contribution to the decrease of CO₂ assimilation rate, calculated from assimilation/intercellular CO₂ curves, ranged from 31 to 46%. Defruiting did not affect leaf water potential, but decreased leaf osmotic potential. Leaf levels of chlorophyll, fructose, glucose, sorbitol and sucrose were not affected by defruiting, whereas starch content increased up to 51% in leaves of defruited trees within 24 h after fruit removal. However, because of the small starch pool present in plum leaves (<1.9% dry weight) it is unlikely that starch accumulation was responsible for the observed decline in CO₂ assimilation rate after fruit removal. The decrease of CO₂ assimilation rate is discussed in relation to the hypothesis of assimilate demand regulating photosynthesis through a feedback mechanism.     

Elevated atmospheric CO2 lowers leaf litter nutritional quality for stream ecosystem food webs

Up to 99% of the carbon fuelling the food webs of temperate woodland streams is derived from inputs of terrestrial leaf litter. Aquatic bacteria, fungi, and detritivore invertebrates directly utilize these inputs, transferring this energy to other components of the food web. Increases in atmospheric CO₂ could indirectly impact woodland stream food webs by chemically altering leaf litter. This study evaluated CO₂-induced chemical changes in aspen ( Populus tremuloides ) leaf litter, and the corresponding effects on stream bacteria, fungi and leaf-shredding cranefly larvae ( Tipula abdominalis : Diptera). Leaf litter from plants grown under elevated CO₂ had decreased nutritional value to aquatic decomposers and detritivores because of higher levels of structural compounds and lower nitrogen content. Consequently, elevated CO₂-grown leaf litter supported 59% lower bacterial production in a stream than litter grown at ambient CO₂ levels, while not affecting fungal biomass. Larval craneflies fed elevated CO₂-grown microbially colonized leaves consumed less, assimilated less, and grew 12 times slower than their ambient fed counterparts.     

Elevated CO2 and ozone reduce nitrogen acquisition by Pinus halepensis from its mycorrhizal symbiont

The effects of 700 μmol mol⁻¹ CO₂ and 200 nmol mol⁻¹ ozone on photosynthesis in Pinus halepensis seedlings and on N translocation from its mycorrhizal symbiont, Paxillus involutus, were studied under nutrient-poor conditions. After 79 days of exposure, ozone reduced and elevated CO₂ increased net assimilation rate. However, the effect was dependent on daily accumulated exposure. No statistically significant differences in total plant mass accumulation were observed, although ozone-treated plants tended to be smaller. Changes in atmospheric gas concentrations induced changes in allocation of resources: under elevated ozone, shoots showed high priority over roots and had significantly elevated N concentrations. As a result of different shoot N concentration and net carbon assimilation rates, photosynthetic N use efficiency was significantly increased under elevated CO₂ and decreased under ozone. The differences in photosynthesis were mirrored in the growth of the fungus in symbiosis with the pine seedlings. However, exposure to CO₂ and ozone both reduced the symbiosis-mediated N uptake. The results suggest an increased carbon cost of symbiosis-mediated N uptake under elevated CO₂, while under ozone, plant N acquisition is preferentially shifted towards increased root uptake.     

Photosynthesis, Soluble and Structural Carbon Compounds in Two Mediterranean Oak Species (Quercus pubescens and Q. ilex) after Lifetime Growth at Naturally Elevated CO2 Concentrations

Abstract: To study physiological responses of mature forest trees to elevated CO₂ after lifetime growth under elevated atmospheric CO₂ concentrations ( p CO₂), photosynthesis, Rubisco content, foliar concentrations of soluble sugars and starch, sugar concentrations in transport tissues (phloem and xylem), structural biomass, and lignin in leaves and branches were investigated in 30- to 50-year-old Quercus pubescens and Q. ilex trees grown at two naturally elevated CO₂ springs in Italy. Ribulose-1,5-bisphosphate carboxylase/oxygenase content was decreased in Q. pubescens grown under elevated CO₂ concentrations, but not in Q. ilex. Photosynthesis was consistently higher in Q. pubescens grown at elevated CO₂ as compared with "control" sites, whereas the response in Q. ilex was less pronounced. Stomatal conductance was lower in both species leading to decreased transpiration and increased instantaneous water use efficiency in Q. pubescens. Overall mean sugar + starch concentrations of the leaves were not affected by elevated p CO₂, but phloem exudates contained higher concentrations of soluble sugars. This finding suggests increased transport to sinks. Qualitative changes in major carbon-bearing compounds, such as structural biomass and lignins, were only found in bark but not in other tissues. These results support the concept that the maintenance of increased rates of photosynthesis after long-term acclimation to elevated p CO₂ provides a means of optimization of water relations under arid climatic conditions but does not cause an increase in aboveground carbon sequestration per unit of tissue in Mediterranean oak species.     

The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

Soil fungal-arthropod responses to Populus tremuloides grown under enriched atmospheric CO2 under field conditions

We investigated the influence of elevated CO₂ and soil N availability on the growth of arbuscular mycorrhizal and non-mycorrhizal fungi, and on the number of mycophagous soil microarthropods associated with the roots of Populus tremuloides . CO₂ concentration did not significantly affect percentage infection of Populus roots by mycorrhizal or non-mycorrhizal fungi. However, the extra-radical hyphal network was altered both qualitatively and quantitatively, and there was a strong interaction between CO₂ and soil N availability. Under N-poor soil conditions, elevated CO₂ stimulated hyphal length by arbuscular mycorrhizal fungi, but depressed growth by non-mycorrhizal fungi. There was no CO₂ effect at high N availability. High N availability stimulated growth by opportunistic saprobic/pathogenic fungi. Soil mites were not affected by any treatment, but collembolan numbers were positively correlated with the increase in non-mycorrhizal fungi. Results indicate a strong interaction between CO₂ concentration and soil N availability on mycorrhizal functioning and on fungal-based soil food webs.     

Effects of elevated CO2 concentration on photosynthesis, respiration and carbohydrate status of coppice Populus hybrids

To determine how increased atmospheric CO₂ will affect the physiology of coppiced plants, sprouts originating from two hybrid poplar clones ( Populus trichocarpa × P. deltoides - Beaupre and P. deltoides × P. nigra - Robusta) were grown in open-top chambers containing ambient or elevated (ambient + 360 μmol mol⁻¹) CO₂ concentration. The effects of elevated CO₂ concentration on leaf photosynthesis, stomatal conductance, dark respiration, carbohydrate concentration and nitrogen concentration were measured. Furthermore, dark respiration of leaves was partitioned into growth and maintenance components by regressing specific respiration rate vs specific growth rate. Sprouts of both clones exposed to CO₂ enrichment showed no indication of photosynthetic down-regulation. During reciprocal gas exchange measurements, CO₂ enrichment significantly increased photosynthesis of all sprouts by approximately 60% ( P < 0.01) on both an early and late season sampling date, decreased stomatal conductance of all sprouts by 10% ( P < 0.04) on the early sampling date and nonsignificantly decreased dark respiration by an average of 11%. Growth under elevated CO₂ had no consistent effect on foliar sugar concentration but significantly increased foliar starch by 80%. Respiration rate was highly correlated with both specific growth rate and percent nitrogen. Long-term CO₂ enrichment did not significantly affect the maintenance respiration coefficient or the growth respiration coefficient. Carbon dioxide enrichment affected the physiology of the sprouts the same way it affected these plants before they were coppiced.     

Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment

Artificial chalk grassland swards were exposed to either ambient air or air enriched to 600 μ mol mol^–1 CO₂, using free-air CO₂ enrichment technology, and subjected to an 8 week simulated grazing regime. After 14 months of treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco) activity ( V _c,max) and electron transport mediated ribulose-1,5-bisphosphate (RuBP) regeneration capacity ( J _max), estimated from leaf gas exchange, were significantly lower in fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop. grown in elevated CO₂. After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulneraria and S. minor, but acclimation continued in the grass Bromopsis erecta (Hudson) Fourr. Changes in net photosynthesis ( P _n) with growth at elevated CO₂ ranged from a 1·6% reduction in precut leaves of A. vulneraria to a 47·1% stimulation in postcut leaves of S. minor . Stomatal acclimation was observed in leaves of A. vulneraria (reduced stomatal density) and B. erecta (reduced stomatal conductance). The results are discussed in terms of whole-plant resource-use optimization and chalk grassland community competitive interactions at elevated CO₂.     

High carbon dioxide and sun/shade effects on isoprene emission from oak and aspen tree leaves

Abstract. Isoprene (2-methyl 1, 3-butadiene) is emitted from many plants, especially trees. We tested the effect of growth at high CO₂ partial pressure and sun versus shade conditions on the capacity of Quercus rubra L. (red oak) and Populus tremuloides Michx. (quaking aspen) leaves to make isoprene. Oak leaves grown at high CO₂ partial pressure (65 Pa) had twice the rate of isoprene emission as leaves grown at 40Pa CO₂. However, aspen leaves behaved oppositely, with high CO₂-grown leaves having just 60-70% the rate of isoprene emission as leaves grown in 40 Pa CO₂. Similar responses were observed from 25 to 35 °C leaf temperature during assay. The stimulation of isoprene emission by growth at high CO₂ and the stimulation in high temperature resulted in isoprene emission consuming over 15% of the carbon fixed during photosynthesis in high-CO₂ grown oak leaves assayed at 35 °C. Leaves from the south (sunny) sides of trees growing in natural conditions had rates of isoprene emission double those of leaves growing in shaded locations on the same trees. This effect was similar in both aspen and oak. The leaves used for these experiments had significantly different chlorophyll a/b ratios indicating they were functionally sun (from the sunny locations) or shade leaves (from the protected locations). Because the metabolic pathway of isoprene synthesis is unknown, we are unable to speculate about how or why these effects occur. However, these effects are more consistent with metabolic control of isoprene release rather than a metabolic leak of isoprene from metabolism. The results are also important for large scale modelling of isoprene emission and for predicting the effect of future increases in atmospheric CO₂ level on isoprene emission from vegetation.     

Enhanced CO2 alters the relationship between photosynthesis and defence in cyanogenic Eucalyptus cladocalyx F. Muell.

The effect of elevated CO₂ and different levels of nitrogen on the partitioning of nitrogen between photosynthesis and a constitutive nitrogen-based secondary metabolite (the cyanogenic glycoside prunasin) was examined in Eucalyptus cladocalyx . Our hypothesis was that the expected increase in photosynthetic nitrogen-use efficiency of plants grown at elevated CO₂ concentrations would lead to an effective reallocation of available nitrogen from photosynthesis to prunasin. Seedlings were grown at two concentrations of CO₂ and nitrogen, and the proportion of leaf nitrogen allocated to photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), protein and prunasin compared. Up to 20% of leaf nitrogen was allocated to the cyanogenic glycoside, although this proportion varied with leaf age, position and growth conditions. Leaf prunasin concentration was strongly affected by nitrogen supply, but did not increase, on a dry weight basis, in the leaves from the elevated CO₂ treatments. However, the proportion of nitrogen allocated to prunasin increased significantly, in spite of a decreasing pool of leaf nitrogen, in the plants grown at elevated concentrations of CO₂. There was less protein in leaves of plants grown at elevated CO₂ in both nitrogen treatments, while the concentration of active sites of Rubisco only decreased in plants from the low-nitrogen treatment. These changes in leaf chemistry may have significant implications in terms of the palatability of foliage and defence against herbivores.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Growth in elevated CO2 enhances temperature response of photosynthesis in wheat

The temperature dependence of C₃ photosynthesis may be altered by the growth environment. The effects of long-term growth in elevated CO₂ on photosynthesis temperature response have been investigated in wheat ( Triticum aestivum L.) grown in controlled chambers with 370 or 700 μmol mol⁻¹ CO₂ from sowing through to anthesis. Gas exchange was measured in flag leaves at ear emergence, and the parameters of a biochemical photosynthesis model were determined along with their temperature responses. Elevated CO₂ slightly decreased the CO₂ compensation point and increased the rate of respiration in the light and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) V_cmax, although the latter effect was reversed at 15°C. With elevated CO₂, J_max decreased in the 15–25°C temperature range and increased at 30 and 35°C. The temperature response (activation energy) of V_cmax and J_max increased with growth in elevated CO₂. CO₂ enrichment decreased the ribulose 1,5-bisphosphate (RuBP)-limited photosynthesis rates at lower temperatures and increased Rubisco- and RuBP-limited rates at higher temperatures. The results show that the photosynthesis temperature response is enhanced by growth in elevated CO₂. We conclude that if temperature acclimation and factors such as nutrients or water availability do not modify or negate this enhancement, the effects of future increases in air CO₂ on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming.     

Effect of enhanced atmospheric CO2 on mycorrhizal colonization by Glomus mosseae in Plantago lanceolata and Trifolium repens

Plantago lanceolata L. and Trifolium repens L. were grown for 16 wk in ambient (360 μmol mol⁻¹) and elevated (610 μmol mol⁻¹) atmospheric CO₂. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and given a phosphorus supply in the form of bonemeal, which would not be immediately available to the plants. Seven sequential harvests were taken to determine whether the effect of elevated CO₂ on mycorrhizal colonization was independent of the effect of CO₂ on plant growth. Plant growth analysis showed that both species grew faster in elevated CO₂ and that P. lanceolata had increased carbon allocation towards the roots. Elevated CO₂ did not affect the percentage of root length colonized (RLC); although total colonized root length was greater, when plant size was taken into account this effect disappeared. This finding was also true for root length colonized by arbuscules. No CO₂ effect was found on hyphal density (colonization intensity) in roots. The P content of plants was increased at elevated CO₂, although both shoot and root tissue P concentration were unchanged. This was again as a result of bigger plants at elevated CO₂. Phosphorus inflow was unaffected by CO₂ concentrations. It is concluded that there is no direct permanent effect of elevated CO₂ on mycorrhizal functioning, as internal mycorrhizal development and the mycorrhizal P uptake mechanism are unaffected. The importance of sequential harvests in experiments is discussed. The direction for future research is highlighted, especially in relation to C storage in the soil.     

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.