High Carbohydrate Availability Research Articles

Nutrition for swimming.

Swimming is a sport that requires considerable training commitment to reach individual performance goals. Nutrition requirements are specific to the macrocycle, microcycle, and individual session. Swimmers should ensure suitable energy availability to support training while maintaining long term health. Carbohydrate intake, both over the day and in relation to a workout, should be manipulated (3-10 g/kg of body mass/day) according to the fuel demands of training and the varying importance of undertaking these sessions with high carbohydrate availability. Swimmers should aim to consume 0.3 g of high-biological-value protein per kilogram of body mass immediately after key sessions and at regular intervals throughout the day to promote tissue adaptation. A mixed diet consisting of a variety of nutrient-dense food choices should be sufficient to meet the micronutrient requirements of most swimmers. Specific dietary supplements may prove beneficial to swimmers in unique situations, but should be tried only with the support of trained professionals. All swimmers, particularly adolescent and youth swimmers, are encouraged to focus on a well-planned diet to maximize training performance, which ensures sufficient energy availability especially during periods of growth and development. Swimmers are encouraged to avoid rapid weight fluctuations; rather, optimal body composition should be achieved over longer periods by modest dietary modifications that improve their food choices. During periods of reduced energy expenditure (taper, injury, off season) swimmers are encouraged to match energy intake to requirement. Swimmers undertaking demanding competition programs should ensure suitable recovery practices are used to maintain adequate glycogen stores over the entirety of the competition period.

P53: the tumour suppressor turns mitochondrial regulator

p53: the tumour suppressor turns mitochondrial regulator

Effects of carbohydrate-hydration strategies on glucose metabolism, sprint performance and hydration during a soccer match simulation in recreational players

This study compared the effects of three carbohydrate-hydration strategies on blood glucose concentration, exercise performance and hydration status throughout simulated soccer match-play. A randomized, double-blind and cross-over study design was employed. After familiarization, 14 recreational soccer players completed the soccer match simulation on three separate occasions. Participants consumed equal volumes of 9.6% carbohydrate-caffeine-electrolyte (∼ 6 mg/kg BW caffeine) solution with carbohydrate-electrolyte gels (H-CHO), 5.6% carbohydrate-electrolyte solution with electrolyte gels (CHO) or electrolyte solution and electrolyte gels (PL). Blood samples were taken at rest, immediately before exercise and every 15 min during exercise (first half: 15, 30, 45 min; second half: 60, 75, 90 min). Supplementation influenced blood glucose concentration (time × treatment interaction: p<0.001); however, none of the supplementation regimes were effective in preventing a drop in blood glucose at 60 min. Mean sprint speed was 3 ± 1% faster in H-CHO when compared with PL (treatment: p=0.047). Supplementation caused a 2.3 ± 0.5% increase in plasma osmolality in H-CHO (p<0.001) without change in CHO or PL. Similarly, mean sodium concentrations were 2.1 ± 0.4% higher in H-CHO when compared with PL (p=0.006). Combining high carbohydrate availability with caffeine resulted in improved sprint performance and elevated blood glucose concentrations throughout the first half and at 90 min of exercise; however, this supplementation strategy negatively influenced hydration status when compared with 5.6% carbohydrate-electrolyte and electrolyte solutions.

Demonstration of diet-induced decoupling of fatty acid and cholesterol synthesis by combining gene expression array and2H2O quantification

The liver is a crossroad for metabolism of lipid and carbohydrates, with acetyl-CoA serving as an important metabolic intermediate and a precursor for fatty acid and cholesterol biosynthesis pathways. A better understanding of the regulation of these pathways requires an experimental approach that provides both quantitative metabolic flux measurements and mechanistic insight. Under conditions of high carbohydrate availability, excess carbon is converted into free fatty acids and triglyceride for storage, but it is not clear how excessive carbohydrate availability affects cholesterol biosynthesis. To address this, C57BL/6J mice were fed either a low-fat, high-carbohydrate diet or a high-fat, carbohydrate-free diet. At the end of the dietary intervention, the two groups received (2)H(2)O to trace de novo fatty acid and cholesterol synthesis, and livers were collected for gene expression analysis. Expression of lipid and glucose metabolism genes was determined using a custom-designed pathway focused PCR-based gene expression array. The expression analysis showed downregulation of cholesterol biosynthesis genes and upregulation of fatty acid synthesis genes in mice receiving the high-carbohydrate diet compared with the carbohydrate-free diet. In support of these findings, (2)H(2)O tracer data showed that fatty acid synthesis was increased 10-fold and cholesterol synthesis was reduced by 1.6-fold in mice fed the respective diets. In conclusion, by applying gene expression analysis and tracer methodology, we show that fatty acid and cholesterol synthesis are differentially regulated when the carbohydrate intake in mice is altered.

Carbohydrates for training and competition

An athlete's carbohydrate intake can be judged by whether total daily intake and the timing of consumption in relation to exercise maintain adequate carbohydrate substrate for the muscle and central nervous system (“high carbohydrate availability”) or whether carbohydrate fuel sources are limiting for the daily exercise programme (“low carbohydrate availability”). Carbohydrate availability is increased by consuming carbohydrate in the hours or days prior to the session, intake during exercise, and refuelling during recovery between sessions. This is important for the competition setting or for high-intensity training where optimal performance is desired. Carbohydrate intake during exercise should be scaled according to the characteristics of the event. During sustained high-intensity sports lasting ∼1 h, small amounts of carbohydrate, including even mouth-rinsing, enhance performance via central nervous system effects. While 30–60 g · h−1 is an appropriate target for sports of longer duration, events >2.5 h may benefit from higher intakes of up to 90 g · h−1. Products containing special blends of different carbohydrates may maximize absorption of carbohydrate at such high rates. In real life, athletes undertake training sessions with varying carbohydrate availability. Whether implementing additional “train-low” strategies to increase the training adaptation leads to enhanced performance in well-trained individuals is unclear.

Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling

We determined the effects of varying daily carbohydrate intake by providing or withholding carbohydrate during daily training on endurance performance, whole body rates of substrate oxidation, and selected mitochondrial enzymes. Sixteen endurance-trained cyclists or triathletes were pair matched and randomly allocated to either a high-carbohydrate group (High group; n = 8) or an energy-matched low-carbohydrate group (Low group; n = 8) for 28 days. Immediately before study commencement and during the final 5 days, subjects undertook a 5-day test block in which they completed an exercise trial consisting of a 100 min of steady-state cycling (100SS) followed by a 7-kJ/kg time trial on two occasions separated by 72 h. In a counterbalanced design, subjects consumed either water (water trial) or a 10% glucose solution (glucose trial) throughout the exercise trial. A muscle biopsy was taken from the vastus lateralis muscle on day 1 of the first test block, and rates of substrate oxidation were determined throughout 100SS. Training induced a marked increase in maximal citrate synthase activity after the intervention in the High group (27 vs. 34 micromol x g(-1) x min(-1), P < 0.001). Tracer-derived estimates of exogenous glucose oxidation during 100SS in the glucose trial increased from 54.6 to 63.6 g (P < 0.01) in the High group with no change in the Low group. Cycling performance improved by approximately 6% after training. We conclude that altering total daily carbohydrate intake by providing or withholding carbohydrate during daily training in trained athletes results in differences in selected metabolic adaptations to exercise, including the oxidation of exogenous carbohydrate. However, these metabolic changes do not alter the training-induced magnitude of increase in exercise performance.

Cinética da frequência cardíaca nos domínios muito pesado e severo após manipulação dietética

Little is known about heart rate (HR) kinetics during exercise in the very heavy (VH) and severe (SE) intensity domains. The objective of this study was to describe mathematically the HR kinetics during exercise performed in these intensity domains and to compare the parameters derived from these models between situations of high (HCHO), low (LCHO) and control (C) carbohydrate availability. Twelve men performed three trials to exhaustion in the VH or SE domains after diet manipulation with HCHO, LCHO and C. The VH intensity was ΔLW75% (75% of the difference between VO2max and LL2) and SE was 115% of VO2max identified in a previous incremental test (20 W/3 min). HR responses were mathematically fitted by mono- and biexponential functions. In the VH domain, the residual sum of squares (RSS) obtained with the biexponential model was significantly lower than that obtained with the monoexponential model (P &lt; 0.05). In the SE domain, no significant difference in RSS was observed between the mathematical models (P &gt; 0.05). In the VH domain, there were no significant differences in biexponential parameters between the HCHO, LCHO and C conditions. In the SE domain, there were no significant differences in monoexponential parameters between the HCHO, LCHO and C conditions, although the time constant of the monoexponential model was significantly reduced in LCHO when compared to HCHO (51.5 ± 26.4 vs 65.4 ± 34.1 s; P &lt; 0.05). The bi- and monoexponential mathematical models seem to be the best description of HR responses during exercise performed in the HV and SE intensity domains, respectively. In addition, carbohydrate availability only seems to affect HR kinetics during exercise performed at SE intensity.

Protein Metabolism and Endurance Exercise

Prolonged endurance exercise stimulates whole-body protein turnover (synthesis and degradation) but it remains contentious whether this translates into an increased net protein oxidation or dietary requirement for protein. Skeletal muscle is the major energy consumer during exercise and the oxidation of branched-chain amino acids (BCAA) is increased several-fold, suggesting an increased requirement for fuel. Increased BCAA oxidation has been proposed to impair aerobic energy provision during prolonged exercise, but there is little evidence to support this theory. Endurance training blunts the acute exercise-induced increase in whole-body protein turnover and skeletal BCAA oxidation at a given work intensity. However, training also increases the maximal capacity for skeletal muscle BCAA oxidation, as evidenced by a higher maximal activity of the rate-determining enzyme branched-chain oxo acid dehydrogenase. Exercise-induced changes in protein metabolism are affected by nutritional status, with high carbohydrate availability (as typically practiced by endurance athletes) generally associated with reduced net protein utilisation. Ingestion of protein with carbohydrate improves net protein balance during exercise and recovery compared with carbohydrate alone, but it remains to be determined whether this practice facilitates the adaptive response to chronic training.

“Fat adaptation” for athletic performance: the nail in the coffin?

endurance athletes have a high capacity for the oxidation of fat during exercise as a legacy of their training. Therefore, it is intriguing that this capacity can be easily upregulated by the chronic consumption of a low-carbohydrate (<2.5 g·kg−1·day−1), high-fat (∼65–70% of energy) diet.

Decreased Skeletal Muscle Pyruvate Dehydrogenase Activation During Cycling Following Short-term High-fat Adaptation With Carbohydrate Restoration

Previous studies have shown that five days of a high-fat diet while training, followed by carbohydrate restoration, results in metabolic adaptations that increase the rate of whole-body fat oxidation during aerobic cycling. No studies have elucidated the mechanisms responsible for this shift of increased fat utilization despite high carbohydrate (CHO) availability. PURPOSE To investigate the effects that a 5-day high-fat diet while training followed by a 1-day CHO restoration has on skeletal muscle pyruvate dehydrogenase activation (PDHa) and hormone sensitive lipase (HSL) activity on CHO and fat metabolism during 70% VO2 peak cycling. METHODS Seven male cyclists rode for 20 min at 70% VO2 peak on two occasions. For 5-days prior to the trials, subjects consumed a random order of high-CHO diet (10.3 g·kg−1·day−1 CHO or ∼70% of total energy intake from CHO) or an isoenergetic high-fat (FAT-adpt) diet (2.5 g·kg−1·day−1 CHO or ∼17% energy; 4.6 g·kg−1·day−1 FAT or 47% energy from fat) while undertaking supervised training. For both diets on day 6, subjects ingested high CHO and rested before their experimental trials on day 7. Blood and expired gas measurements were obtained throughout the trials and muscle biopsy samples were obtained immediately prior to exercise and at 1 and 20 min of cycling. RESULTS Following either diet, 1-day of CHO restoration resulted in similar resting glycogen contents (FAT-adpt: 873.1 ± 121.4 vs. 867.9 ± 119.5 umoles glucosyl units·g−1 dw) and similar rates of glycogenolysis during exercise. Despite similar glycogen contents, FAT-adpt reduced the respiratory exchange ratio (RER) during steady state cycling at 70% VO2 peak (0.85 ± 0.02 vs. 0.91 ± 0.01; P<0.05) which resulted in a ∼45% increase and a ∼30% decrease in fat and carbohydrate oxidation respectively. Average exercise PDHa values were 29% lower (P<0.05) during the FAT-adpt compared to CHO (1.69 ± 0.25 vs 2.39 ± 0.19 mmol · kg−1 ww · min−1). Muscle contents of pyruvate, acetyl CoA, and ATP/ADPf, all known regulators of PDH, were unaffected. There was a ∼20% increase (p=0.116) in HSL after the FAT-adpt compared to CHO during rest and cycling (5.32 ± 0.79 vs 4.26 ± 0.28 mmol · kg−1 dw · min−1). CONCLUSIONS Results indicate that the metabolic shifts incurred by the FAT-adpt diet, despite carbohydrate restoration, resulted in decreased whole body and skeletal muscle CHO oxidation and increased whole body FAT oxidation during steady state cycling. The mechanisms behind the decreased activation of PDHa (CHO oxidation) or the 20% increase in HSL after a high-fat diet are unknown.

DICHLOROACETATE IS ASSOCIATED WITH AN INCREASE IN MALONYL-COA BUT NO CORRESPONDING DECREASE IN FA OXIDATION

We have shown that high carbohydrate (CHO) availability is associated with a decrease in fatty acid (FA) oxidation in perfused muscle at rest. PURPOSE: To determine whether an increase in glycolytic flux is the mechanism by which FA utilization is altered with high CHO availability. METHODS: Rat hindquarters were perfused with 500 μM palmitate,[1–14C]palmitate, 6 mM glucose, 10 μU/mL insulin, and without dichloroacetate (DCA) (C: n = 7, BW = 333.6±12.0 g) or with DCA, a drug that is known to stimulate pyruvate dehydrogenase (DCA: n = 7, BW = 316.9 ± 6.6 g). RESULTS: Glucose uptake was not significantly different between groups (3.9 ± 0.4 μmol/g/hr, p > 0.05). In line with the action of DCA on PDH, lactate release was significantly lower in DCA vs.C (0.3 ± 0.5 vs. 2.6 ± 0.3 μmol/g/hr) and post-perfusion malonyl-CoA levels were significantly higher in DCA vs. C (1.5 ± 0.4 vs. 0.87 ± 0.1 pmol/mg). At an equal FA delivery rate of 90.2 ± 8 nmol/min/g, fractional and total rates of palmitate uptake were not different (p > 0.05) between C (0.074 ± 0.008 and 5.46 ± 0.71 nmol/min/g) and DCA (0.080 ± 0.014 and 7.24 ± 1.29 nmol/min/g). Contrary to the observed increase in malonyI-CoA, percent and total rates of palmitate oxidized were not significantly different (p > 0.05) between groups (C: 9.7 ± 1.1% and 0.49 ± 0.07 nmol/min/g vs DCA: 7.3 ± 2.1% and 0.49 ± 0.12 nmol/min/g). CONCLUSIONS: Our results show that under some conditions, changes in malonyl-CoA levels are not associated with reciprocal changes in FA oxidation. This suggests that cellular factors other than malonyl-CoA also play a role in the regulation of muscle FA oxidation. Supported by NIH AR45168 and ACSM Foundation Grant FRG26.

High carbohydrate availability increases LCFA uptake and decreases LCFA oxidation in perfused muscle.

To determine whether changes in long-chain fatty acid (LCFA) oxidative metabolism induced by elevated intracellular carbohydrate availability are due to changes in LCFA uptake or in mitochondrial transport capacity, rat hindquarters were perfused with 500 microM palmitate and [1-14C]palmitate or [1-14C]octanoate as well as with either low (LG) or high (HG) carbohydrate availability. Glucose uptake was higher in the HG vs. LG group (23.6 +/- 1.5 vs 4.7 +/- 0.9 micromol x g(-1) x h(-1), P < 0.05). Palmitate delivery was not significantly different between groups and averaged 97.1 +/- 4.6 nmol x min(-1) x g(-1) (P > 0.05). Fractional and total palmitate uptake values were 60% higher (P < 0.05) in the HG (0.125 +/- 0.012 and 7.4 +/- 1.2 nmol x min(-1) x g(-1)) vs. LG (0.079 +/- 0.009 and 11.8 +/- 1.5 nmol x min(-1) x g(-1)) group. Values of percent and total palmitate oxidized were significantly lower (P < 0.05) in the HG (9.1 +/- 1.1% and 1.31 +/- 0.16 nmol x min(-1) x g(-1)) vs. LG (23.4 +/- 5.2% and 0.76 +/- 0.08 nmol x min(-1) x g(-1)) group. Conversely, values of fractional uptake and percent oxidation of octanoate were not significantly different between groups (P > 0.05). Malonyl-CoA levels were inversely correlated with LCFA oxidation (P < 0.05). These results demonstrate that high carbohydrate availability in muscle is associated with a decrease in LCFA oxidation that is not due to a parallel decrease in LCFA uptake; rather, the decrease in LCFA oxidation could be due to malonyl-CoA inhibition of mitochondrial LCFA transport.

Carbohydrate Accumulation in Relation to Heat Stress Tolerance in Two Creeping Bentgrass Cultivars

Understanding physiological factors that may confer heat tolerance would facilitate breeding for improvement of summer turf quality. The objective of this study was to investigate whether carbohydrate availability contributes to changes in turf quality and root mortality during heat stress in two creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw. (syn. A. palustris Huds.)] cultivars, `L-93' and `Penncross', that contrast in heat tolerance. Grasses were grown at 14-hour days and 11-hour nights of 22/16 °C (control) and 35/25 °C (heat stress) for 56 days in growth chambers. Turf quality decreased while root mortality increased under heat-stress conditions for both cultivars, but to a greater extent for `Penncross' than `L-93'. The concentrations of total nonstructural carbohydrate (TNC), fructans, starch, glucose, and sucrose in shoots (leaves and stems) and roots decreased at 35/25 °C. The reduction in carbohydrate concentrations of shoots was more pronounced than that of roots. Shoot glucose and sucrose concentrations were more sensitive to heat stress than other carbohydrates. `L-93' maintained significantly higher carbohydrate concentrations, especially glucose and sucrose, than `Penncross' at 35/25 °C. Results suggest that high carbohydrate availability, particularly glucose and sucrose, during heat stress was an important physiological trait associated with heat-stress tolerance in creeping bentgrass.

ROLE OF FATS IN EXERCISE: Types and Quality

Plasma TGs, FFAs, and muscle TG are oxidizable lipid fuel sources for skeletal muscle metabolism during prolonged exercise. Plasma FFAs are a major fuel oxidized by skeletal muscle, and their rate of use by muscle depends on several factors, including plasma FFA availability, transport from plasma to the mitochondria, and intracellular metabolism. Mobilization of FFAs from adipose tissue is the first committed step in FFA metabolism, and it depends on the rate of adipose tissue lipolysis. Adipose tissue lipolysis increases with exercise duration and exercise intensity up to intensities of approximately 60% to 65%. Evidence suggests that FFAs are transported from plasma to the mitochondria by FFA transporter proteins that include the plasma membrane and cytosolic FABPPM and FABPC. Plasma FFA use can also be regulated at the mitochondrial transport step by changing the activity of carnitine palmitoyltransferase (CPT-1). Although results from biopsy and tracer studies indicate that muscle TG contribute to skeletal muscle oxidative metabolism during exercise, their exact contribution is difficult to ascertain. Evidence shows that muscle TG use depends on exercise intensity, duration, and mode. The contribution of plasma TG to skeletal muscle metabolism is small. The rate of use of plasma TG is dependent on lipoprotein lipase activity, which is correlated with the oxidative capacity of the muscle fibers. Dietary manipulations can modulate substrate use during exercise and can potentially affect exercise performance. High carbohydrate availability before exercise is associated with an increase in blood glucose and plasma insulin concentrations, which can ultimately decrease the rate of adipose tissue lipolysis and the availability of plasma FFAs. Increased glucose flux has also been shown to decrease lipid oxidation by directly inhibiting the transport of FFAs across the mitochondrial membranes. High lipid availability can be changed by short-term or long-term exposure to high-fat diets. Because carbohydrate reserves are diminished with exposure to high-fat diets, improvements in exercise performance have been difficult to measure under these conditions.

Enumeration of amino acid fermenting bacteria in the human large intestine: effects of pH and starch on peptide metabolism and dissimilation of amino acids

Proteins and trichloroacetic acid-soluble peptides were present in high concentrations in human intestinal contents and faeces. Free amino acids were also detected in millimolar amounts in proximal and distal colon contents, with hydroxyproline, alanine, lysine and valine predominating, showing that a wide variety of organic N-containing compounds was available for fermentation by intestinal bacteria. Measurements of products of dissimilatory amino acid metabolism (ammonia, branched chain fatty acids) demonstrated that these substances occurred in all regions of the large bowel. Amino acid fermenting populations were enumerated in faeces obtained from five healthy donors by most probable number analysis. Counts ranged from 10 10 to 10 11 per gram dry weight faeces. Acetate, propionate and butyrate were the principal fermentation acids in the most probable number tubes. Bacteria forming branched chain fatty acids as major end products of metabolism ranged from 0.6% (isovalerate/2-methylbutyrate) to 40% (isobutyrate) of total peptide and amino fermenting populations. Plate counts also gave high values for peptide fermenting communities in the region of 10 11 per gram dry weight faeces, though considerably lower numbers of organisms grew on plates containing either single amino acids or Stickland pairs. Clostridia and anaerobic Gram-positive cocci were the predominant isolates in these studies. Physiological investigations on the effects of pH and carbohydrate availability on peptide and amino acid fermentation by intestinal bacteria showed that two environmental characteristics of the proximal colon (low pH, high carbohydrate availability), reduced the rate and net ammonia production from peptides, while carbohydrate (starch) was more important in this respect in amino acid fermentation vessels. Starch reduced initial rates of production of branched chain fatty acids by approximately 35% in peptide fermentations, however, culture pH was a more significant determinant affecting formation of these metabolites. Comparisons of branched chain fatty acid formation by faecal bacteria at pH 6.8 and 5.5 showed that their production was reduced by over 60% in pH 5.5 cultures. These data demonstrate that by increasing bacterial requirements for organic N-containing compounds for use in biosynthetic reactions, and through fermentation acid production, carbohydrate availability plays a major role in regulating dissimilatory metabolism of peptides and amino acids in the human large intestine.

Enumeration of amino acid fermenting bacteria in the human large intestine: effects of pH and starch on peptide metabolism and dissimilation of amino acids

Proteins and trichloroacetic acid-soluble peptides were present in high concentrations in human intestinal contents and faeces. Free amino acids were also detected in millimolar amounts in proximal and distal colon contents, with hydroxyproline, alanine, lysine and valine predominating, showing that a wide variety of organic N-containing compounds was available for fermentation by intestinal bacteria. Measurements of products of dissimilatory amino acid metabolism (ammonia, branched chain fatty acids) demonstrated that these substances occurred in all regions of the large bowel. Amino acid fermenting populations were enumerated in faeces obtained from five healthy donors by most probable number analysis. Counts ranged from 1010 to 1011 per gram dry weight faeces. Acetate, propionate and butyrate were the principal fermentation acids in the most probable number tubes. Bacteria forming branched chain fatty acids as major end products of metabolism ranged from 0.6% (isovalerate/2-methylbutyrate) to 40% (isobutyrate) of total peptide and amino fermenting populations. Plate counts also gave high values for peptide fermenting communities in the region of 1011 per gram dry weight faeces, though considerably lower numbers of organisms grew on plates containing either single amino acids or Stickland pairs. Clostridia and anaerobic Gram-positive cocci were the predominant isolates in these studies. Physiological investigations on the effects of pH and carbohydrate availability on peptide and amino acid fermentation by intestinal bacteria showed that two environmental characteristics of the proximal colon (low pH, high carbohydrate availability), reduced the rate and net ammonia production from peptides, while carbohydrate (starch) was more important in this respect in amino acid fermentation vessels. Starch reduced initial rates of production of branched chain fatty acids by approximately 35% in peptide fermentations, however, culture pH was a more significant determinant affecting formation of these metabolites. Comparisons of branched chain fatty acid formation by faecal bacteria at pH 6.8 and 5.5 showed that their production was reduced by over 60% in pH 5.5 cultures. These data demonstrate that by increasing bacterial requirements for organic N-containing compounds for use in biosynthetic reactions, and through fermentation acid production, carbohydrate availability plays a major role in regulating dissimilatory metabolism of peptides and amino acids in the human large intestine.

PEACH: A simulation model of reproductive and vegetative growth in peach trees.

The hypothesis that carbohydrate partitioning is driven by competition among individual plant organs, based on each organ's growth potential, was used to develop a simulation model of the carbon supply and demand for reproductive and vegetative growth in peach trees. In the model, photosynthetic carbon assimilation is simulated using daily minimum and maximum temperature and solar radiation as inputs. Carbohydrate is first partitioned to maintenance respiration, then to leaves, fruits, stems and branches, then to the trunk. Root activity is supported by residual carbohydrate after aboveground growth. Verification of the model was carried out with field data from trees that were thinned at different times. In general, the model predictions corresponded to field data for fruit and vegetative growth. The model predicted that resource availability limited fruit and stem growth during two periods of fruit growth, periods that had been identified in earlier experimental studies as resource-limited growth periods. The model also predicted that there were two periods of high carbohydrate availability for root activity. The fit between model predictions and field data supports the initial hypothesis that plants function as collections of semiautonomous, interacting organs that compete for resources based on their growth potentials.

Plant Factors Controlling Seed Set in Maize

In maize (Zea mays L.) large decreases in kernel number result when low water potentials (Psi(w)) and high temperatures occur during pollination. To gain insight into the basis for the decreased seed set, silk, pollen, and ear-leaf Psi(w), the capability for silk osmotic adjustment, and pollen appearance were measured to determine their relationship to seed set. A multiple-eared or prolific (high carbohydrate availability to the pistillate inflorescence) hybrid (B73 x FR25), a heat sensitive hybrid (WF9 x A632), and a commercial hybrid (B73 x Mo17) were studied. A cross-pollination experiment, with pollination limited by pollen amount, was conducted to determine the impact on seed set of water and heat stressing the tassel and water stressing the ear. At low Psi(w), silk Psi(w) and seed set were decreased whereas pollen Psi(w), appearance, and viability were unaffected. High temperature resulted in a 2 megapascal decrease in pollen Psi(w), visually damaged pollen being shed, decreased pollen viability, and, in two of the hybrids, substantially decreased pollen shed. Prolificacy did not result in increased silk solute accumulation but did result in superior seed production by the pistillate inflorescence at low Psi(w). The magnitude of the decrease in silk solute potential was small (0.2 megapascal) and similar for all genotypes. One hybrid maintained a relatively high silk turgor but this hybrid also decreased the most in seed production when the pistillate inflorescence was water deficient. These results indicated an adverse effect of high temperature on pollen development, a positive relationship between seed production and silk water status, and no advantage to high silk turgor after silk emergence in maintaining seed production. Additionally, there was no evidence of variation in silk solute regulation capability among hybrids which varied in prolificacy, a trait important in drought tolerance, but the seed production of the pistillate inflorescence of the prolific hybrid was least affected by water deficit.