Utilization Of Photosynthate Research Articles

Assessment of sweet potato (Ipomoea batatas) accessions for growth, yield and quality traits

From the study it was noticed that winter sweet potato crop (first season) recorded higher tuber yield due to more utilization of photosynthates, and optimum temperatures for tuberisation and tuber bulking. Second season crop (summer to early monsoon crop) recorded higher vegetative growth which was due to more sunlight and higher temperatures during the crop period which promoted vegetative growth. The pooled analysis of two season’s data revealed that genotype Ib 73 recorded higher tuber length, number of tubers per vine, single tuber weight, tuber yield/vine and marketable tuber yield per hectare, maximum dry matter content, which had a positive correlation with tuber yield. Less weevil infestation percentage was recorded in Ib 73, it might be due to the characteristic feature of thin vine with more latex content. One of the important quality parameters is β-Carotene content in the tuber, which was recorded maximum in the variety Sree Kanaka, thus it may be recommended for the alleviation of malnutrition in children. However, Ib 73 excelled other genotypes in terms of yield and cooking quality characters with low TSS and total sugar content, hence it can be recommended for commercial cultivation among the farmers by replacing potatoes for culinary purpose.

The impact of different soil water levels on nitrogen, phosphorus, and potassium uptake, photosynthetic performance, and sugar distribution of sago palm seedlings

AbstractUnder high soil water levels, an imbalance occurs in the utilization of photosynthate between supporting vegetative growth and storage as starch. This results in a reduction in the density of dry matter in the stem and increased plant size. On the other hand, biomass yield and starch yield are considered to be low. This study examined the response of sago palm (Metroxylon sagu Rottb.) seedlings to 50 and 80% soil water levels. To determine the response of the plant, several parameters were observed during the experiment: N, P, and K uptake, leaf photosynthesis, plant morphological growth, and sugar content in plants. It was found that the sago palm was able to show good growth performance under both 50 and 80% soil water levels. However, 50% soil water level is preferable due to trends in N, P, and K uptake, leaf photosynthetic rate, morphological growth performance, and also because no loss of starch is evident in the petiole or root.

Assessing Long-Distance Carbon Partitioning from Photosynthetic Source Leaves to Heterotrophic Sink Organs with Photoassimilated [14C]CO2.

Phloem loading and long-distance transport of photoassimilate from source leaves to sink organs are essential physiological processes that contribute to plant growth and yield. At a minimum, three steps are involved: phloem loading in source organs, transport along the phloem path, and phloem unloading in sink organs. Each of these can have variable rates contingent on the physiological state of the plant, and thereby influence the overall transport rate. In addition to these phloem transport steps, rates of photosynthesis and photosynthate movement in the pre-phloem path, as well as photosynthate utilization in post phloem tissues of sink organs also contribute to phloem transport. The protocol described here estimates carbon allocation along the entire path from initial carbon fixation to delivery to sink organs after a labeling pulse: [14C]CO2 is photoassimilated in source leaves and loading and transport of the 14C label to heterotrophic sink organs (roots) is quantified by scintillation counting. This method is flexible and can be adapted to quantify long-distance transport in many plant species.

Terpenes and isoprenoids: a wealth of compounds for global use.

Role of terpenes and isoprenoids has been pivotal in the survival and evolution of higher plants in various ecoregions. These products find application in the pharmaceutical, flavor fragrance, and biofuel industries. Fitness of plants in a wide range of environmental conditions entailed (i) evolution of secondary metabolic pathways enabling utilization of photosynthate for the synthesis of a variety of biomolecules, thereby facilitating diverse eco-interactive functions, and (ii) evolution of structural features for the sequestration of such compounds away from the mainstream primary metabolism to prevent autotoxicity. This review summarizes features and applications of terpene and isoprenoid compounds, comprising the largest class of secondary metabolites. Many of these terpene and isoprenoid biomolecules happen to be high-value bioproducts. They are essential components of all living organisms that are chemically highly variant. They are constituents of primary (quinones, chlorophylls, carotenoids, steroids) as well as secondary metabolism compounds with roles in signal transduction, reproduction, communication, climatic acclimation, defense mechanisms and more. They comprise single to several hundreds of repetitive five-carbon units of isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In plants, there are two pathways that lead to the synthesis of terpene and isoprenoid precursors, the cytosolic mevalonic acid (MVA) pathway and the plastidic methylerythritol phosphate (MEP) pathway. The diversity of terpenoids can be attributed to differential enzyme and substrate specificities and to secondary modifications acquired by terpene synthases. The biological role of secondary metabolites has been recognized as pivotal in the survival and evolution of higher plants. Terpenes and isoprenoids find application in pharmaceutical, nutraceutical, synthetic chemistry, flavor fragrance, and possibly biofuel industries.

Comparative Effectiveness of Organic Substitution in Fertilizer Schedule: Impacts on Nitrous Oxide Emission, Photosynthesis, and Crop Productivity in a Tropical Summer Rice Paddy

Studies on replacement of inorganic fertilizer with organic residues to improve crop productivity and their impact on greenhouse gas emission from agricultural soil merit more attention. Two-year field experiments were conducted to study the impact of different organic residues with varied carbon (C)/nitrogen (N) ratios as substitutes of chemical fertilizer on emission reduction of nitrous oxide (N2O) and crop yield from a tropical summer rice paddy of India. Five treatments comprising of conventional N fertilizer (NPK), cow manure (CD), rice straw (RS), poultry manure (PM), and sugarcane bagasse (SCB) were applied in a rice field to estimate N2O emission. Application of CD (at 10 t ha−1) resulted in maximum reduction of seasonal N2O emissions (15 %) over NPK, RS, PM, and SCB. Application of CD and RS enhanced leaf photosynthetic rate and caused maximum utilization of photosynthates towards developing grains as evident from grain filling ability and higher grain yield. Substitution of NPK with organic residues enhanced soil nutrient availability in terms of C and N resulting in improved soil fertility and to some extent influenced soil nitrogen processes which in turn reduced N2O emissions. We conclude that suitable management of soil in agricultural ecosystem can reduce the emission of N2O and protect and preserve the soil health without compromising the agronomic productivity reducing the use of chemical fertilizer and maintaining the sustainability of rice ecosystem as evident from lower carbon equivalent emissions (CEE) and higher carbon efficiency ratio (CER) at CD in rice paddies in the present study.

Chemical composition of essential oils in the leaves of Delairea odorata during flowering and post-flowering periods

on their mass spectra. The percentage of essential oil isolated was 1.6%, and the major compounds identified were germacrene D (17.9%), caryophyllene (11.9), trans-isocroweacin (9.6), acoradiene (7.6), and -curcumene (6.5%) during the flowering season. During the post -flowering period trans--bergamotene (36.8%), trans--ocimene (16.5), bicyclo[7.2.0]undec-4-ene (11.5) and carene (9.3) were the major compounds identified, and the extract percentage was 1.9%. The chemical composition of D. odorata was completely different from Senecio scandens [4]. Though there is confusion in differentiating D. odorata from S. scandens with this chemical variation in the volatile oil composition, we suggest that D. odorata is a different species. The differences in the yield and composition can partly be attributed to the different types of secretory structures [5]. Both in the flowering and post-flowering periods, the essential oils were dominated by sesquiterpenes, which constitute about 89.1 and 42.1%, respectively. There was a considerable amount of alkenes (11.9%) and alkanes (6.6%) recorded during the post-flowering period. The terpene turnover depends on photosynthesis and the utilization of photosynthates [6]. Bergamotene was the major compound during the vegetative phase and plays an important role as an insect repellent [7]. The higher concentration of bergamotenes in the essential oil during the vegetative phase may be the reason why D. odorata has no natural insects feeding on them in the study area. The decrease in the concentration of bergamotenes during the flowering period may be an evolutionary strategy adapted by the plant to attract pollinating agents. These variations obtained in the essential oil constitution may be due to the fact that flowering and fruiting events were influenced by biotic and abiotic factors.

Local and Systemic Proteomic Changes in Medicago Truncatula at an Early Phase of Sinorhizobium meliloti Infection

A symbiotic association with N-fixing bacteria facilitates the growth of leguminous plants under nitrogen-limiting conditions. The establishment of the symbiosis requires signal exchange between the host and the bacterium, which leads to the formation of root nodules, inside which bacteria are hosted. The formation of nodules is controlled through local and systemic mechanisms, which involves root-shoot communication. Our study was aimed at investigating the proteomic changes occurring in shoots and concomitantly in roots of Medicago truncatula at an early stage of Sinorhizobium meliloti infection. The principal systemic effects consisted in alteration of chloroplast proteins, induction of proteins responsive to biotic stress, and changes in proteins involved in hormonal signaling and metabolism. The most relevant local effect was the induction of proteins involved in the utilization of photosynthates and C-consuming processes (such as sucrose synthase and fructose-bisphosphate aldolase). In addition, some redox enzymes such as peroxiredoxin and ascorbate peroxidase showed an altered abundance. The analysis of local and systemic proteome changes suggests the occurrence of a stress response in the shoots and the precocious alteration of energy metabolism in roots and shoots. Furthermore, our data indicate the possibility that ABA and ethylene participate in the communicative network between root and shoot in the control of rhizobial infection.

【Short Report】Influence of the Improved System of Rice Intensification (SRI) on Rice Yield, Yield Components and Tillering Characteristics under Different Rice Establishment Methods

The impacts of the system of rice intensification (SRI) and conventional management (CM) on grain yield, yield components and tillering capacity were examined under 4 rice establishment methods transplanting (TP), seedling casting (SC), mechanical transplanting (MT) and direct seeding (DS). SRI produced significantly higher grain yield than CM under TP and MT but not under DS or SC. DS and SC produced much higher seedling quality than TP or MT, suggesting that robust seedlings with vigorous roots weaken the positive effect of SRI on rice yield. SRI produced a higher tillering rate than CM, but did not affect ear-bearing tiller rate significantly. Moreover, the net photosynthetic rate of the recent fully expanded leaf at mid-tillering stage was significantly higher in SRI than in CM under MT and TP. The obtained results also indicated that SRI increased biomass accumulation before heading and improved utilization of photosynthates in the grain-filling stage.

A parameter condition for ruling out multiple equilibria of the photosynthetic carbon metabolism

AbstractIn this paper, we propose a reduced molecular network for the photosynthetic carbon metabolism, which can describe the following key characteristics: Calvin cycle, utilization of photosynthate, and photorespiration. Taking the concentrations of the nine major metabolites as variables, we represent the reduced network by deriving a nonlinear differential‐algebraic system with 48 parameters, and theoretically study the multi‐equilibrium property in the photosynthetic carbon metabolism. Specifically, we equivalently transform the original 9‐dimensional system into an independent 2‐dimensional subsystem with ten parameters, and show that the original system has no more than one physiologically feasible equilibrium when the ten parameters of the subsystem stay in a certain field around the nominal value of each parameter, no matter what values the other 38 parameters in the original model are taken. Such a theoretical result not only provides profound insights for qualitatively understanding of the dynamic features of the photosynthetic carbon metabolism, but also can be used to make an accurate judgement on a correct strategy for improving the photosynthesis in plants.Copyright © 2011 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society

LINKING TIME-DEPENDENT CARBON-FIXATION EFFICIENCIES IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) TO UNDERLYING METABOLIC PATHWAYS(1).

The chl-specific short-term (14) C-based production (P(b) ) measurement is a widely used tool to understand phytoplankton responses to environmental stresses. However, among the metabolic consequences of these stresses is variability in lifetimes of newly fixed carbon that cause P(b) to range between chl-specific net primary production (NPP*) and chl-specific gross photosynthetic electron flow that is available for carbon reduction () depending on growth rate. To investigate the basis for this discrepancy, photosynthate utilization was characterized in Dunaliella tertiolecta Butcher grown at three different growth rates in N-limited chemostats. P(b) was measured throughout a 2 min to 24 h time course and showed clear growth-rate-dependent differences in lifetimes of newly fixed carbon. (14) C pulse-chase experiments revealed differences in patterns of carbon utilization between growth rates. At high growth rate, the majority of (14) C was initially fixed into polysaccharide and lipid, but the relative contribution of each labeled biochemical pool to the total label changed over 24 h. In fast-growing cells, labeled polysaccharides decreased 50%, while labeled lipids increased over the first 4 h. At low growth rate, (14) C was initially incorporated primarily into protein, but the contribution of labeled protein to the total label increased over the next 24 h. Together, time-resolved measurements of P(b) and cellular NAD and NADP content suggest an enhanced role for alternative dissipation pathways at very low growth rate. Findings of this study contribute to an integrated understanding of growth-rate-dependent shifts in metabolic processes from photosynthesis to net growth.

Amending Soils with Hydrogels Increases the Biomass of Nine Tree Species under Non‐water Stress Conditions

AbstractThe classical aim of the application of super absorbent polyacrylate (SAPs) hydrogels is the prolonging of plant survival under water stress. Their effect on plant growth during non‐water stress conditions is not known. This study examined the root and shoot biomass of seedlings of nine tree species; Eucalyptus grandis, Eucalyptus citriodora, Pinus caribaea, Araucaria cunninghamii, Melia volkensii, Grevillea robusta, Azadirachta indica, Maesopsis eminii and Terminalia superba. The seedlings were potted in five soil types; sand, sandy loam, loam, silt loam and clay. These were amended at two hydrogel levels: 0.2 and 0.4% w/w and grown under controlled conditions in a green house. Root and shoot growth responses of the seedlings were determined by measuring the dry weight of the roots, stems, leaves and twigs. The addition of either 0.2 or 0.4% hydrogel to the five soil types resulted in a significant increase of the root dry weight (p < 0.001) in eight tree species compared to the controls after 8 wk of routine watering. Also, the dry weight of stems and leaves and twigs were significantly (p < 0.001) higher in the nine tree species potted in hydrogel amended soil types than in the hydrogel free controls. These results suggested that hydrogel amendment enhances the efficiency of water uptake and utilization of photosynthates of plants grown in soils which have water contents close to field capacity.

Testing the “source–sink” hypothesis of down-regulation of photosynthesis in elevated [CO 2] in the field with single gene substitutions in Glycine max

Acclimation of photosynthesis to elevated atmospheric carbon dioxide concentration was tested in lines of soybean ( Glycine max) that differed by single genes that altered either the capacity to nodulate or growth habit (determinate or indeterminate growth). Both genetic changes provided, within a uniform genetic background, a test of the “source–sink” hypothesis that down-regulation of photosynthesis in elevated carbon dioxide is a result of inability to form sufficient “sinks” for the additional photosynthate. Plants were grown under ambient and elevated [CO 2] (550 μmol mol −1) in the field, using free air gas concentration enrichment (FACE). Mutation of the determinate cultivar, Elf, to an indeterminate form did not result in increased responsiveness to elevated [CO 2]. This may reflect a large sink capacity in the selection of determinate cultivars. In elevated [CO 2] only the determinate isoline of the indeterminate cultivar (Williams- dt1) and the non-nodulating genotype showed down-regulation of photosynthesis. This resulted from decreases in apparent in vivo Rubisco activity ( V c,max) and maximum rate of electron transport ( J max). Increase in total non-structural carbohydrate (TNC) content, which is often correlated with down-regulation of photosynthesis, in Williams- dt1 was 80% greater in elevated [CO 2] than in ambient [CO 2] controls, compared to 40% in the indeterminate line. The results from mutations of the Williams line are consistent with the hypothesis that genetic capacity for the utilization of photosynthate is critical to the ability of plants to sustain increased photosynthesis when grown at elevated [CO 2].

Interactions of nitrate and CO2 enrichment on growth, carbohydrates, and rubisco in Arabidopsis starch mutants. Significance of starch and hexose.

Wild-type (wt) Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were grown in hydroponics under two levels of nitrate, 0.2 versus 6 mM, and two levels of CO(2), 35 versus 100 Pa. Growth (fresh weight and leaf area basis) was highest in wt plants, lower in TL46, and much lower in TL25 plants under a given treatment. It is surprising that the inability to synthesize starch restricted leaf area development under both low N (N(L)) and high N (N(H)). For each genotype, the order of greatest growth among the four treatments was high CO(2)/N(H) > low CO(2)/N(H), > high CO(2)/N(L), which was similar to low CO(2)/N(L). Under high CO(2)/N(L), wt and TL46 plants retained considerable starch in leaves at the end of the night period, and TL25 accumulated large amounts of soluble sugars, indicative of N-limited restraints on utilization of photosynthates. The lowest ribulose-1,5-bisphosphate carboxylase/oxygenase per leaf area was in plants grown under high CO(2)/N(L). When N supply is limited, the increase in soluble sugars, particularly in the starch mutants, apparently accentuates the feedback and down-regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase, resulting in greater reduction of growth. With an adequate supply of N, growth is limited in the starch mutants due to insufficient carbohydrate reserves during the dark period. A combination of limited N and a limited capacity to synthesize starch, which restrict the capacity to use photosynthate, and high CO(2), which increases the potential to produce photosynthate, provides conditions for strong down-regulation of photosynthesis.

The role of growth rate, redox-state of the plastoquinone pool and the trans-thylakoid ΔpH in photoacclimation of Chlorella vulgaris to growth irradiance and temperature

The long-term photoacclimation of Chlorella vulgaris Beijer (UTEX 265) to growth irradiance and growth temperature under ambient CO2 conditions was examined. While cultures grew at a faster rate at 27 than at 5 degrees C, growth rates appeared to be independent of irradiance. Decreases in light-harvesting polypeptide accumulation, increases in xanthophyll pool size and changes in the epoxidation state of the xanthophyll cycle pigments were correlated linearly with increases in the relative reduction state of QA, the primary quinone receptor of photosystem II, when estimated as 1-qP under steady-state growth conditions. However, we show that there is also a specific temperature-dependent component, in addition to the redox-state of the QA, involved in regulating the content and composition of light-harvesting complex II of C. vulgaris. In contrast, modulation of the epoxidation state of the xanthophyll pool in response to increased 1-qP in cells grown at 5 degrees C was indistinguishable from that of cells grown at 27 degrees C, indicating that light and temperature interact in a similar way to regulate xanthophyll cycle activity in C. vulgaris. Because C. vulgaris exhibited a low-light phenotype in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), but a high-light phenotype upon addition of 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone, we conclude that the plastoquinone pool acts as a sensor regulating the accumulation of light-harvesting polypeptides in C. vulgaris. However, concomitant measurements of non-photochemical fluorescence quenching (qN) and the epoxidation state of the xanthophyll pool appear to indicate that, in addition to the redox-state of the plastoquinone pool, the trans-thylakoid deltapH may also contribute to sensing changes in irradiance and temperature that would lead to over-excitation of the photosynthetic apparatus. We suggest that sink capacity as reflected in photosynthate utilization and cell growth ultimately regulate photoacclimation in C. vulgaris.

Contrasting Effects of Carbon Dioxide and Irradiance on the Acclimation of Photosynthesis in Developing Soybean Leaves

Leaves developed at high irradiance (I) often have higher photosynthetic capacity than those developed at low I, while leaves developed at elevated CO2 concentration [CO2] often have reduced photosynthetic capacity compared with leaves developed at lower [CO2]. Because both high I and elevated [CO2] stimulate photosynthesis of developing leaves, their contrasting effects on photosynthetic capacity at maturity suggest that the extra photosynthate may be utilized differently depending on whether I or [CO2] stimulates photosynthesis. These experiments were designed to test whether relationships between photosynthetic income and the net accumulation of soluble protein in developing leaves, or relationships between soluble protein and photosynthetic capacity at full expansion differed depending on whether I or [CO2] was varied during leaf development. Soybean plants were grown initially with a photosynthetic photon flux density (PPFD) of 950 µmol m-2 s-1 and 350 µmol [CO2] mol-1, then exposed to [CO2] ranging from 135 to 1400 µmol mol-1 for the last 3 d of expansion of third trifoliolate leaves. These results were compared with experiments in which I was varied at a constant [CO2] of 350 µmol mol-1 over the same developmental period. Increases in area and dry mass over the 3 d were determined along with daily photosynthesis and respiration. Photosynthetic CO2 exchange characteristics and soluble protein content of leaves were determined at the end of the treatment periods. The increase in leaflet mass was about 28 % of the dry mass income from photosynthesis minus respiration, regardless of whether [CO2] or I was varied, except that very low I or [CO2] increased this percentage. Leaflet soluble protein per unit of area at full expansion had the same positive linear relationship to photosynthetic income whether [CO2] or I was varied. For variation in I, photosynthetic capacity varied directly with soluble protein per unit area. This was not the case for variation in [CO2]. Increasing [CO2] reduced photosynthetic capacity per unit of soluble protein by up to a factor of 2.5, and photosynthetic capacity exhibited an optimum with respect to growth [CO2]. Thus CO2 did not alter the relationship between photosynthetic income and the utilization of photosynthate in the net accumulation of soluble protein, but did alter the relationship between soluble protein content and photosynthetic characteristics in this species.

A polypeptide with similarity to phycocyanin alpha-subunit phycocyanobilin lyase involved in degradation of phycobilisomes.

To optimize the utilization of photosynthate and avoid damage that can result from the absorption of excess excitation energy, photosynthetic organisms must rapidly modify the synthesis and activities of components of the photosynthetic apparatus in response to environmental cues. During nutrient-limited growth, cyanobacteria degrade their light-harvesting complex, the phycobilisome, and dramatically reduce the rate of photosynthetic electron transport. In this report, we describe the isolation and characterization of a cyanobacterial mutant that does not degrade its phycobilisomes during either sulfur or nitrogen limitation and exhibits an increased ratio of phycocyanin to chlorophyll during nutrient-replete growth. The mutant phenotype was complemented by a gene encoding a polypeptide with similarities to polypeptides that catalyze covalent bond formation between linear tetrapyrrole chromophores and subunits of apophycobiliproteins. The complementing gene, designated nblB, is expressed at approximately the same level in cells grown in nutrient-replete medium and medium devoid of either sulfur or nitrogen. These results suggest that the NblB polypeptide may be a constitutive part of the machinery that coordinates phycobilisome degradation with environmental conditions.

Evidence for the Occurrence of Feedback Inhibition of Photosynthesis in Rice

The response of photosynthesis in the flag leaf of rice (Oryza sativa) to elevated CO2 or reduced O2 was investigated relative to other environmental factors using steady-state gas exchange techniques. We found under moderate conditions of temperature and photosynthetic flux density (PFD) (26°C and 700μmol quanta m−2s−1 similar to growth conditions) photosynthesis in the flag leaf of rice during heading and grain filling saturated at near ambient levels of CO2, with a concomitant loss of O2 sensitivity, when a high stomatal conductance was maintained by high humidity (low vapor pressure deficit). Under 18°C there was near complete loss of O2 sensitivity of photosynthesis at normal ambient levels of CO2. This is in contrast to the large enhancement of photosynthesis by supra-atmospheric levels of CO2 and sub-atmospheric levels of O2 by suppression of photorespiration when there is no limitation on utilizing the initial product of CO2 assimilation (triose-P) as predicted from Ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic properties. Thus, loss of sensitivity to CO2 and O2 has been previously explained as a limitation on utilization of triose-P to synthesize carbohydrates. Under high PFD at 25°C, the rate of photosynthesis in rice declined over a period of hours around midday, while the intercellular levels of CO2 remained constant suggesting a limitation on utilization of photosynthate. Short-term fluctuations in climatic factors including temperature, light and humidity could result in a feedback limitation on photosynthesis in rice which may be exacerbated by rising CO2.

A source-sink framework for coupling water, carbon, and nutrient dynamics of vegetation.

Feedbacks between photosynthesis and growth and the influence on these relationships of environmental factors are reviewed. The evidence indicates a strong dependence of photosynthesis on photosynthate utilization. It also indicates that growth is more sensitive than photosynthesis to water and mineral nutrient stress. As a consequence, many relationships between photosynthesis and external driving variables are confounded by internal feedback effects that reflect the influence of external factors on growth. A source-sink framework for modeling carbon dynamics with coupled water and nutrient interactions in soil-plant systems is outlined. Simulations based on these concepts suggest that sink feedback regulation of photosynthesis at various times in diurnal, wetting-drying, and annual cycles is probably a common occurrence in soil-plant systems.

Tansley Review No. 27 The control of carbon partitioning in plants.

This review reports on the processes associated with carbon transfer and metabolism in leaves and growing organs and the role of long-distance transport and vascular links in the regulation of carbon partitioning in plants. Partitioning is clearly influenced by both the supply and demand for photosynthate and is moderated by vascular connections and the storage capacity of the leaves and pathway tissues. However there appears to be little more than circumstantial evidence either that short distance transfer of carbon within either the source or the sink, or that long-distance transport in the phloem, are limiting photosynthesis or growth directly. Although individual biochemical and physiological processes relating to photosynthesis and growth may be well understood, the factors primarily responsible for the control of carbon partitioning in plants have not been clearly identified. There is a need for a greater understanding of organ initiation and development (source and sink formation and potential size), the clear identification of whether growth is sink or source limited (including possible sink-controlled photosynthesis) and a detailed assessment of the role of storage in buffering developmental and environmental changes in sink and source activity. Also more information is needed on the role of hormonal and nutritional factors in regulating source and sink activity (organ interactions not directly associated with carbon transfer). CONTENTS Summary 341 I. Introduction 342 II. General source-sink relationships 342 III. Control at the source 345 IV. The utilization of photosynthate: sink characteristics and limitations 353 V. Vascular constraints and temporary storage 360 VI. Concluding comments 366 Acknowledgements 366 References 367.

Influence of Leaf Age on Photosynthesis, Enzyme Activity, and Metabolite Levels in Wheat

The rate of photosynthesis under high light (1000 micromole quanta per square meter per second) and at 25 degrees C was measured during development of the third leaf on wheat plants and compared with the activity of several photosynthetic enzymes and the level of metabolites. The rate of photosynthesis reached a maximum the 7th day after leaf emergence and declined thereafter. There was a high and significant correlation between the rate of photosynthesis per leaf area and the activities of the enzymes ribulose 5-phosphate kinase (r = 0.91), ribulose 1,5-bisphosphate (RuBP) carboxylase (r = 0.94), 3-phosphoglycerate (PGA) kinase (r = 0.82), and fructose 1,6-bisphosphatase (r = 0.80) per leaf area. There was not a significant correlation of photosynthesis rate with chlorophyll content. The rate of photosynthesis was strongly correlated with the level of PGA (r = 0.85) and inversely correlated with the level of triose phosphate (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate) (r = 0.92). RuBP levels did not change much during leaf development; therefore photosynthesis rate was not correlated with the level of RuBP. The rate of photosynthesis was at a maximum when the ratio of PGA/triose phosphate was high, and when the ratio of RuBP/PGA was low. Although several enzymes change in parallel with leaf development, the metabolite changes suggest the greatest degree of control may be through RuBP carboxylase. The sucrose content of the leaf was highest under high rates of photosynthesis. There was no evidence that later in leaf development, photosynthesis (measured under high light and at 25 degrees C) was limited by utilization of photosynthate.