Sink Demand Research Articles

Fast and simple fluorometric measurement of phloem loading exposes auxin-dependent regulation of Arabidopsis sucrose transporter AtSUC2.

The rate of sucrose export from leaves is a major factor in balancing whole-plant carbon and energy partitioning. A comprehensive study of its dynamics and relationship to photosynthesis, sink demand, and other relevant processes is hampered by the shortcomings of current methods for measuring sucrose phloem loading. We utilize the ability of sucrose transporter proteins, known as SUCs or SUTs, to specifically transport the fluorescent molecule esculin in a novel assay to measure phloem loading rates. Esculin was administered to source leaves and its fluorescence in the leaf extract was measured after 1 or 2 h. Dicot plants with an active phloem loading strategy showed an export-dependent reduction of esculin fluorescence. Relative leaf esculin export rates correlated with leaf export rates of isotopic carbon and phloem exudate sucrose levels. We used esculin experiments to examine the effects of phytohormones on phloem loading in Arabidopsis, showing, for example, that auxin induces phloem loading while cytokinin reduces it. Transcriptional regulation of AtSUC2 by AUXIN RESPONSE FACTOR1 (ARF1) corroborated the link between auxin signaling and phloem loading. Unlike established methods, the esculin assay is rapid and does not require specialized equipment. Potential applications and limitations of the esculin assay are discussed.

A Hydroponic System to Study the Effects of Root and Meristem Night Temperature on Growth, Photosynthesis Carbon Balance, and Antioxidant Enzymes in Rice

Nocturnal root and meristem temperature (RMT) can have a strong effect on rice growth and yield. However, underlying mechanisms are not well understood. To investigate the effects of night-time RMT on photosynthesis biomass allocation and activities of antioxidant enzymes, we designed a hydroponic system that maintained the following daily patterns of day/night temperature: 18/28 °C (HNT) or 28/18 °C (LNT). Rice plants cv. IR64 were grown in the greenhouse and subjected to either HNT or LNT. HNT stimulated growth and tillering but did not affect biomass allocation. HNT plants increased total biomass by 16 and 35%, depending on time of exposure. HNT increased rates of photosynthesis (Pn) compared to LNT plants in leaves of different ages. Overnight carbohydrate remobilisation was larger in HNT than in LNT plants, particularly at 16 days after treatment (dat), when Pn and relative growth rates were highest. Leaf soluble protein concentrations and specific leaf area were not affected by RMT, indicating higher photosynthetic nitrogen use efficiency in HNT plants. Super Oxide Dismutase, Ascorbate Peroxidase, and Glutathione Reductase activities did not respond to RMT, indicating no change in the production of reactive oxygen species in LNT plants despite lower photosynthesis rates. HNT increased sink demand by stimulating tillering, the increased sink demand upregulated the source activity through a larger leaf area per plant and a higher Pn throughout the canopy. The hydroponic system described here was able to control the temperature of the nutrient solution effectively, the installation of a second pump directly circulating the nutrient solution from and back to the reservoir through the cooling system allowed reaching the target temperature within 1 h. This system opens new opportunities to characterise plant responses to RMT alone or in combination with other environmental drivers.

A Waste Heat Assessment of a Manufacturing Facility With Consideration for Demand Matching Through Thermal Energy Storage

Abstract A waste heat assessment of a manufacturing facility was conducted. A physical survey identified potential waste heat sources. For these sources, the temporal fluctuations in temperature and flowrate, and the frequency of occurrence of the waste heat, were determined from measurements, calculations, and company records. The energy and exergy of the waste heat were calculated. The total annual waste heat is 36.5 TJ of energy, which represents 0.84 TJ of exergy. The boiler, plant vacuums, and chiller comprise 96% of the energy rejected and are the major contributors to the rejected exergy. Using the waste heat for space heating is evaluated. Based on fuel consumption in the boiler, the annual energy for space heating is estimated at 8.5 TJ. Theoretically, waste heat from the boiler flue, polypropylene dryers, and plant vacuums could meet up to 39% of this energy without storage. Simply allowing the vacuum exhausts to cool in the building could theoretically meet up to 27%. Models are developed to investigate using waste heat recovery and thermal energy storage (TES) to provide space heating for a prototype warehouse building, and estimates for the initial costs of the recovery system are developed. Modeling indicates that TES is highly beneficial for matching short-term demand fluctuations and for smoothing the temporal oscillations in the waste heat. A nine-month heating season is simulated to determine the fraction of the load met by the recovery system. A small amount of thermal storage, e.g., 12.5–25% of daily waste heat, improved the fraction of demand that could be met. Larger sizes had diminishing returns and significantly increased the initial cost and payback period. To reduce initial cost, a refined recovery system design that uses a TES as an intermediate and for storage is proposed. The study highlights the need to consider fluctuations in the waste heat supply and sink demand, for thermal storage, and to identify relatively simple modifications to recover waste heat.

Smart nutrient management Nutrient Expert® enhances rice productivity through adjusting source-sink relationships during grain filling

Smart nutrient management optimises chemical fertiliser application while maintaining crop yields. Nutrient Expert® (NE) is a user-friendly smart nutrient management system designed to improve the productivity of smallholder farmers. However, the physiological mechanisms underlying the effects of nutrient management have yet to be fully understood. We analysed data from field experiments conducted in South and Northeast China to explore the effects of different nitrogen (N) fertiliser inputs and nutrient management systems on panicle weight and post-flowering source-sink relationships in various cropping seasons of rice. Treatments included NE recommendation, local farmers’ practices (FP), soil test based fertiliser recommendation (ST), and four N rate treatments with plus or minus 25 % and plus or minus 50 % of NE. Our analysis indicated that rice panicle weight and N accumulation increased with increasing N application rate up to a recommended rate by NE in most cases, beyond which there was little further increase. Moreover, NE significantly enhanced rice panicle weight and N accumulation compared to FP and ST in the single-season rice in Northeast China. In all experiments, both in South and Northeast China, the post-flowering source-sink difference gradually increased from negative to positive values with an increase in N application rate. Too low N application rate resulted in insufficient source supply, while too high N application rate gave excessive source supply since the sink demand reached its upper limit. Compared with FP and ST, NE achieved higher source supply and sink demand in the single-season rice, and mobilised more N from source organs to sink organs. Our analysis provides evidence for positive impacts of integrated nutrient management strategies on rice source-sink relationships and offers a physiological basis for improved rice productivity of designed NE nutrient management strategies.

Sugar sensing in C4 source leaves: a gap that needs to be filled.

Plant growth depends on sugar production and export by photosynthesizing source leaves and sugar allocation and import by sink tissues (grains, roots, stems, and young leaves). Photosynthesis and sink demand are tightly coordinated through metabolic (substrate, allosteric) feedback and signalling (sugar, hormones) mechanisms. Sugar signalling integrates sugar production with plant development and environmental cues. In C3 plants (e.g. wheat and rice), it is well documented that sugar accumulation in source leaves, due to source-sink imbalance, negatively feeds back on photosynthesis and plant productivity. However, we have a limited understanding about the molecular mechanisms underlying those feedback regulations, especially in C4 plants (e.g. maize, sorghum, and sugarcane). Recent work with the C4 model plant Setaria viridis suggested that C4 leaves have different sugar sensing thresholds and behaviours relative to C3 counterparts. Addressing this research priority is critical because improving crop yield requires a better understanding of how plants coordinate source activity with sink demand. Here we review the literature, present a model of action for sugar sensing in C4 source leaves, and suggest ways forward.

Beyond source and sink control-toward an integrated approach to understand the carbon balance in plants.

A conceptual understanding on how the vegetation's carbon (C) balance is determined by source activity and sink demand is important to predict its C uptake and sequestration potential now and in the future. We have gathered trajectories of photosynthesis and growth as a function of environmental conditions described in the literature and compared them with current concepts of source and sink control. There is no clear evidence for pure source or sink control of the C balance, which contradicts recent hypotheses. Using model scenarios, we show how legacy effects via structural and functional traits and antecedent environmental conditions can alter the plant's carbon balance. We, thus, combined the concept of short-term source-sink coordination with long-term environmentally driven legacy effects that dynamically acclimate structural and functional traits over time. These acclimated traits feedback on the sensitivity of source and sink activity and thus change the plant physiological responses to environmental conditions. We postulate a whole plant C-coordination system that is primarily driven by stomatal optimization of growth to avoid a C source-sink mismatch. Therefore, we anticipate that C sequestration of forest ecosystems under future climate conditions will largely follow optimality principles that balance water and carbon resources to maximize growth in the long term.

Pod Production Dynamics and Pod Size Distribution of Theobroma cacao L. Clone CCN 51 in Full Sunlight

Cacao fruit production dynamics vary from one location to another and are conditioned by the number of pods produced per tree. During cocoa pod development, the strength of the carbon sink varies depending on the demand exerted by the pods, which is proportional to the size. The relationship between cocoa pod production dynamics and size distribution is still poorly understood. Dissecting this relationship is an important step toward further improving cocoa crop management. In this study, the annual yield dynamics and quantity of cocoa pods produced by popular, highly productive, and widespread clone CCN 51 were investigated, based on six size classes observed during its fructification. Growth parameters were determined as weekly increments of pod length and diameter, whereas daily increments were estimated using the logistic Richards model. The fruiting cycle was characterized by the coexistence of fruits of various sizes where the number of pods belonging to each size class changes throughout the fruiting season. Fruit production varied following a seasonal pattern, reaching a maximum of 36 pods/tree, in trees cultivated in full sunlight, of which approximately 55% matured and were harvested. The peak carbon sink demand occurs when the tree pods have the highest numbers of pods. During this period, 65% of the pods had lengths between 5 and 15 cm, which corresponds to the period of the highest pod growth rate. The average length values of the harvested pods were generally below 23 cm and rarely exceeded 7 pods/tree. The Richard model proved to describe accurately the pod growth rates for CCN 51. This represents a promising tool to determine pod growth in other cultivars of relevance for the cocoa industry, which is essential to improve cocoa crop management.

A time-window within the grain filling period accounted for early leaf senescence in maize under sink limitation

In annual crops such as maize, assimilates demand exerted by grains converges with increasing leaf senescence. As sink demand changes as grains growth, leaf senescence could also be affected during the grain filling period. In this work, we evaluated the effect of a decrease in sinks demand at different moments of the grain filling period on maize leaf senescence. Sinks were modified in field experimentation via restriction of pollination or ear removal at four different times during the grain filling period. These growth and senescence-related-variables were periodically assessed: dry matter, stem carbohydrates, leaf initiation, leaf area, chlorophyll, sap flow and photosynthesis. Restriction of pollination and ear removal increased stem carbohydrates concentration and stem dry matter. Leaf senescence was accelerated only when ear was removed at R3 and R4 phenological stages. The magnitude of the effect was leaf position-dependent. A simple model relating leaf area duration or the area under the chlorophyll evolution curve with leaves age accounted for the observed results. Protecting grains during this window would avoid a decrease in grain yield both due to direct damage to the grains and the source reduction for advanced leaf senescence. These results would be of interest to producers, breeders and modelers of this crop.

Genetic architecture of source-sink-regulated senescence in maize.

Source and sink interactions play a critical but mechanistically poorly understood role in the regulation of senescence. To disentangle the genetic and molecular mechanisms underlying source-sink-regulated senescence (SSRS), we performed a phenotypic, transcriptomic, and systems genetics analysis of senescence induced by the lack of a strong sink in maize (Zea mays). Comparative analysis of genotypes with contrasting SSRS phenotypes revealed that feedback inhibition of photosynthesis, a surge in reactive oxygen species, and the resulting endoplasmic reticulum (ER) stress were the earliest outcomes of weakened sink demand. Multienvironmental evaluation of a biparental population and a diversity panel identified 12 quantitative trait loci and 24 candidate genes, respectively, underlying SSRS. Combining the natural diversity and coexpression networks analyses identified 7 high-confidence candidate genes involved in proteolysis, photosynthesis, stress response, and protein folding. The role of a cathepsin B like protease 4 (ccp4), a candidate gene supported by systems genetic analysis, was validated by analysis of natural alleles in maize and heterologous analyses in Arabidopsis (Arabidopsis thaliana). Analysis of natural alleles suggested that a 700-bp polymorphic promoter region harboring multiple ABA-responsive elements is responsible for differential transcriptional regulation of ccp4 by ABA and the resulting variation in SSRS phenotype. We propose a model for SSRS wherein feedback inhibition of photosynthesis, ABA signaling, and oxidative stress converge to induce ER stress manifested as programed cell death and senescence. These findings provide a deeper understanding of signals emerging from loss of sink strength and offer opportunities to modify these signals to alter senescence program and enhance crop productivity.

The investigation of treatment design parameters on carbon integration networks

Carbon Integration methods help identify the appropriate allocation of captured carbon dioxide (CO2) streams into CO2-using sinks, and are especially useful when a number of CO2 sink options are present simultaneously. The method helps identify CO2 allocation scenarios when subjected to an emission target on the CO2 overall network. Many carbon dioxide sink options are costly, and more often than not, require a high purity carbon dioxide source to satisfy the sink demand. Hence, it is imperative to effectively incorporate treatment units in such networks, to obtain high-purity CO2 streams. In fact, it has been previously reported in many studies that the most expensive step in Carbon Capture, Utilization and Sequestration (CCUS) is the treatment system. As a result, this paper focuses on reassessing the performance of carbon integration networks using a more rigorous cost model for the treatment design stage. The effect of utilizing different treatment operating conditions on the overall cost of the treatment stage of CO2 (before allocation) is first captured using a detailed cost model. Subsequently, this information is then fed into a network design problem that involves a CO2 source-sink allocation network problem, and different CO2 net capture targets within the network. For this, an enhanced treatment model that captures all necessary treatment design parameters has been utilized alongside the original model. The original carbon integration formulation has been adopted from previous work. Many of the cost items have been lumped into single parameters in the original formulation, and lack the necessary depth required to carry out the necessary investigations for this work. Hence, the treatment model introduced in this paper is more rigorous, as it accounts for important technical performance constraints on the system to be assessed. Utilizing a more detailed cost model was found to be very helpful in understanding several effects of varying parameters on the overall source-sink allocations, when subjected to different CO2 net emission reduction targets. The cost of the carbon network increases when the solvent temperatures are increased. However, there was a noticeable linear trend at lower temperatures compared to higher temperatures, where the increase became non-linear. Furthermore, it was discovered that for net capture targets of 20% and 25%, no revenue from carbon storage could be generated beyond a solvent temperature of 25 °C. Additionally, the optimal diameter of the treatment column was more responsive to changes in solvent temperature for cases with low net capture targets (below 10%), while its sensitivity decreased for higher capture targets (above 10%).Graphical

Effects of Source Strength and Sink Size on Starch Metabolism, Starch Properties and Grain Quality of Rice (Oryza sativa L.)

The source strength and sink demand as well as their interaction have been demonstrated to co-regulate the synthesis of starch and determine the grain quality, but the knowledge of the underlying physiological mechanisms is limiting. An indica variety, Yangdao 6, and a japonica variety, Jinxiangyu 1, were planted with three treatments, including normal growth plant (CK), leaf-cutting (LC) and spikelet-thinning (ST). The transcript levels of starch metabolic genes, physicochemical characteristics of starch and appearance, milling, cooking and tasting qualities of rice under different treatments were determined. The ST treatment increased the relative expression of genes related to the synthesis of short branch-chains of amylopectin (SSI, BEI, BEIIb) and amylose (GBSSI) and reduced the relative expression of medium-long to long branch-chains of amylopectin synthesis genes (SSIIa, SSIIIa, SSIIIb, ISA1). When comparing ST with the CK treatment, starch granules became smoother with higher contents of short branch-chains and lower ratios of medium-long and long branch-chains of amylopectin; the crystallinity and the value of 1045/1022 cm−1 was decreased; for pasting properties, the setback and pasting temperature were decreased; the peak viscosity, hot viscosity, breakdown and final viscosity were significantly increased. Meanwhile, the ST treatment improved the appearance, milling and cooking and tasting qualities. The opposite results were observed under the LC treatment. These results indicated that source strength and sink size would regulate expression levels of starch metabolic genes, which is pivotal for the contents of amylose and short/long branch chains ratio of amylopectin, thus changing the structure and physicochemical properties of starch and grain quality. Here, we speculated that the improved source strength in terms of higher leaf/canopy photosynthesis and small sink size, such as small panicle size, would be preferred traits in high grain quality rice breeding.

Combination of suitable planting density and nitrogen rate for high yield maize and their source-sink relationship in Northwest China.

Increasing crop yield per unit area by increasing planting density is essential to ensure food security. However, the optimal combination of planting density and N application for high-yielding maize and its source-sink characteristics need to be more clearly understood. A two-year field experiment was conducted combining three planting densities (D1: 70,000 plants ha-1 ；D2: 100,000 plants ha-1 ；D3: 130,000 plants ha-1 ) and three nitrogen rates (N1: 150 kg N hm-2 , N2: 350 kg N hm-2 , N3: 450 kg N hm-2 ). The results showed that increasing planting density significantly increased leaf area index and grain yield, but negatively affected ear traits. The Richards model was used to fit the dynamic changes of dry matter accumulation of maize under different treatments, and the fitting results were good. Increasing planting density increased population yield while limiting the development of individual plants, bringing the period of rapid dry matter accumulation to an early end and accelerating leaf senescence. Appropriate N rate could prolong the period of rapid accumulation of dry matter in maize, increase the 100-kernel weight. Increasing planting density enhanced post-silking dry matter accumulation to a lesser extent, and the source-sink relationship of the maize population gradually developed from sink limitation to source limitation with increasing planting density. The decrease in yield due to the insufficient source strength to meet the sink demand at too high densities was the reason that limited further improvement of the optimal planting density. Appropriate N rate facilitated the realization of yield potential at high density. This article is protected by copyright. All rights reserved.

Relative contribution of soil N availability and grain sink demand to the control of post-anthesis N uptake by field-grown spring barley

Relative contribution of soil N availability and grain sink demand to the control of post-anthesis N uptake by field-grown spring barley

Does constant or changing light give the best rooting of hibiscus cuttings of two sizes?

In this experiment the effect of light intensity (photosynthetic photon flux density, PPFD) and cutting size (10 and 7 cm) was investigated during five weeks rooting of hibiscus (Hibiscus rosa-sinensis L. ‘Adonicus Yellow’). The light regimes included three constant PPFD: 16, 76 and 151 µmol m−2 s−1; and three changing PPFD after two weeks: 16 → 76, 16 → 151 and 76 → 151 µmol m−2 s−1. The light intensity has a dual effect, promoting photosynthesis and carbohydrate production, but also water loss through transpiration with the potential for desiccating the cutting. The treatments starting with 16 µmol m−2 s−1 insufficiently supported the rooting process, illustrated by the lack shoot growth after two weeks. Within each treatment type (constant or changing PPFD) the root formation was linearly correlated to the total light integral for the propagation period (r2 = 0.835–1.000) for respective cutting size. The changing PPFD treatments showed the strongest response to increasing light integral with the [76 → 151] treatment producing most roots for both cutting sizes, despite the constant 151 µmol m−2 s−1 having the highest total light integral. This indicates that conserving water is of crucial importance before root emergence in the third week of rooting. The 7 cm cuttings produced more root biomass and had higher root:shoot ratio than the 10 cm cuttings. The 7 cm cuttings had higher rates of photosynthesis one week after rooting, probably due to higher sink demand from the growing roots and mobilised more soluble sugars from the leaves. The rooting process takes five weeks and does not demand high light, so the results are promising for moving propagation of hibiscus from the greenhouse to a vertical production system based on light emitting diodes (LEDs) to save production area for final production.

Terminal heat stress in Indian mustard (Brassica juncea L.): Variation in dry matter accumulation, stem reserve mobilization, carbohydrates translocation and their correlation with seed yield

The rapeseed mustard is one of the most important sources of edible oil in India and contributes 28.6% in total oilseed production. The mustard growing areas in India are experiencing the vast diversity in the agro climatic conditions. Here, we studied forty-nine advanced breeding lines of Brassica juncea L. for two consecutive years (2016-18) to examine the variations in the remobilization of assimilates from flowering to maturity stage and their contribution to seed filling under stressed environment. Further, we investigated the impact of high temperature on dry matter accumulation and partitioning from source to sink in Brassica germplasm. The synchronization between the seed filling stage and the onset of heat spell is critical event that determines the overall yield. Imbalances caused due to miss-matching of above events created hindrance in source-sink translocation, thus resulted in yield losses. Amount of remobilized dry matter, remobilization efficiency and remobilization percentage increased significantly, while the dry matter accumulation, total carbohydrates content and seed yield per plant declined in the late sown genotypes during both crop seasons. Reduced accumulation of photo assimilates under stress and higher sink demand resulted in more number of shriveled seeds leading to yield depression. The higher remobilization efficiency in late sown genotypes was strongly associated with dry matter at flowering that consequently tended to affect the final seed weight. This study will provide insights for better understanding of source-sink relationships in Indian mustard under heat stress and the differential remobilization efficiencies in the advanced breeding lines.

Elucidating the role of SWEET13 in phloem loading of the C4 grass Setaria viridis.

Photosynthetic efficiency and sink demand are tightly correlated with rates of phloem loading, where maintaining low cytosolic sugar concentrations is paramount to prevent the downregulation of photosynthesis. Sugars Will Eventually be Exported Transporters (SWEETs) are thought to have a pivotal role in the apoplastic phloem loading of C4 grasses. SWEETs have not been well studied in C4 species, and their investigation is complicated by photosynthesis taking place across two cell types and, therefore, photoassimilate export can occur from either one. SWEET13 homologues in C4 grasses have been proposed to facilitate apoplastic phloem loading. Here, we provide evidence for this hypothesis using the C4 grass Setaria viridis. Expression analyses on the leaf gradient of C4 species Setaria and Sorghum bicolor show abundant transcript levels for SWEET13 homologues. Carbohydrate profiling along the Setaria leaf shows total sugar content to be significantly higher in the mature leaf tip compared with the younger tissue at the base. We present the first known immunolocalization results for SvSWEET13a and SvSWEET13b using novel isoform-specific antisera. These results show localization to the bundle sheath and phloem parenchyma cells of both minor and major veins. We further present the first transport kinetics study of C4 monocot SWEETs by using a Xenopus laevis oocyte heterologous expression system. We demonstrate that SvSWEET13a and SvSWEET13b are high-capacity transporters of glucose and sucrose, with a higher apparent Vmax for sucrose, compared with glucose, typical of clade III SWEETs. Collectively, these results provide evidence for an apoplastic phloem loading pathway in Setaria and possibly other C4 species.

Changes in photosynthesis and chlorophyll a fluorescence in relation to leaf temperature from just before to after harvest of Vitis vinifera cv. Shiraz vines grown in outdoor conditions.

Harvesting fruit from horticultural species causes a down-regulation of photosynthesis but some species can recover after harvest. The objective of this study was to assess the hypothesis that the impact of fruit removal on the photosynthetic performance of Shiraz grapevines, in relation to CO2 concentration and leaf temperature, would contribute to a depreciation in photosynthetic assimilation. To assess this hypothesis, vines that were continuously vegetative were compared with vines that were harvested when fruit were ripe. These fruiting vines had higher rates of CO2 -limited photosynthesis at all leaf temperatures compared to vegetative vines before harvest but after, photosynthetic rates were highest in vegetative vines. There were few treatment differences in CO2 -saturated photosynthesis before harvest but after, below about 30°C, the harvested vines had higher photosynthesis than the vegetative vines. Maximum rates of ribulose 1,5 bisphosphate (RuBP) carboxylation and regeneration and responses to temperature were unaffected by differences in sink demand but after harvest, maximum rates increased, but markedly more in the vegetative vines, especially at higher temperatures. This conformed to higher photosynthetic rates in the vegetative vines. There were no sink demand effects on chlorophyll a fluorescence, consistent with the evidence that the fruit sink removal probably affected Rubisco activity and performance. The conclusion that sink removal caused a depreciation in photosynthesis was sustained but the temperature had a strong modulating effect through both stomatal and non-stomatal limitations driving the depreciation in assimilation. What was less clear was why assimilation of continuously vegetative vines increased during the harvest time when there were no apparent changes in sink demand.

Cotton phloem loads from the apoplast using a single member of its nine-member sucrose transporter gene family.

Phloem loading and transport are fundamental processes for allocating carbon from source organs to sink tissues. Cotton (Gossypium spp.) has a high sink demand for the cellulosic fibers that grow on the seed coat and for the storage reserves in the developing embryo, along with the demands of new growth in the shoots and roots. Addressing how cotton mobilizes resources from source leaves to sink organs provides insight into processes contributing to fiber and seed yield. Plasmodesmata frequencies between companion cells and flanking parenchyma in minor veins are higher than expected for an apoplastic loader, and cotton's close relatedness to Tilia spp. hints at passive loading. Suc was the only canonical transport sugar in leaves and constituted 87% of 14C-labeled photoassimilate being actively transported. [14C]Suc uptake coupled with autoradiography indicated active [14C]Suc accumulation in minor veins, suggesting Suc loading from the apoplast; esculin, a fluorescent Suc analog, did not accumulate in minor veins. Of the nine sucrose transporter (SUT) genes identified per diploid genome, only GhSUT1-L2 showed appreciable expression in mature leaves, and silencing GhSUT1-L2 yielded phenotypes characteristic of blocked phloem transport. Furthermore, only GhSUT1-L2 cDNA stimulated esculin and [14C]Suc uptake into yeast, and only the GhSUT1-L2 promoter caused uidA (β-glucuronidase) reporter gene expression in minor vein phloem of Arabidopsis thaliana. Collectively, these results argue that apoplastic phloem loading mediated by GhSUT1-L2 is the dominant mode of phloem loading in cotton.

Physiology and whole-plant carbon partitioning during stem sugar accumulation in sweet dwarf sorghum.

A greater rate of phloem unloading and storage in the stem, not a higher rate of sugar production by photosynthesis or sugar export from leaves, is the main factor that results in sugar accumulation in sweet dwarf sorghum compared to grain sorghum. At maturity, the stem internodes of sweet sorghum varieties accumulate high concentrations of fermentable sugars and represent an efficient feedstock for bioethanol production. Although stem sugar accumulation is a heritable trait, additional factors that drive sugar accumulation in sorghum have not been identified. To identify the constraints on stem sugar accumulation in sweet sorghum, we used a combination of carbon-11 (11C) radiotracer, physiological and biochemical approaches, and compared a grain sorghum and sweet dwarf sorghum line that have similar growth characteristics including height. Photosynthesis did not increase during development or differ between the sorghum lines. During the developmental transition to the reproductive stage, export of 11C from leaves approximately doubled in both sorghum lines, but 11C export in the sweet dwarf line did not exceed that of the grain sorghum. Defoliation to manipulate relative sink demand did not result in increased photosynthetic rates, indicating that the combined accumulation of C by all sink tissues was limited by the maximum photosynthetic capacity of source leaves. Nearly 3/4 of the 11C exported from leaves was transported to the lower stem in sweet sorghum within 2h, whereas in grain sorghum nearly 3/4 of the 11C was in the panicle. Accordingly, the transcripts of several sucrose transporter (SUT) genes were more abundant in the stem internodes of the sweet dwarf line compared to the grain sorghum. Overall, these results indicate that sugar accumulation in sweet sorghum stems is influenced by the interplay of different sink tissues for the same sugars, but is likely driven by elevated sugar phloem unloading and uptake capacity in mature stem internodes.

Low sink demand caused net photosynthetic rate decrease is closely related to the irrecoverable damage of oxygen-releasing complex and electron receptor in peach trees

Source sink balance is one of the major determinants of carbon partitioning in plants. However, its effects on photosynthesis in fruit trees are largely unknown. In this work, the effects of low sink demand on net photosynthetic rate (Pn) and chlorophyll fluorescence after fruit removal (-fruit) in peach (Prunus persica (L.) Batsch cv. ‘Zaojiubao’) trees were investigated. The stepwise energy flow through photosystem II (PSII) at the reaction center (RC) was analyzed with quantitative analyses of fluorescence transient, also called JIP-test. We found that Pn was significantly lower and closely correlated to the leaf stomatal conductance (Gs) of -fruit trees than that of fruit retained (+fruit) trees. Leaf temperature (Tleaf) of -fruit trees was remarkably higher than that of +fruit trees. Day-time-period assays of chlorophyll (Chl) fluorescence revealed that, in the leaves of -fruit trees, the fluorescence parameters, such as NPQ (non-photochemical quenching coefficient) and ΦD0 (maximum quantum yield of non-photochemical de-excitation), decreased in the morning and recovered to the normal level in the afternoon, whereas other parameters, such as ΦE0 (quantum yield for electron transport at t = 0), Ψ0 (probability that a trapped exciton moves an electron to QA pool), F0 (minimum fluorescence, when all PSII RCs are open) and Wk (relative variable fluorescence at 300 μs of the chlorophyll fluorescence transient), did not. These results suggest that OEC complex and QA pool were irreversibly affected by low sink demand, whereas light harvest antenna and PSII potential efficiency retained a strong ability to recover.