Sink Demand Research Articles

INFLORESCENCE SINK DEMAND AND LEAF BLACKENING IN PROTEA NERIIFOLIA.

A major postharvest problem of Protea neriifolia is premature leaf blackening. Carbohydrate stress, due to floral sink demand, may lead to cellular disorganization and leaf blackening. Leaf blackening, nonstructural carbohydrates, ethylene, carbon exchange rates, stomatal conductance and lipid peroxidation were measured on leaves of vegetative and floral stems preharvest, and during a 7 day dark postharvest period. Postharvest treatments were: 0 or 0.5% sucrose in the vase solution, 20% sucrose pulse, or floral decapitation. Leaf blackening was significantly reduced in vegetative stems and floral stems in the 20% pulse treatment, in comparison to all other treatments. Ethylene production and lipid peroxidation were not associated with leaf blackening in any treatment and leaf respiration rates declined for all treatments over time. The magnitude and rate of leaf blackening was inversely related to leaf starch concentrations, with greatest carbohydrate depletion occurring within 24 h of harvest (by 75-85%). Leaf starch from the 20% pulse treatment increased by 300%, in contrast to declining starch concentrations in all other treatments. The data suggest that the flowerhead functions as the major sink for carbohydrate depletion leading to subsequent leaf blackening.

Changes in Post-anthesis Assimilates in Stem and Spike Components of Italian Ryegrass (Lolium multiflorum Lam.). I. Water Soluble Carbohydrates*

Stem carbohydrate reserves, in ryegrass grown for seed, may play a vital role in maintaining seed growth, especially under conditions of limited photosynthesis. Little is known concerning the processes controlling stem carbohydrate utilization and partitioning in ryegrass with respect to seed growth. The objective of this investigation was to determine detailed post-anthesis changes in stem and spikelet carbohydrates as affected by modification of source and sink strength. Source-sink relations were altered by imposing detillering or detillering-defoliation treatments at anthesis. Patterns of carbohydrate distribution of the ryegrass stem were different, both among positions within the stem and with age. Stem carbohydrates accumulated during early stages of seed growth and then declined as seeds matured. Reducing sugars comprised only a small fraction of the stem's total water soluble carbohydrates. Detillering induced the formation of new tiller sinks, thus increasing sink strength and reversing the carbohydrate gradient from spikelet (seed) sinks to new tiller sinks. Defoliation, combined with detillering, decreased source strength by reducing total stem carbohydrate. In control plants, carbohydrate levels appeared adequate to support maximum seed set, whereas conditions for reduced carbohydrate levels, resulting from detillering or detillering plus defoliation, lowered seed set. Results suggest that under conditions of limited source strength (e.g. reduced photosynthetic capacity), the stem plays a major role in partitioning assimilates to compensate for sink demand. New tiller growth during the period of seed development may out-compete seeds for available carbohydrate and thus reduce seed set

Carbohydrate Depletion and Leaf Blackening in Protea neriljiolia

Leaf blackening on cut flower Protea nerii[olia R. Br. stems was dramatically reduced under a 12-hour photosynthetic light period (120 μmol·m-2·s-1) at 25C for 15 days compared with stems kept in the dark. In the light, addition of 0.5% exogenous sugar to the vase solution resulted in a maximum of 2.5% leaf blackening, while stems with no exogenous sugar had a maximum of 16.5%. Continuous darkness resulted in 94% leaf blackening by day 7, irrespective of sugar treatment. Starch and sucrose concentrations were markedly lower in leaves on dark-held stems than in leaves on stems held in the light; thus, carbohydrate depletion could be the primary stress that initiates leaf blackening. In the light, rates of carbon exchange and assimilate export were similar, indicating that the amount of carbon fixed maybe regulated by sink demand. The pattern of carbon partitioning changed in light-held leaves of the 0% sugar treatment during rapid floral expansion and senescence. Inflorescence expansion appears to influence partitioning of photoassimilates and storage reserves into transport carbohydrates; under decreased sink demand, the assimilate export rate decreases and photoassimilates are partitioned into starch. The data suggest that sink strength of inflorescences held in darkness may be responsible for the depletion of leaf carbohydrates and. consequently, blackening.

Diurnal Changes in Photosynthesis in Wheat Under Different Source‐Sink Relationships

AbstractThe effect of alteration in source‐sink ratio by defoliation and ear removal on diurnal changes in photosynthesis was examined in wheat var. Kalyansona. The photosynthesis rate was maximum during initial few hours in the forenoon and decreased towards evening. The increase in sink demand for a particular leaf by excising other sources leaves, sustained higher rate of photosynthesis during the day. On the other hand de‐eared plants where sink demand was decreased by removal of main shoot ears and all the tillers, photosynthesis rate showed a more rapid decline towards evening, although, the photosynthesis rates were more or less similar to control plants in the morning. These results indicate the influence of sink demand on sustenance of photosynthesis during the day.

Crop Growth Rate and Seeds per Unit Area in Soybean

The number of seeds per unit area is an important yield component in soybean [Glycine max (L.) Merr.]; however, the mechanisms responsible for the regulation of this yield component are not well understood. Field experiments were conducted at Lexington, KY (3 yr), and at Taian, China (1 yr), to investigate the relationship between net canopy photosynthesis and seeds per unit area using genotypes with differences in individual seed growth rates (SGR). At Lexington, shades (30 and 63% reduction in insolation) were placed over plots from growth stage Rl until maturity to create differences in canopy photosynthesis. Planting dates (early and late) and row spacing (wide and narrow) were used at Taian to create differences in canopy photosynthesis. Crop growth rate (CGR) was measured between growth stage Rl and RS as an estimate of net canopy photosynthesis. Yield, seeds per m2, and SGR were also measured. Within each genotype, there was a linear relationship between CGR and seeds per m2 across treatments and years. Within an experiment, seeds per m2 at a constant CGR was inversely related to genotypic differences in SGR. A partitioning coefficient (γ) was estimated by dividing the total sink demand (seeds per m2 ✕ SGR) by CGR. There were no apparent genotypic differences in γ however, γ decreased linearly as CGR increased. The data suggest that the model proposed by Charles‐Edwards, which describes seeds per m2 as a direct function of canopy photosynthesis and a partitioning coefficient and an inverse function of assimilate flux to individual seeds, accurately describes the regulation of seeds per m2 in soybean.

Effects of Sink Demand on Photosynthesis in Cucumber

In Cucumis sativus L. net photosynthesis of leaf number 12 increased until 12 d after its unfolding and then decreased gradually. Sink demand was varied by imposing the number of fruits per plant without changing the number of leaves per plant. Net leaf photosynthesis did not respond to sink demand during the first 16 d of fruit growth. Subsequently, a substantial reduction in photosynthesis was found only when all fruits were removed. This reduction was accompanied by a decrease in the rate of transpiration, indicating a higher stomatal resistance. Neither starch nor monoand disaccharides accumulated noticeably at reduced sink demand, suggesting that there was no end product inhibition of photosynthesis. Dry matter content of the leaves increased and specific leaf area decreased when sink demand was reduced. When modelling cucumber production, effects of sink demand on leaf photosynthesis can be ignored.

The Effect of Cooling on Photosynthesis, Amounts of Carbohydrate and Assimilate Export in Sunflower

Sunflower plants (Helianthus annuus L.) grown at 30 °C were cooled to 13 °C in the light in atmospheric C02 or low C02, or in darkness. Photosynthetic rate at 30 °C after cooling whole plants in atmospheric C02 for 12 h during a photoperiod was significantly lower than at the start of the photoperiod compared to plants cooled at low C02, those cooled in the dark and those maintained at 30 °C. Amounts of sucrose, hexoses and starch in leaves at 13 °C increased throughout a 14 h photoperiod to levels higher than in leaves at 30 °C, where amounts of sucrose and hexoses were stable or falling after 4 h. Carbohydrate accumulation at 13 °C during this photoperiod was more than twice that at 30 °C. After three photoperiods and two dark periods at 13 °C carbohydrate levels in leaves were still as high as at the end of the first photoperiod, but less carbohydrate accumulated during the photoperiods than during the first photoperiod, and more was partitioned as starch. Amounts of soluble carbohydrate in roots were greater after 14 h at 13 °C than in roots of plants at 30 °C. Loss of 14C from leaves at 30 °C as a proportion of 14C02 fixed by them at 30 °C, decreased after exposure of plants to 13 °C in the light for 30 min prior to 14C02feeding. Results indicate an effect of cold on the transport process that was light-dependent. It is inferred that the reduction in the proportion of 14C lost from leaves after 10 h cooling was due to reduced sink demand, whereas the rise in the proportion of 14C lost from leaves after 24 h reflects reduced photosynthetic rate. The coincidence of reduced photosynthetic rate with raised carbohydrate levels in leaves maintained at 30 °C throughout, whilst the rest of the plant was cooled to 13 °C in the light implies feedback inhibition of photosynthesis. This may reduce the imbalance between source and sink in sunflower during the first days of long-term cooling.

Effects of cross-pollination on dry matter accumulation, nutrient partitioning and grain yield of maize hybrids grown under different levels of N fertility

This study was initiated to investigate the relationship between increased kernel sink capacity and fertiliser N use efficiency of maize (Zea mays L). Two hybrids differing in yield potential, Pioneer brand 3732 (P3732) and B73 × Mo17, were grown on field plots with four levels of N fertiliser. The kernel sink of P3732 was modified by cross-pollinating P3732 with pollen from B73 × Mo17. Vegetation and kernal samples collected from the N treatments at mid-silk and at 30, 45 and 60 days post-pollination (DPP) were analysed for accumulation and partitioning of dry matter and N content. Grain yield was also determined. Modification of the P3732 endosperm genotype through cross-pollination significantly increased kernel weight, kernel protein content and grain yield at each level of N treatment, indicating an improvement in fertiliser N use efficiency due to an increased sink demand. All three levels of N application produced similar effects on the accumulation and partitioning of dry matter and N in plant parts for both endosperm genotypes. Since the harvest index observed at 30, 45 and 60 DPP was similar between sib- and cross-pollinated plants, the additional nutrients translocated into developing kernels of P3732 cross-pollinated plants were mainly derived from increases in duration of dry matter production and N uptake by vegetative tissues rather than from improved partitioning efficiency.

Response of wheat growth and CO2 assimilation to altering root-zone temperature

Wheat plants (Triticum aestivum L. var. Chisholm) grown at an air temperature of 23 °C and a root-zone temperature of 3 °C exhibited a significant reduction in shoot and root dry weight and leaf area compared with plants grown at a root-zone temperature of 23 °C. This reduction was correlated with a significantly lower CO2 assimilation rate that was associated with lower leaf conductance, lower internal CO2 concentration, and more negative water potential. Low CO2 assimilation rate was also associated with high starch and total soluble sugar levels in the shoot, less translocation of photosynthate, and possibly less sink demand. Leaf chlorophyll concentration was not affected by altering the root-zone temperature, whereas water use efficiency of plants grown at a root-zone temperature of 3 °C was as much as 1.5 times higher as those grown at 23 °C. Key words: carbohydrate, chlorophyll, photosynthate partitioning, leaf conductance, water potential, water use efficiency.

Root growth and root-shoot interaction in transplants and direct seeded pepper plants

The objectives of this work were to assess and describe pepper ( Capsicum annuum L.) root distribution in the soil profile and to relate root to shoot growth. Plants were field-grown from transplants, produced with either top or bottom irrigation, or from direct seeding using either primed or raw seeds. Dry weights were determined every 2 weeks beginning at 30 days post-planting. Root growth was measured at two soil levels, 0–10 cm and 10–20 cm and in three 10-cm wide positions within each level. Transplants and direct seeded plants had 150 and 100%, respectively, greater root mass in the upper than in the lower soil level. Root growth increase over 56 days was linear for transplants while in direct seeded plants root growth had a lag phase of approximately 14 days, with a sharp increase thereafter. At the end of the growth period, fruit, stem, leaf and root dry weight accounted for 66, 11, 19 and 4% for transplants and 39, 16, 33 and 12% for direct seeded plants. The coordination of growth between root and shoot changed after fruit set only in transplants, which indicates that transplants exhibited a greater fruit sink demand and fruit production than seeded plants.

Photosynthetic Decline from High Temperature Stress during Maturation of Wheat

High temperature stress reduces grain growth in wheat (Triticum aestivum L.) by altering source activity and sink capacity. The impact of stress on source and sink interactions in two wheat cultivars of differing source thermotolerance was monitored by analysis of chlorophyll fluorescence transients, Fv (variable fluorescence) and PSM (peak, stationary, maximum), of attached flag leaves on intact and decapitated tillers grown at optimum (20 degrees C) and stress (35 degrees C) temperatures after anthesis. The thermotolerant cultivar Waverly had reduced Fv and PS quenching and a large increase of SM during heat stress. The less thermotolerant cultivar, Len, exhibited increased Fv and PS quenching and a small increase of SM. Fluorescence induction was similar in intact and decapitated tillers of Len, indicating diminished sinksource interaction during heat stress. The present results and previous observations of photosynthetic activities indicate that cyclic electron transport and photophosphorylation in flag leaves of the thermotolerant cultivar were stimulated by sink demand (increased SM in intact plants). Reduced grain development in the thermolabile cultivar resulted from limited capacity to support cyclic electron transport and photophosphorylation (slight increase in SM of intact plants and large reduction of Cytochrome f/b(6)-mediated electron transport capacity). It was concluded that heat stress injures the photosynthetic apparatus during reproductive growth of wheat and that diminished source activity and sink capacity may be equally important in reducing productivity.

Effect of 2-(3,4-dichlorophenoxy)triethylamine (DCPTA) on the growth and development of sugarbeet

Foliar application of 2-(3,4-dichlorophenoxy)triethylamine (DCPTA) significantly increased the taproot development, leaf development, and the photosynthetic productivity of sugarbeet ( Beta vulgaris L. cv. 811X-S). Photosynthate partitioning in DCPTA-treated sugarbeet plants appeared to be balanced between the demands of plant growth and taproot sucrose accumulation. When compared to controls, the increased taproot fresh weight of 30 μM DCPTA-treated taproots at 88 days after treatment was statistically significant ( P < 0.01), and the percentage sucrose content was maintained resulting in an 81% increase in sucrose yield per taproot. Chloroplasts isolated from mature leaves of 30 μM DCPTA-treated plants, as compared with that of controls, showed a 23% increase in the total soluble protein to chlorophyll ratio that parallel an observed increase in activated ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity in vitro per unit chlorophyll. The total activated Rubisco activity per dm 2 leaf area of 30 μM DCPTA-treated plants was increased 87% when compared to the value of controls. Increased Rubisco activity largely accounted for the observed increase in net photosynthesis in DCPTA-treated plants. However, intense sink demand for photosynthate and the delayed leaf senescence of older leaves may increase net-carbon assimilation in mature (source) leaves of DCPTA-treated plants.

The Effect of Temperature on Photosynthesis and Carbon Fluxes in Sunflower and Rape

Sunflower (Helianthus annuus L.) and oilseed rape (Brassica napus L.) were grown at constant temperatures of 30 °C (warm) and 13 °C (cold). Maximal rates of photosynthesis between 5 °C and 35 °C were at higher temperatures in sunflower than rape. Photosynthetic rate over 4 h at the growth temperature declined in warm-and cold-grown rape and cold-grown sunflower, but remained constant in warm-grown sunflower. The stimulation of photosynthesis by 2 0 kPa O2 compared to 21 kPa 02 declined with decreasing temperature. At 10 °C in warm-grown rape photosynthesis was insensitive to 2-0 kPa 02. However, sensitivity to low O2 continued at 10 °C in warm-grown sunflower. Carbohydrates accumulated in the cold, particularly fructose, glucose and sucrose in warm-grown sunflower transferred to 13 °C. By monitoring changes of 14C in leaves after the assimilation of 14C02, the rates of carbon export from leaves, pool sizes and carbon fluxes between them were estimated. The transfer of warm- and cold-grown rape to 13 °C and 30 °C, respectively, had little effect on these parameters over 22 h. However, export of carbon from sunflower leaves at 13 °C was markedly less than at 30 °C, irrespective of the growth temperature, due to slower export from the transport pool. The rapid suppression of carbon export at 13 °C in warm-grown sunflower may be due to inhibited translocation rather than reduced sink demand in the cold. It is concluded that assimilate utilisation is more depressed in the cold than is photosynthesis; this imposes a greater restriction on biomass production in sunflower than in rape.

Photosynthetic carbon partitioning: its regulation and possibilities for manipulation

The regulation of photosynthetic sucrose synthesis and partitioning is reviewed with particular emphasis on the role of fructose‐2,6‐bisphosphate and sucrose phosphate synthase. It is concluded that a hierachy of regulatory mechanisms exist, which allows partitioning to be changed without this necessarily leading to a reduction in the rate of photosynthesis. Nevertheless, experimental conditions can be defined in which photosynthesis is limited by the rate of end‐product synthesis. These systems can be used to screen for genetic mutants or specific inhibitors, and to investigate a shift in sink demand or environmental factors that specifically act on carbon partitioning and/or the rate of endproduct synthesis.

Regulation of assimilation and senescence by the fruit in monocarpic plants

For most monocarpic species studied, blocking fruit development prevents or at least delays the death of the plants. However, it usually does not prevent the decline in photosynthetic rate, while it may or may not maintain the photosynthetic components (machinery) such as chlorophyll and ribulose 1,5‐bisphosphate carboxylase. Since sink demand influences photosynthetic rate and the fruit‐less plants have limited sink capacity, their reduced photosynthesis may represent a metabolic adjustment but not senescence. The exact causes of the variable decreases in photosynthetic components in fruit‐less plants are uncertain. The causes of metabolic decline may be different in reproductive‐phase monocarpic plants with and without fruit. In any case, assimilation rate, i.e. photosynthesis, may not always provide a reliable index of senescence. Whether or not the developing fruits of monocarpic plants actually control the early and intermediate decline of metabolism is not clear; however, at least in soybean under non‐stress conditions, they cause the final decrease and death.

INTERRELATIONSHIPS BETWEEN PLANT GROWTH REGULATORS AND FRUIT TREE MINERAL NUTRITION

Information is presented for fruit trees to show the effect of applied growth substances on mineral uptake and tissue mineral levels. The effects of nutrient supply to the roots on hormone synthesis and levels in the plant are also considered. The data are discussed in terms of a direct effect of applied growth substances on the actual nutrient absorption occurring in the roots, and of an indirect influence occurring via growth effects induced by growth substances which weaken or strengthen sink demand and consequently affect mineral nutrition. In a few cases, the evidence for a direct effect on mineral uptake by the root is quite sound, but most experiments are inconclusive with respect to the explanation of the observed phenomena.

Whole Plant and Leaf Steady State Gas Exchange during Ethylene Exposure in Xanthium strumarium L.

The effects of ethylene evolved from ethephon on leaf and whole plant photosynthesis in Xanthium strumarium L. were examined. Ethylene-induced epinasty reduced light interception by the leaves of ethephon treated plants by up to 60%. Gas exchange values of individual, attached leaves under identical assay conditions were not inhibited even after 36 hours of ethylene exposure, although treated leaves required a longer induction period to achieve steady state photosynthesis. The speed of translocation of recently fixed (11)C-assimilate movement was not seriously impaired following ethephon treatment; however, a greater proportion of the assimilate was partitioned downward toward the roots. Within 24 hours of ethephon treatment, the whole plant net carbon exchange rate expressed on a per plant basis or a leaf area basis had dropped by 35%. The apparent inhibition of net carbon exchange rate was reversed by physically repositioning the leaves with respect to the light source. Ethylene exposure also inhibited expansion of young leaves which was partially reversed when the leaves were repositioned. The data indicated that ethylene indirectly affected net C gain and plant growth through modification of light interception and altered sink demand without directly inhibiting leaf photosynthesis.

Main Stem Sink Manipulation in Wheat

The role of main stem (MS) sink size on N use by field-grown soft red winter wheat (Triticum aestivum L. cv Hart) was determined. At Feeke's growth stage 8 (last leaf just visible), 100 micromoles of 99 atom% (15)N-ammonium was injected into the lower MS. At anthesis, MS sink size was adjusted by removal of 0, 33, 66, and 100% of the MS spikelets; tiller spikes were left intact. The MS and tiller average kernel size was unaffected by MS sink manipulations. The MS kernel N concentration increased when MS spikelets were removed. Tiller kernel N concentrations were unaffected except when the entire MS reproductive sink was removed, which caused an increase in tiller kernel N concentration. Net losses of MS vegetative N during grain fill were similar for all treatments except for plants lacking MS spikelets, which mobilized 30% less N from the MS. Labeled N was predominately (>90%) associated with the insoluble reduced N fraction of plant tissues at anthesis. Allocation of labeled N to tillers was not proportional to reduction in MS sink size. These results indicate that the reproductive sink on an individual culm has first priority for vegetative N mobilized during grain fill even when sink demand is reduced substantially.

The role of sink demand in carbon partitioning and photosynthetic reinvigoration following shoot decapitation

Photosynthesis, growth, and carbon partitioning of vigorous coppice shoots were compared with the slower growing intact shoots of Populus maximowiczii×nigra L. MN9 to determine the relationship between carbon partitioning and photosynthetic rate. Relative height growth rate of coppice shoots was 2.2 times that of intact shoots with net photosynthetic rate 1.9 times that of intact shoots. Coppice leaves exported a larger proportion of newly‐fixed assimilate (11% compared with 6%) after a 4‐h chase. The greater export from coppice leaves was correlated with a greater proportion of [14C]‐labelled photosynthate deposited as starch in stems 4 cm below the point of label application. Coppice leaf assimilate levels were reduced to 15% that of leaves on intact plants, but coppice leaves had twice the concentration of labelled sucrose. Carbohydrates constituted 55% of the water‐soluble [14C]‐labelled photosynthate in leaves of coppice shoots compared with 40% in intact shoots. The results suggest that carbon allocation and partitioning in coppice shoots were altered towards production and export of new assimilate, and support the hypothesis that photosynthetic rate is responsive to sink demand for assimilates.

Relationship of Enhanced Sink Demand with Photosynthesis and Amount and Activity of Ribulose 1,5-bisphosphate carboxylase in Soybean Leaves

Summary The presence of reproductive sinks is known to enhance the photosynthetic rate of soybean [ Glycine max (L.) Merr.] leaves. Thinning the plant stand at early podding to 25 % of normal plant density increased pod set and pod (plus seed) growth rate per plant, thereby increasing sink size of the remaining plants. CO 2 -exchange rate (CER), the contents of soluble protein and ribulose 1,5-bisphosphate carboxylase (Rubisco), as well as Rubisco activity, were determined for leaves of differing ontogeny. Leaf CER of plants with enhanced sink demand peaked at a higher level during the season, or the rates declined less rapidly during senescence than for control plants (normal density). CER was positively correlated with that of soluble protein and Rubisco content per unit leaf area. The percentage of protein that was Rubisco did not change in comparison with controls and, as the soluble protein increased after thinning, this led to an increased Rubisco activity per unit leaf area that was not due to increased Rubisco activity on a soluble protein basis. Thus, the greater sink demand increased CER or delayed its decline in soybean in parallel with the amount of Rubisco protein, not with the activity of the enzyme.