Effect of Source-to-sink Ratio on Partitioning of Dry Matter and14C-photoassimilates in Wheat during Grain Filling

Increasingly, wheat (Triticum aestivumL.) is being grown intropical environments, but there is inadequate information aboutthe physiological processes limiting yield. In this investigation,the source:sink ratio was manipulated to examine the performanceof source-sink interactions after anthesis and the factor(s)limiting grain filling in tropical conditions. Plants of threewheat cultivars, Cuba C-204, Candeias and IAC-60, were artificiallymodified to give different source:sink ratios. The treatmentswere: I, Control; II, all spikelets on one side of the spikeremoved; III, all spikelets removed except the four centralspikelets of the spike; and IV, flag leaf blade removed. Thedistribution of dry matter between kernels and stem internodeswas analysed at harvest in all three cultivars. Partitioningof¹⁴C-photoassimilates was measured on three occasions afteranthesis in the cultivar Cuba C-204. Modifications of source:sinkratio led to different patterns of allocation of dry matterbetween cultivars and sowing dates. The reduction in sink sizein treatment II produced no significant change in the mass pergrain in the January sowing, but this was enhanced in two cultivarsin the November sowing. In treatment III, both mass per grainand translocation of¹⁴C-photoassimilates declined, apparentlydue to feedback inhibition of photosynthesis. The participationof stem reserves in grain filling and the existence of genotypicdifferences in response to availability of photoassimilateswere corroborated. The pattern of partitioning of dry matterobserved in plants in this investigation suggests a source limitation,particularly during the November sowing. This pattern differedmarkedly from that in other studies, most of which have beenmade in temperate areas.Copyright 1999 Annals of Botany Company Photoassimilates, sink, source, partitioning, grain filling, wheat.     

Postanthesis allocation of photosynthates and grain growth in wheat cultivars as affected by source/sink change

Two wheat cultivars, Hesheng 2 with large grain yield potential, and Shannong 505 with small grain yield potential, were used for investigating the responses of postanthesis photosynthesis, dry matter accumulation and allocation, and grain growth to source/sink changes. At the initial grain filling stage, Hesheng 2 was sensitive to source reduction leading to an increase of net photosynthetic rate (P_N) by 10 %; however, little effect of sink reduction was observed. In Shannong 505, P_N was obviously decreased by sink reduction, and changed a little after source reduction. At the rapid grain filling stage, Hesheng 2 was sensitive to both source and sink reduction resulting in the increase or decrease of P_N, respectively. However, the response of P_N in Shannong 505 to source/sink changes was similar to that in previous stage. The dry matter (DM) accumulation after anthesis was affected by source/sink changes. In Hesheng 2, the decrease in DM was higher than that in Shannong 505 after the same source or sink reduction. Source reduction caused a decrease in the allocation of DM to the sheath and stem, and promoted the reserve photosynthates to be reallocated to grain. The effect of sink reduction was contrary. The grain mass of Hesheng 2 was more easily regulated by source/sink changes than that of Shannong 505. The effect source/sink changes on grain mass was in order upper > basal > middle spikelets on spike. As for a spikelet, the effect was found mainly in the grain mass at the positions 3 and 4 from base of the spikelet. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of source/sink manipulation on net photosynthetic rate and photosynthate partitioning during grain filling in winter wheat

Source-sink relationship, which was influenced by both genotype and environmental factors, contributed to the variation in photosynthesis and photosynthate partitioning of wheat. Source reduction by partial defoliation increased leaf net photosynthetic rate (PN), and sink reduction decreased PN of irrigated wheat. However, the change in PN varied among genotypes. Source reduction enhanced photosynthate translocation into grain in irrigated wheat. However, the enhancement was more evident in cv. Lumai 215953 than incv. Lumai 15. Sink reduction had little effect on the translocation of photosynthate into grain in cv. Lumai 15, but decreased the translocation of photosynthate into grain and increased it into stem in cv. Lumai 215953. In rainfed, non-irrigated wheat, the source or sink manipulation influenced PN only slightly. The source reduction decreased the partitioning of photosynthates into the upper parts (including grains) of plant. However, very little effects of sink reduction on the production of photosynthates occurred in rainfed wheat. This showed that grain sink size was not a factor limiting the production of photosynthates, but controlled the partitioning of photosynthates. Sink reduction decreased photosynthate translocation into grains, and increased it into upper parts of rainfed wheat plant. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grain filling in relation to monocarpic senescence of wheat in varying source-sink ratios

Flag leaf removal at any stage of grain growth hastened senescence (reduction in chlorophyll content) of the sterile glumes whereas a removal of the latter did not alter senescence of the former. Kernel mass, grain mass per ear, harvest index and sink activity reduced more conspicuously by the removal of glumes than by the flag leaf removal. Removal of grains hastened senescence of the glumes only, although protein content increased at a later grain removal in the majority of source organs. Moisture contents of the developing grains were mostly reduced by the removal of either the flag leaf or the glumes, though flag leaf removal at anthesis affected it the most. Protein levels of the developing grains reduced at harvest only when the glumes were removed at anthesis but sugar levels in later phases always decreased irrespective of the time of removal and nature of the source organs.     

Hormones in the grains in relation to sink strength and postanthesis development of spikelets in rice

Inferior spikelets usually exhibit a slower grain filling rate and lower grain weight than superior spikelets in a rice (Oryza sativa L.) panicle. This study investigated whether the variations in grain filling between the two kinds of spikelets were attributed to their sink strength and whether the sink strength was regulated by the hormonal levels in the grains. Using two field-grown rice genotypes, the division rate of endosperm cells, hormonal levels in the grains, and grain weight of both superior and inferior spikelets were determined during the grain filling period. The results showed that superior spikelets had dominance over inferior spikelets in endosperm cell division rate and cell number, grain filling and grain weight. Changes in zeatin (Z) and zeatin riboside (ZR) contents paralleled and were very significantly correlated with the cell division rate and cell number. Cell division rate and the content of indole-3-acetic acid (IAA) in the grains were also significantly correlated. Gibberellin (GAs; GA₁+ GA₄) content of the grains was high but ABA levels were low at the early grain filling stage. ABA increased substantially during the linear phase of grain growth and was very significantly correlated with grain dry weight during this period. Application of kinetin at 2 through 6 days post anthesis (DPA) significantly increased cell number, while spraying ABA at 11 through 15 DPA significantly increased the grain filling rate. The results suggest that differences in sink strength are responsible for variations in grain filling between superior and inferior spikelets. Both cytokinins and IAA in the grains may mediate cell division in rice endosperm at early grain filling stages, and therefore regulate the sink size of the grain, whereas ABA content correlates with sink activity during the linear period of grain growth.     

Effects of foliar application of BAP on source and sink strength in four six-rowed barley (Hordeum vulgare L.) cultivars

A field experiment was conducted to investigate the effects of foliar application of a synthetic cytokinin (BAP) on source and sink strength of four different six-rowed barley (Hordeum vulgare L.) cultivars. Different spraying treatments consisting of spraying on whole plant, spraying only on leaves and spraying only on ears started at anthesis and continued for 7 days. One additional spraying was carried out on late period of grain filling. Results showed that spraying only on leaves did not affect ear weight, grain yield and 1,000-grain weight, while the two other treatments increased all above mentioned traits. Neither of treatments affected stem weight, biological yield and contribution of stem reserves in grain filling. Exogenous cytokinin did not increase photosynthetic rate and chlorophyll content in treated leaves until late period of grain filling, although there was no significant increase in final grain weight due to late application of BAP. Our results suggested that effects of foliar application of BAP were mostly due to increased sink size soon after anthesis and increased sink demand probably met by current photosynthesis of organs other than leaves, like ear green tissues. An erratum to this article is available at .     

Sink-limitation to yield and biomass: a summary of some investigations in spring wheat

Yield potential can be expressed as a product of light interception, radiation use efficiency (RUE), and the partitioning of biomass to grain yield, or harvest index (HI). Traits related to early or late light interception have not been shown to be associated with genetic improvement of spring wheat yield in favourable environments. It is, however, well established that yield improvement is largely a result of increased HI, although the most recent studies comparing genetic progress in HI over time in spring wheat indicate that it has not made any additional progress since the mid 1980s. These observations suggest that future genetic progress in yield will most likely be achieved by focusing on constraints to RUE. Considering the possibility that RUE may be influenced indirectly by sink limitation, it is apparent that biomass may be increased by increasing grain number, for example. Experiments with high yielding spring wheat lines containing the alien translocation 7DL.7Ag showed increased grains m‐2 (15%), yield (12%), and biomass (9%) compared with controls. The translocation was also associated with a larger investment in spike mass at anthesis (15%), more grains/spike (10%), and increased flag‐leaf photosynthetic rate during grain‐filling (20%). The data suggest that increased biomass in 7DL.7Ag lines was due to significantly increased RUE post‐anthesis, as a result of a larger kernel number (sink) that increased the demand for photosynthesis during grain‐filling. The hypothesis that increased photosynthesis and RUE may respond directly to a larger number of grains/spike was tested experimentally by imposing a light treatment during boot stage. The treatment was associated with a small increase (5%) in the proportion of biomass invested in spike mass at anthesis, reflected by on average three extra grains/spike at maturity. The treatment was associated with 25% more yield and 22% more biomass than controls, while carbon assimilation rate measured on flag‐leaves during grainfilling was 10% higher than controls. The results suggest that RUE can be increased indirectly by increasing sink strength and that the current yield limiting process in spring wheat is the determination of kernel number. Experimental data are presented on how spike fertility may be increased through breeding, for example by introgression of the multi‐ovary trait to increase grain number per spikelet. In addition, results of analysis of the physiological bases of genotype × year interaction in high yield environments are presented in the context of how such information can provide a focus for genetic studies of sink limitation.     

源库关系改变对小麦灌浆期植株光合速率及~(14)C同化物运转分配的影响

选用千粒重大小不同的小麦品种,研究了去除顶端两个小穗对两类品种（大粒品种和小粒品种）千粒重、穗粒数、穗粒重、籽粒平均灌浆速率、单穗平均增重速率、植株光合速率及１４Ｃ同化物运转分配的影响。试验结果表明,去除顶端两个小穗后,两类品种的千粒重提高,穗粒数降低或基本不受影响,但降低比例明显低于去除小穗数的比例。籽粒平均灌浆速率和单穗平均增重速率（分别表征籽粒库容活性和穗粒库容活性）相应提高。结果,两类品种的穗粒重表现为补偿性增长。两类品种比较,小粒品种的增长幅度大于大粒品种。穗粒库容活性增强使得小粒品种灌浆中后期的植株光合速率提高,使两类品种分配到籽粒中的１４Ｃ同化物比例增加。从而表明,无论是植株光合速率还是同化物的运转分配皆受穗粒库容活性的调控,调控方式和幅度因品种类型而不同。但提高其穗粒重的着眼点都应当是提高穗粒库容活性。     

小麦不同品种光合速率和14 C同化物分配对源库比改变的影响

 选用千粒重大小不同的小麦品种,研究了去除顶端两个小穗对两类品种(大粒品种和小粒品种)穗部性状、籽粒平均灌浆速率、单穗平均增重速率、植株光合速率及14C同化物运输分配的影响。试验结果表明,去除顶端两个小穗后,两类品种的籽粒平均灌浆速率和单穗平均增重速率(分别表征籽粒库容活性和穗粒库容活性)相应提高,穗粒重表现为补偿性增长。两类品种比较,小粒品种的增长幅度大于大粒品种。穗粒库容活性增强使得小粒品种灌浆中后期的植株光合速率提高,使两类品种分配到籽粒中的14C同化物比例增加。从而表明,无论是植株光合速率还是同化物的运转分配皆受库容活性的调控,调控方式和幅度因品种类型而不同。     

Deciphering the environmental impact on spike architectural traits for grain yield consolidation in bread wheat (T. aestivum L.)

The realization of grain yield in wheat is decided by source-sink balance under prevailing environmental conditions. Management conditions like changing the sowing time influence the source-sink capacity through modification in agronomic traits. Therefore, this experiment was conducted to decipher the influence of spike architectural traits (SATs) on grain yield and to open avenues for further grain yield enhancement. Comparatively early sowing over timely sowing gives the advantage of realizing higher grain yield with a positive relationship with SATs namely spike length, spikelets per spike, individual spike weight, individual grain weight, number of grains per spikelet, grain length, and grain width of upper and lower spike portion. Confirmatory factorial analysis revealed that spike length, spikelets per spike, individual spike weight, grains per spikelet were having a significant effect in deciding grain yield in early sown. The presence of a significant effect of genotype by environment interaction over grain yield and SATs allows the exploitation of available genotypic and environmental variability for further yield enhancement. GGE analysis on transformed and standardized grain yield-trait (GY-trait) combinations was used in the selection of genotypes having high GY-trait combinations for both sowing times. In early sowing, WG 11 was the best for high GY with high individual spike weight; grain length and grain width at lower and upper parts of the spike; and shorter days to 50% flowering. Genotypes exclusively having the high GY-trait combination along with low values of remaining GY-trait combinations were also selected with genotype focused GGE approach.     

小麦不同品种光合速率和~(14)C同化物分配对源库比改变的响应

选用千粒重大小不同的小麦品种,研究了去除顶端两个小穗对两类品种（大粒品种和小粒品种）穗部性状、籽粒平均灌浆速率、单穗平均增重速率、植株光合速率及１４Ｃ同化物运输分配的影响。试验结果表明,去除顶端两个小穗后,两类品种的籽粒平均灌浆速率和单穗平均增重速率（分别表征籽粒库容活性和穗粒库容活性）相应提高,穗粒重表现为补偿性增长。两类品种比较,小粒品种的增长幅度大于大粒品种。穗粒库容活性增强使得小粒品种灌浆中后期的植株光合速率提高,使两类品种分配到籽粒中的１４Ｃ同化物比例增加。从而表明,无论是植株光合速率还是同化物的运转分配皆受库容活性的调控,调控方式和幅度因品种类型而不同。     

Survival and recovery of perennial forage grasses under prolonged Mediterranean drought: II. Water status, solute accumulation, abscisic acid concentration and accumulation of dehydrin transcripts in bases of immature leaves

Effects of ozone on spring wheat ( Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992–1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered air+35 nl l⁻¹ ozone for 8 h d⁻¹ (NF⁺) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF⁺, 45 nl l⁻¹ 1000–1800 hours, seasonal mean) the net CO₂ uptake of wheat flag leaves was decreased by c . 40% relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO₂ uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF⁺ treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.     

Main stem sink manipulation in wheat : effects on nitrogen allocation to tillers

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% ¹⁵N-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.     

Ontogenetic changes in photosynthetic capacity and dry matter production of flag wheat leaves during the grain filling period

The relationships between photosynthetic capacity and dry matter accumulation during the grain filling period have been studied in flag leaves of Triticum aestivum L., cv. Kolibri grown in Mediterranean field conditions. Particular importance has been given to assimilate accumulation in relation to the onset of senescence. During grain filling, the time course of specific dry weight (SDW) was similar in the blade and in the sheath. Variations in SDW were about six times larger in the sheath than in the blade. Minimum blade SDW values occurred during heading and at anthesis. Maximum blade SDW values were observed two weeks after anthesis. After this, SDW values decreased sharply. The dry matter increase per grain in the period from two weeks after anthesis to the end, was only about 25% of final grain dry weight. The importance of environmental constraints on maximum SDW values are discussed. Maximum SDW values occurred at the beginning of the period of rapid decline in blade net CO₂ assimilation rate and leaf nitrogen content, that is, at the beginning of senescence. On the other hand, the stomatal resistance to CO₂ and the development of senescence are not apparently related. The maximum blade dry weight increase (considering a value of zero at heading) was about 60 mg dry weight per g fresh weight. The possible relationships between dry matter accumulation and senescence onset are discussed.     

Effect of source-sink relations and nitrogen nutrition on senescence and N remobilization in the flag leaf of wheat

The relation between the source-sink ratio and nitrogen nutrition on grain yield of wheat ( Triticum aestivum L. cv. Klein Chamaco) was studied in a greenhouse experiment. Plants were grown until anthesis in pots with soil fertilized with 0.16 mmol N per plant twice a week. At anthesis, all leaves but the flag leaf were excised in a group of plants. In another group the treatment consisted in a similar defoliation plus the longitudinal excision of half the ear, while a third group was left untouched as a control. At the same time, the N supply to half of the plants in each group was interrupted, while the other half continued receiving 16 m M N. The defoliated plants showed a longer functional life of the flag leaf than the control, retaining the chlorophyll, soluble proteins and total reduced nitrogen for a longer time. The ear-excised plants showed an intermediate behavior. The plants with the interrupted N supply showed a faster leaf senescence than the N supplied ones, and this coincided with an increase in the proteolytic activity and nitrogen transport to the ear. However there were no differences in ear weight between the two nitrogen treatments. It is concluded that leaves and ear compete for the nitrogen, and that a low level of carbohydrates in the flag leaf, due to a low source-sink ratio, delays leaf senescence.     

Stay‐green: A consequence of the balance between supply and demand for nitrogen during grain filling?

Retention of green leaf area in grain sorghum under post‐anthesis drought, known as stay‐green, is associated with greater biomass production, lodging resistance and yield. The stay‐green phenomenon can be examined at a cell, leaf, or whole plant level. At a cell level, the retention of chloroplast proteins such as LHCP2, OEC33 and Rubisco until late in senescence has been reported in sorghum containing the KS19 source of stay‐green, indicating that photosynthesis may be maintained for longer during senescence in these genotypes. At a leaf level, longevity of photosynthetic apparatus is intimately related to nitrogen (N) status. At a whole plant level, stay‐green can be viewed as a consequence of the balance between N demand by the grain and N supply during grain filling. To examine some of these concepts, nine hybrids varying in the B35 and KS19 sources of stay‐green were grown under a post‐anthesis water deficit. Genotypic variation in delayed onset and reduced rate of leaf senescence were explained by differences in specific leaf nitrogen (SLN) and N uptake during grain filling. Matching N supply from age‐related senescence and N uptake during grain filling with grain N demand found that the shortfall in N supply for grain filling was greater in the senescent than stay‐green hybrids, resulting in more accelerated leaf senescence in the former. We hypothesise that increased N uptake by stay‐green hybrids is a result of greater biomass accumulation during grain filling in response to increased sink demand (higher grain numbers) which, in turn, is the result of increased radiation use efficiency and transpiration efficiency due to higher SLN. Delayed leaf senescence resulting from higher SLN should, in turn, allow more carbon and nitrogen to be allocated to the roots of stay‐green hybrids during grain filling, thereby maintaining a greater capacity to extract N from the soil compared with senescent hybrids.     

The developing reproductive 'sink' induces oxidative stress to mediate nitrogen mobilization during monocarpic senescence in wheat

Removal of reproductive 'sink,' i.e., spikelets from wheat, after anthesis delays the rate of flag leaf senescence. Oxidative stress and the oxidative damage to proteins were studied in relation to nitrogen mobilization in wheat plants showing normal and delayed senescence. Wheat plants lacking a reproductive sink showed decreased oxidative stress, lower lipid peroxidation and maintained higher protein, oxidatively damaged proteins, and nitrogen levels as compared to plants with reproductive sink during monocarpic senescence. Oxidative damage to the proteins when not followed by high proteolytic activities led to a slower nitrogen mobilization in wheat plants lacking a reproductive sink. Thus, the influence of the reproductive sink was due to its ability to drive forward the nitrogen mobilization process through high ROS levels which mediated both damage to the proteins and influenced proteolytic activities.     

小麦籽粒灌浆期冠层温度分异动态及其与源库活性的关系

选用18个小麦品种为材料,从开花期到乳熟末期,对品种间冠层温度进行比较,并对冠层温度与旗叶源活性指标(丙二醛含量、蔗糖磷酸合成酶活性、净光合速率、叶绿素含量、单茎有效绿叶数、顶三叶绿叶面积比率)和库活性指标(籽粒蔗糖合成酶活性、ADPG焦磷酸化酶活性、可溶性蛋白含量、籽粒淀粉积累速率)进行相关分析.结果表明:小麦品种间冠层温度在各生育时期均有显著性差异,且随灌浆进程的推进差异程度越来越大;从乳熟初期到乳熟末期,冠层温度与各源活性指标的相关系数均有增加的趋势,并在乳熟后期和末期均达到极显著水平;冠层温度与各库活性指标的相关系数在乳熟末期均达到显著水平.研究发现,小麦籽粒灌浆期品种间冠层温度差异随生育期推进而愈加显著,冠层温度与源库活性的关系以乳熟后期和末期最密切.     

Monocarpic senescence in wheat: Influence of sterile glumes and ear

The senescence of sterile glumes, flag leaf and the other two leaves below the ear of wheat ( Triticum aestivum L. cv. Sonalika) was studied in relation to grain development and surgical manipulation. The senescence of sterile glumes was faster than that of the leaves in terms of chlorophyll and protein degradation. The flag leaf senesced later than the other two leaves below it. Removal of sterile glumes markedly reduced the harvest index (crop: straw ratio) and average dry weight per grain as compared to removal of the flag leaf. Maximum grain weight was achieved after the glumes had senesced completely. Removal of the ear delayed senescence of all the three leaves. It is concluded that sterile glumes are important suppliers of assimilate for grain filling and that nutrient drainage is the primary cause of the monocarpic senescence in wheat.