Branching Shoots and Spikes from Lateral Meristems in Bread Wheat

Wheat grain yield consists of three components: spikes per plant, grains per spike (i.e. head or ear), and grain weight; and the grains per spike can be dissected into two subcomponents: spikelets per spike and grains per spikelet. An increase in any of these components will directly contribute to grain yield. Wheat morphology biology tells that a wheat plant has no lateral meristem that forms any branching shoot or spike. In this study, we report two novel shoot and spike traits that were produced from lateral meristems in bread wheat. One is supernumerary shoot that was developed from an axillary bud at the axil of leaves on the elongated internodes of the main stem. The other is supernumerary spike that was generated from a spikelet meristem on a spike. In addition, supernumerary spikelets were generated on the same rachis node of the spike in the plant that had supernumerary shoot and spikes. All of these supernumerary shoots/spikes/spikelets found in the super wheat plants produced normal fertility and seeds, displaying huge yield potential in bread wheat.     

Invertase-mediated Interconversion of Sucrose and Hexoses During Their Translocation in Growing Pearl Millet Plant

Metabolism of free sugars, particularly sucrose, in various plant tissues enroute from leaf sheaths to grains in growing pearl millet was studied. With the enhancement in growth, the levels of both reducing and non-reducing sugars declined in middle and basal leaf sheaths but increased in flag leaf sheath towards plant maturity. In sheath, wall-bound invertase was more active than soluble invertases and the activities of all these enzymes rose towards maturity. Besides hexoses and sucrose, some fructose polymers were also detected in the internodes. In contrast with internodes, where the levels of total free sugars declined till around anthesis, in penultimate node their levels continuously increased, but attained peak values at 65 CAS in middle- and basal nodes. In both these tissues, arriving sucrose encounters invertases but in nodes wall-bound invertase appears to be the pivotal one. On feeding (U-¹⁴C)-sucrose to the detached ear-heads a large proportion of ¹⁴C was incorporated into hexoses alone in peduncle and rachis. PCMBS and HgCl₂ inhibited the metabolism of sucrose supplied to peduncle and rachis pointing to the involvement of invertases in sucrose cleavage in these organs. Through the regulated operation of invertase(s), the nodes seem to maintain a controlled flow of free sugars from source to sink tissue.     

Dynamics of light and nitrogen distribution during grain filling within wheat canopy

In monocarpic species, during the reproductive stage the growing grains represent a strong sink for nitrogen (N) and trigger N remobilization from the vegetative organs, which decreases canopy photosynthesis and accelerates leaf senescence. The spatiotemporal distribution of N in a reproductive canopy has not been described in detail. Here, we investigated the role of the local light environment on the spatiotemporal distribution of leaf lamina N mass per unit leaf area (SLN) during grain filling of field-grown wheat (Triticum aestivum). In addition, in order to provide some insight into the coordination of N depletion between the different vegetative organs, N dynamics were studied for individual leaf laminae, leaf sheaths, internodes, and chaff of the top fertile culms. At the canopy scale, SLN distribution paralleled the light gradient below the flag leaf collar until almost the end of grain filling. On the contrary, the significant light gradient along the flag leaf lamina was not associated with a SLN gradient. Within the top fertile culms, the time course of total (alive + necrotic tissues) N concentration of the different laminae and sheaths displayed a similar pattern. Another common pattern was observed for internodes and chaff. During the period of no root N uptake, N depletion of individual laminae and sheaths followed a first-order kinetics independent of leaf age, genotype, or N nutrition. The results presented here show that during grain filling, N dynamics are integrated at the culm scale and strongly depend on the local light conditions determined by the canopy structure.     

Combining Next Generation Sequencing with Bulked Segregant Analysis to Fine Map a Stem Moisture Locus in Sorghum (Sorghum bicolor L. Moench)

Sorghum is one of the most promising bioenergy crops. Stem juice yield, together with stem sugar concentration, determines sugar yield in sweet sorghum. Bulked segregant analysis (BSA) is a gene mapping technique for identifying genomic regions containing genetic loci affecting a trait of interest that when combined with deep sequencing could effectively accelerate the gene mapping process. In this study, a dry stem sorghum landrace was characterized and the stem water controlling locus, qSW6, was fine mapped using QTL analysis and the combined BSA and deep sequencing technologies. Results showed that: (i) In sorghum variety Jiliang 2, stem water content was around 80% before flowering stage. It dropped to 75% during grain filling with little difference between different internodes. In landrace G21, stem water content keeps dropping after the flag leaf stage. The drop from 71% at flowering time progressed to 60% at grain filling time. Large differences exist between different internodes with the lowest (51%) at the 7^th and 8^th internodes at dough stage. (ii) A quantitative trait locus (QTL) controlling stem water content mapped on chromosome 6 between SSR markers Ch6-2 and gpsb069 explained about 34.7-56.9% of the phenotypic variation for the 5^th to 10^th internodes, respectively. (iii) BSA and deep sequencing analysis narrowed the associated region to 339 kb containing 38 putative genes. The results could help reveal molecular mechanisms underlying juice yield of sorghum and thus to improve total sugar yield.     

Complementary patterns of stiffness in stem and leaf sheaths of Triticale

The variation in the stiffness of stem and leaf sheaths along the shoot axis of Triticale (Triticosecale W., cv. Jago) was examined, using an ultrasonic method, at two stages of development, (i) at the stage of high stem mechanical instability when upper internodes are forming (heading), and (ii) at milk maturity when development of strengthening tissues is completed (three weeks after anthesis). The squared velocity of low-frequency longitudinal pulse waves was used as a measure of the specific modulus of elasticity, averaged over the whole cross section of the structures and related to the unit density of the material. Structural material of varying effective stiffness was found to be utilized along leaf sheaths with a pattern complementary to that in growing stems. The stiffness increased basipetally along leaf sheaths in the direction of increasing flexibility of internodes. Maximum values of the specific modulus of elasticity in particular leaf sheaths were enhanced acropetally, ensuring the stronger mechanical protection of those meristematic zones which were actively elongating and were located at the upper internodes. The stiffest material present in leaf sheaths covered only those stem sections which could be the most critical for plant safety during stem elongation. This characteristic and very regular pattern of stiffness alteration along the shoot axis was structurally determined as it remained similar after air drying the specimens. It is concluded that adaptation of cereals to withstand environmental loads is realized not only on morphological and anatomical levels but is also reflected in a specific heterogeneity in the material properties of the cell walls which support the plant.This research was supported by the Ministry of Agriculture, Poland (Project CPBR 10.1 C 29).     

The utilization of pre-anthesis reserves in grain filling of wheat. Assessment by steady-state 13CO2/12CO2 labelling

δ, C isotope composition relative to Pee Dee Belemnite
WSC, water-soluble carbohydrates
N, nitrogen
C, carbon
cv, cultivar
ME, efficiency of mobilized pre-anthesis C utilization in grain filling (g C g^–1C)

Significant mobilization of protein and carbohydrates in vegetative plant parts of wheat regularly occurs during grain filling. While this suggests a contribution of reserves to grain filling, the actual efficiency of mobilized assimilate conversion into grain mass (ME) is unknown. In the present study the contribution of pre-anthesis C (C fixed prior to anthesis) to grain filling in main stem ears of two spring wheat (Triticum aestivum L.) cultivars was determined by ¹³C/¹²C steady-state labelling. Mobilization of pre-anthesis C in vegetative plant parts between anthesis and maturity, and the contributions of water-soluble carbohydrates (WSC) and protein to pre-anthesis C mobilization were also assessed. Experiments were performed with two levels of N fertilizer supply in each of 2 years. Pre-anthesis reserves contributed 11–29% to the total mass of C in grains at maturity. Pre-anthesis C accumulation in grains was dependent on both the mass of pre-anthesis C mobilized in above-ground vegetative plant parts (r² = 0·87) and ME (defined as g pre-anthesis C deposited in grains per g pre-anthesis C mobilized in above-ground vegetative plant parts; r² = 0·40). ME varied between 0·48 and 0·75. The effects of years, N fertilizer treatments and cultivars on ME were all related to differences in the fractional contribution of WSC to pre-anthesis C mobilization. Multiple regression analysis indicated that C from mobilized pre-anthesis WSC may be used more efficiently in grain filling than C present in proteins at anthesis and mobilized during grain filling. Possible causes for variability of ME are discussed.     

Gas exchange of ears of cereals in response to carbon dioxide and light

One cultivar each of spring wheat (Triticum aestivum L. cv. Arkas), oat (Avena sativa L. cv. Lorenz), and barley (Hordeum vulgare L. cv. Aramir) was chosen in order to study the relative contributions of individual bracts to the gas exchange of whole ears. The distribution and frequency of the stomata on the bracts were examined. Gas exchange was measured at normal atmospheric CO₂ (330 bar) and at high CO₂ (2000 bar) on intact ears and on ears from which glumes or lemmas and pleae (wheat and oat) or awns (barley) had been removed.The relative contribution to the gas exchange of the whole organ is highest for the awns of barley ears. In wheat, the contribution of the glumes is slightly higher than that of the inner bracts before anthesis. Two weeks after anthesis the inner bracts contribute more than the glumes. This tendency of increasing importance of the inner bracts is also found in oat ears, but the relative amount of CO₂ uptake by the glumes is higher than in wheat. These changes during ontogeny result from the better supply of light to the inner bracts caused by opening of the ears' structures during grain filling, which in part compensates for the decreasing photosynthetic capacity.The ratio of the photosynthesis rate at high CO₂ to that at normal CO₂ is lower for the glumes of oat and for the awns of barley than for the other bracts.Abbreviations A³³⁰, A²⁰⁰⁰ net photosynthesis rate, A³³⁰ at normal atmospheric CO₂ (330 bar), A²⁰⁰⁰ at high CO₂ (2000 bar) - PPFD photosynthetic photon flux density - p^c intercellular partial pressure of CO₂     

Remobilization of cadmium in maturing shoots of near isogenic lines of durum wheat that differ in grain cadmium accumulation.

Cadmium accumulation in grain of durum wheat (Triticum turgidum L. var. durum) represents a concern to consumers. In an effort to understand the regulation of Cd accumulation in maturing grain, the remobilization of 109Cd applied to stem and flag leaves was examined in two near-isogenic lines that differ in grain Cd accumulation. Absorbed 109Cd was primarily retained in the labelling flap (50-54% and 65-80% for stem and flag leaves, respectively). Cadmium exported from the stem flap initially (3 d) accumulated in the stem in a declining gradient towards the head. Subsequent remobilization of Cd deposited in the stem was associated with Cd accumulation in the grain. Cadmium exported from the flag leaf flap was primarily directed to the grain. Little (<1%) Cd accumulated in the glumes or rachis, and transport of Cd to shoot tissues below the flag leaf node was low (<1%). On average, 9% and 17% of absorbed 109Cd accumulated in the grain 14 d after labelling the stem and flag leaf, respectively. Irrespective of labelling position, the low Cd-accumulating isoline averaged 1.5-2-fold lower Cd accumulation per grain and Cd concentration in the grain than the high Cd-accumulating isoline. Cadmium accumulation in the grain was inversely correlated with Cd retention in the stem (stem labelled) and labelling flap (flag leaf labelled) for both isolines. Cadmium translocation to the grain was not inhibited by Zn when both were applied simultaneously (50 pM 109Cd; 0.5 microM 65Zn) to the flag leaf. These results show that elevated remobilization of Cd from the leaves and stem to the maturing grain may be partially responsible for the high accumulation of Cd in durum wheat grain.     

Carbon mobilization in shoot parts and roots of wheat during grain filling: assessment by 13C/12C steady-state labelling, growth analysis and balance sheets of reserves

cv, cultivar
δ, deviation of C isotope composition from a standard
Δ, C isotope discrimination
WSC, water soluble carbohydrates

Steady-state labelling of all post-anthesis photosynthate of wheat was performed to assess the mobilization of pre-anthesis C (C fixed prior to anthesis) in vegetative plant parts during grain filling. Results were compared with estimates obtained by indirect approaches to mobilization of pre-anthesis C: ‘classical’ growth analysis and balance sheets of water soluble carbohydrates (WSC) and protein. Experiments were performed with two spring wheat cultivars grown with differential nitrogen fertilizer supply in 1991 and 1992. The fraction of pre-anthesis C mobilized in above-ground vegetative biomass ranged between 24 and 34% of total C present at anthesis. Treatment effects on mobilization of pre-anthesis C in total above-ground vegetative biomass were closely related (r² = 0·89) to effects on mobilization of WSC-C plus protein-C (estimated as N mobilized × 3·15). On average, 81% of pre-anthesis C mobilization was attributable to the balance of pre-anthesis WSC (48%) and protein (33%) between anthesis and maturity. In roots, WSC and protein mobilization accounted for only 29% of the loss of pre-anthesis C. Notably, mobilization of pre-anthesis C was 1·4–2·6 times larger than the net loss of C from above-ground vegetative biomass between anthesis and maturity. This discrepancy was mainly due to post-anthesis C accumulation in glumes and stem. Post-anthesis C accumulation was related to continued synthesis of structural biomass after anthesis and accounted for a mean 15% of total C contained in above-ground vegetative plant parts at maturity. A close correspondence between net loss of C and mobilization of pre-anthesis C was only apparent in leaf blades and leaf sheaths. Although balance sheets of WSC and protein also underrated the mobilization of pre-anthesis C by ≈ 19%, they gave a much better estimate of pre-anthesis C mobilization than growth analysis.     

panicle phytomer 1 mutations affect the panicle architecture of rice

 We have characterized three panicle phytomer 1 (pap1) mutations from the phytomer viewpoint. In pap1 mutants, rachis phytomers were strongly affected involving a severe reduction of rachis internode length and an increase in the number of rachis internodes (number of phytomers), resulting in a large number of primary branches. In addition, bracts were frequently over-developed. By contrast, pap1 differently affected primary branch phytomers resulting in a reduction in both the number and length of internodes. Spikelets were also modified. Rudimentary and empty glumes were frequently elongated. Floral organs were mostly normal. However, a double mutation between pap1 and fon1 markedly increased the number of floral organs compared with the single fon1 mutation, suggesting that PAP1 has a distinct role in the differentiation of floral organs. The functions of PAP1 on panicle architecture are: (1) the negative regulation of the number of phytomers on the rachis but a positive regulation of the number on primary branches, (2) an elongation of internodes, and (3) the negative regulation of bract development. Received: 5 October 1997 / Accepted: 27 January 1998     

The Vascular System in the Rachis of a Wheat Ear

The vascular system in the rachis of ears of wheat (Triticumaestivum L. cvs Gamenya, Olympic and Bungulla) was examinedon material grown in the field and in a growth cabinet. In theinternodes, central and peripheral bundles were observed andtheir mean number and size were determined. A significant 1:1 relationship between the number of spikelets on the ear andthe number of central bundles at the base of the rachis wasestablished. The number of both central and peripheral bundlesdeclined acropetally along thé length of the rachis.The decline in peripheral bundles occurred mainly between internodes1 and 6, numbered from the base. The decline in central bundlesoccurred at a rate of less than one bundle per internode betweeninternodes 1 and 4, though in some ears, there was no decline;in larger ears, central bundles declined at a rate of one totwo bundles per internode between internodes 5 and 11. Aboveinternode 11, the rate of decline varied with ear size. Threecentral bundles consistently reached the terminal spikelet.The number and cross-sectional surface area of xylem vesselsand sieve tubes and the total vascular size also declined acropetallyalong the rachis. The decline in total vascular size was dueto (a) some bundles branching and to reductions in size, (b)the diversion or dropping of bundles into spikelets, or (c)a combination of (a) and (b). These observations are discussedin relation to the distribution of grain number and weight onthe ear. Triticum aestivum L., wheat, rachis, spikelets, vascular anatomy, xylem, sieve tubes     

Responses of various crop species and cultivars to fertilizer application

Summary Crop response to fertilizer application depends not only on the level of available plant nutrients in the soil but is also related to crop physiology and morphology. For a well balanced nutrition the rate of nutrient supply to the roots must correspond with the rate of nutrient required for growth. Species or cultivars with a high growth rate generally respond more favourably to fertilizer application than those with low growth rates. An analogous relationship holds for the biomass produced per unit soil surface. Thus modern rice and wheat cultivars tolerate a more dense spacing than older ones. Due to the dense stand the yield and particularly the grain yield of the modern varieties may be several times higher than those of older cultivars, and therefore also the nutrient requirement, especially the demand for N and P, is higher for the modern cultivars.Modern cereal cultivars are characterized by a high crop index which means that after flowering a high proportion of grain filling material must be produced by photosynthesis. Assimilation and translocation of photosynthates are favoured by K⁺. Thus in particular modern cultivars require a high K⁺ content for optimum grain filling.Nutrient exploitation of soils by plant roots depends on root morphology and root physiology. Grasses generally have much longer roots than dicots. Thus the rate of K⁺ and phosphate uptake per unit root length is lower for grasses than for dicots. It is for this reason that dicots respond earlier to a K⁺ and phosphate dressing than grasses.Species living symbiotically with Rhizobium may depress the rhizosphere pH considerably and thus promote the dissolution of phosphate rock.     

Demand for Assimilates Determines the Productivity of Intensive and Extensive Rice Crops in Primorskii Krai

Intensive and extensive rice crops (Oryza sativa L.), regionally cultivated in Primorskii krai (maritime territory), were grown under full and 50% attenuated solar radiation. Plants of different varieties were used to examine the supply of newly synthesized and reutilized ¹⁴C-assimilates to caryopses and to estimate the dry weight dynamics of whole plants, vegetative organs, and grains. Cultivar-specific differences were revealed with respect to the sink capacity of caryopses, the export of photosynthates from the upper leaf and their delivery to the panicle, and the contributions of newly produced and reutilized assimilates to grain filling. In rice plants of all varieties grown under full insolation, the amount of photosynthates produced during grain filling was insufficient to satisfy the demand of caryopses; one-fourth or one-fifth of this demand was satisfied at the expense of mobilization of stored metabolites. The mobilization was accelerated by the elevated demand for assimilates and by attenuated insolation. In artificially shaded plants of intensive varieties, the pool of newly produced assimilates was lower and reutilization of previously gained assimilates started earlier than in shaded plants of extensive varieties. It is concluded that the higher grain yield of intensive rice varieties, cultivated in Primorskii krai, is determined by a higher demand for assimilates and by a higher production and accelerated supply of newly formed photosynthates to caryopses during the first half of the grain-filling stage. The potential productivity of these varieties is constrained by the deficit of assimilates during the second half of grain-filling stage. The low grain productivity of extensive varieties is caused by the insufficient number of grains in panicles and by low demand for assimilates throughout the period of grain filling.     

Contribution of the wheat (Triticum aestivum L.) flag leaf to grain yield in response to plant growth regulators

In West-Europe, intensive cereal management uses plant growth regulators (PGRs) especially for wheat. A green-house experiment compared the effects of two PGRs on flag leaf characteristics and yield of winter wheat. Chlormequat chloride + choline chloride (CCC) and chlormequat chloride + choline chloride + imazaquin (CCC+I) were applied to winter wheat at growth stage 5 (Feekes Large scale). CCC and CCC+I significantly increased flag leaf surface area at anthesis. Both treatments also enhanced chlorophyll content of the main stem flag leaf. The grain filling period was extended with PGR application by 2 days. CCC and CCC+I significantly increased net CO₂ assimilation rates during the flag leaf life. No effects of PGR spraying were observed on the pattern of ¹⁴C labelled assimilate distribution. Increased grain yield was due to the increase in average grain weight. The results indicate that PGR treatments increased flag leaf contribution to grain filling. The addition of imazaquin (I) to chlormequat (CCC) improved the effects of CCC.     

Influence of reproductive organs on plant senescence in rice and wheat

The chlorophyll and protein contents of the flag, second and third leaves gradually decreased during the reproductive development of rice (Oryza sativa L. cv. Rasi) and wheat (Triticum aestivum L. cv. Sonalika) plants, whereas proline accumulation increased up to the grain maturation stage and slightly decreased thereafter. In rice plant, the rate of decrease in chlorophyll and protein and increase in proline level were higher in the flag leaf than in the second leaf. It was opposite in wheat plant. The export of [³²P]-phosphate from leaves to grains gradually increased reaching a maximal stage at the grain development stage, and then declined. The export of this radioisotope was greater in rice than in wheat. Removal of panicle at the anthesis and grainfilling stages delayed leaf senescence of rice plant, while in wheat the ponicle removal at any stage did not have a marked effect on delaying leaf senescence. The contents of chlorophyll and protein of glumes were higher in wheat than in rice. The variation of such source-sink relationship might be one of the possible reasons for the above effect on leaf senescence.     

Carbon isotope ratios in ear parts of triticale : influence of grain filling

Four triticale (×Triticosecale Wittmack) genotypes were grown under rainfed conditions with limited irrigation support in Lleida in northeast Spain. For each variety, samples consisting of 10 tillers with half-sterilized spikes were taken three times from anthesis to maturity. Carbon isotope ratios (δ¹³C) were then determined in water extracts from ear bracts (glumes, paleas, and lemmas), awns and flag leaves, and in powdered kernels. For the half-sterilized spikes, carbon isotope analysis was carried out separately in bracts and awns from fertile and nonfertile spikelets. The δ¹³C in the water-soluble fraction of awns, glumes, and glumells from fruitless spikelets was significantly higher than that from fertile spikelets sampled at mid-grain filling. Differences in δ¹³C among sterile and fertile spikelets were not significant in samples taken a few days after anthesis or at maturity. These results are in accordance with some degree of refixation by awns and ear bracts of the CO₂ respired by grains during grain filling. There was progressively higher δ¹³C from flag leaf blades to awns, glumes, and glumells. This variation in δ¹³C along plant parts may be caused by differences in the ratio of assimilation rate to CO₂-diffusive conductance. Values of δ¹³C of mature kernels were between the values at anthesis and mid-grain filling for the water-soluble fraction of flag leaves and inner bracts and were fairly similar to those of glumes and awns.     

Changes in Endogenous Gibberellins and the Metabolism of [H]GA(4) after Geostimulation in Shoots of the Oat Plant (Avena sativa)

The recovery from “lodging,” or bending over, by shoots of 42-day-old Avena sativa plants is controlled primarily by a negatively geotropic differential growth of the lower halves of the p-1 node-pulvinus and the base of the p-1 internode, relative to the upper halves. Although geostimulation causes a significant reduction in p-1 internode length, dry matter accumulation in the p-1 node-pulvinus is increased, apparently at the expense of the sheath. Recovery to an angle of 30° is associated with changes in endogenous gibberellin-like substances (GAs), and in differential metabolism of applied [³H]GA₄ (1.4 Curie per millimole). Although geostimulation depressed total GAs (relative to upright plant parts) to 0.40 and 0.13 for node-pulvini and sheaths, respectively, it increased them 2-fold for internodes. Within the plant part geostimulation increased GAs (relative to upper halves) 29- and 7-fold in lower halves of node-pulvini and internodes, respectively, but reduced GAs to 0.3 in lower halves of sheaths. At age 42 days a GA_4/7-like (nonpolar) substance predominates, with lesser amounts of a GA₃-like (polar) substance. Native GAs of Avena include GA₃, GA₄, and GA₇. Geostimulation enhanced the ratio of nonpolar to polar GAs for both halves of internodes, but tended to depress it for sheaths and nodepulvini.     

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.     

Bioregulation of carbohydrate metabolism in relation to source-sink operation during grain-filling phase of growth in wheat

Mobilization of free sugars from vegetative tissues to grain and their transformation to starch in relation to activities of some relevant enzymes during growth and development were investigated in wheat (Triticum aestivum L.). Vegetative tissues, viz. flag-leaf, flag-leaf sheath, nodes and internodes contained high concentration of free sugars from 70 DAS to 18 DPA and that was in the order of accumulation--flag-leaf sheath> flag-leaf and internodes > nodes. In these tissues, major portion of 14C appeared in endogenous sucrose, irrespective of the nature of (U-14C]-sugars supplied. In photosynthetic structures above flag-leaf node, namely peduncle, rachis and bracts, the free sugar make-up was maximum at anthesis (90 DAS). Activity of soluble acid invertase (EC 3.2.1.26) was high in these tissues during early stages of grain growth but reverse was true for soluble neutral invertase (EC 3.2.1.27) activity. In apical and basal portions of grain, free sugars were more or less similarly distributed in concentration. Linear and rapid accumulation of starch in endosperm paralleled with a decline in accumulation of this polymer in pericarp-aleurone. In the latter tissue, the activities of starch hydrolyzing enzymes, i.e alpha- and beta-amylases (3.2.1.1 and 3.2.1.2) were high during initial stages of grain growth. During active grain-filling, alkaline inorganic pyrophosphatase (EC 3.6.1.1) seemed to play a vital role during starch accumulation in endosperm, whereas the involvement of 3-PGA phosphatase (EC 3.1.3.38) was almost confined to pericarp-aleurone. Impairement of ear head photosynthesis by shading depressed starch synthesis (approximately 50%) indicating, thereby, the significant role of current photosynthates during grain-filling. The results suggested that grain growth in wheat was influenced by an efficient operation of source as well as regulatory factors, including enzymes, constituting intrinsic potential of grain sink.     

Canopy Apparent Photosynthetic Characteristics and Yield of Two Spike-Type Wheat Cultivars in Response to Row Spacing under High Plant Density

In northern China, large-spike wheat (Triticum aestivum L) is considered to have significant potential for increasing yields due to its greater single-plant productivity despite its lower percentage of effective tillers, and increasing the plant density is an effective means of achieving a higher grain yield. However, with increases in plant density, the amount of solar radiation intercepted by lower strata leaves is decreased and the rate of leaf senescence is accelerated. Row spacing can be manipulated to optimize the plant spatial distribution under high plant density, therefore improving light conditions within the canopy. Consequently, field experiments were conducted from 2010 to 2012 to investigate whether changes in row spacing under high plant density led to differences in canopy apparent photosynthesis (CAP), individual leaf photosynthesis and grain yield. Two different spike-type winter wheat cultivars, Jimai22 (a small-spike cultivar as a control cultivar) and Wennong6 (a large-spike cultivar), were grown at a constant plant density of 3,600,000 plants ha^–1 (a relatively higher plant density) over a wide range of row spacing as follows: 5-cm row spacing (R₀), 15-cm row spacing (R₁), 25-cm conventional row spacing (R₂), and 35-cm row spacing (R₃). The two-year investigations revealed that increased row spacing exhibited a significantly higher light transmission ratio (LT), which improved light conditions within the canopy; however, excessive light leakage losses in R₂ and R₃ treatments were not favorable to improved irradiation energy utilization efficiency. Aboveground biomass accumulation was influenced by row spacing. Two spike-type wheat accumulated greater biomass under 15-cm row spacing compared to other row spacing treatments, although a markedly improved photosynthetic rate (P_N), effective quantum yield of photosystem II (Φ_PSII) and maximal efficiency of photosystem II photochemistry (F_v/F_m) in the penultimate and third leaves were observed in R₂ and R₃ treatments. At the same time, a longer duration of CAP and green leaf area was maintained in R₁ during grain filling. Compared with conventional row spacing, Wennong6 in R₁ treatment obtained 21.0% and 19.1% higher grain yield in 2011 and 2012, respectively, while for Jimai22 it increased by 11.3% and 11.4%, respectively. A close association of yield with CAP and LAI at mid-grain filling was observed. In conclusion, for the tested growing conditions, decreasing the row spacing to an optimal distance (15 cm) maintained a longer duration of LAI and CAP during grain filling, made a better coordination of group and individual leaf photosynthesis, and accumulated higher aboveground biomass, leading to a greater grain yield. In addition, Wennong6 had a more rational canopy architecture than Jimai22 (improved LT and higher LAI) and CAP under 15-cm row spacing, leading to a higher grain yield, which indicated that the large-spike type cultivar has the potential to obtain higher yields by increasing plant density through optimum row spacing allocation (15 cm).