Pod photosynthesis and seed dark CO2 fixation support oil synthesis in developingBrassica seeds

Rate of photosynthesis and activities of photosynthetic carbon reduction cycle enzymes were determined in pods (siliqua), whereas rate of dark CO₂ fixation, oil content and activities of enzymes involved in dark CO₂ metabolism were measured in seeds ofBrassica campestris L. cv. Toria at different stages of pod/seed development. The period between 14 and 35 days after anthesis corresponded to active phase of seed development during which period, seed dry weight and oil content increased sharply. Rate of pod photosynthesis and activities of photosynthetic carbon reduction cycle enzymes were maximum in younger pods but sufficiently high levels were retained up to 40 days after anthesis. The rate of dark¹⁴CO₂ fixation in seeds increased up to 21 days after anthesis and declined thereafter but maintaining sufficiently high rates till 35 days after anthesis. Similarly various enzymes viz., phosphoenolpyruvate carboxylase, NAD⁺-malate dehydrogenase and NADP⁺-malic enzyme, involved in dark CO₂ metabolism retained sufficient activities during the above period. These enzyme activities were more than adequate to maintain the desired supply of malate which mainly arises from dark CO₂ fixation in seeds and further translocated to leucoplasts for onward synthesis of fatty acids. Enzyme localization experiments revealed phosphoenolpyruvate carboxylase and enzymes of sucrose metabolism to be present only in cytosol, whereas enzymes of glycolysis were present both in cytosolic and leucoplastic fractions. These results indicated that oil synthesis in developingBrassica seeds is supported by pod photosynthesis and dark CO₂ fixation in seeds as the former serves as the source of sucrose and the latter as a source of malate     

Carbon Partitioning among Leaves, Fruits, and Seeds during Development of Phaseolus vulgaris L

Development of vegetative and floral buds was found to be a key factor in establishing the way carbon is distributed among growing leaves and fruits in Phaseolus vulgaris L. plants. Leaves emerged principally during a period 14 to 32 days after planting while flowers were produced during a 10- to 12-day period near the end of leaf emergence. Timing of anthesis established the sigmoidal time course for dry weight accumulated by the composite of all fruits on the plant. During the first 12 days following anthesis, fruit growth mainly consisted of elongation and dry weight accumulation by the pod wall. Thereafter, seed dry weight increased for about 1 week, decreased markedly for several days, and then increased again over the next 2 weeks. Accumulation of imported carbon in individual seeds, measured by steady-state labeling, confirmed the time course for dry weight accumulation observed during seed development. Seed respiration rate initially increased rapidly along with dry weight and then remained nearly steady until seed maturation. A number of developmental events described in the literature coincided with the different phases of diauxic growth. The results demonstrated the feasibility of relating current rates of carbon import in individual seeds measured with tracer ¹⁴C to the rates of conversion of imported sucrose and use of the products for specific developmental processes. The resulting data are useful for evaluating the roles of conversion and utilization of imported sucrose in regulating import by developing seeds.     

Fixation of CO2 and Export of Photosynthate by the Carpel in Pisum sativum

Pods of Pisum sativum cv. Meteor were supplied with ¹⁴CO₂ and the fixation and distribution of the radiocarbon studied. A high fixation rate was found in all of the treated pods. In intact control plants about 60 per cent of the total radiocarbon fixed was exported from the carpet into the seeds of the treated pod in the 24 hour period following fixation. Almost all of the remainder was relained in the carpel itself. It was found that the seeds in the treated pod were the major sink for these fixation products and that the weight of the seed determined the level of import of radiocarbon from the carpel in both attached and detached pods. Removal of the seeds from the treated pod prior to ¹⁴CO₂ treatment reduced export from the fed carpel from 60 to 15 per cent, this being distributed mainly in the other pods and stem fractions. It appears that carpet photosynthate is likely to be of great importance in the growth and development of seeds enclosed by the carpel. control of fixation and export of radiocarbon seemed to be a function of the carpel and the seeds enclosed by the carpel and to be relatively independent of the rest of the plant. This could explain the absence of effect of GA on these processes.     

Growth and Development of Soybean (Glycine max [L.] Merr.) Pods: CO(2) Exchange and Enzyme Studies

The rates of CO₂ exchange and ¹⁴CO₂ incorporation in the light and dark and the activities of several photosynthetic, photorespiratory, and respiratory enzymes of soybean (Glycine max [L.] Merr. cv. Wye) reproductive structures were determined at weekly intervals from anthesis to pod maturity. At all stages of pod development soybean reproductive structures were found to be incapable of net photosynthesis under the experimental conditions employed, but capable of gross photosynthesis and light-induced ¹⁴CO₂ uptake. Consistent with the lack of net photosynthesis throughout the development of the reproductive structure, the maximum in vitro activity of ribulose 1,5-bisphosphate carboxylase (EC 4.1.1.39) in pod tissue was only 3% of that in leaf extracts when expressed on a fresh weight basis. We concluded that the major role of the reproductive structure of the soybean with respect to photosynthetic carbon metabolism is the reassimilation of its respiratory CO₂.     

Photosynthesis of bract and its contribution to seed maturity inCarpinus laxiflora

Light saturated net photosynthesis was measured in bracts and leaves ofCarpinus laxiflora, the major species in secondary forests in cool and intermediate temperate zones in Japan. The maximum net photosynthesis of leaves and bracts was essentially constant from May to early August and decreased gradually thereafter. For bracts, it was 3.2 μmol m⁻²s⁻¹, approximately half that for the leaves. The photosynthesis of bracts would thus appear to contribute significantly to seed maturity. The estimated production of bract based on the photosynthesis would make seeds (3 mg dry weight) mature for 37 days, assuming all photosynthate of the bracts to have been distributed in the seeds only. This was quite consistent with the growth curve for the seeds. A mast year phenomenon is discussed in relation to bract photosynthesis and leaf number.     

‘兰箭3号’箭筈豌豆荚果发育动态及腹缝线结构研究

箭筈豌豆(Vicia sativa)是高海拔地区重要的一年生豆科牧草,但荚果成熟时的开裂现象会造成种子的严重损失。该研究以栽培品种‘兰箭3号’为对象,对其荚果在发育过程中的形态特征、水分含量、腹缝线表面结构及腹缝线横截面解剖结构的动态变化进行观察分析,以探讨箭筈豌豆荚果的裂荚机理,为生产中确定种子收获的适宜时间提供理论依据。结果显示:(1)‘兰箭3号’约在盛花后25~30d荚果变为浅棕色,此时荚果已完成生理成熟,且荚果的大小和干重均达到最大值,含水量降到最小值;盛花后25d荚果腹缝线出现裂缝,盛花后35d腹缝线完全裂开。(2)‘兰箭3号’于盛花后20d腹缝线处离层细胞开始解体;盛花后25d,内、中、外果皮的薄壁细胞均开始失水皱缩,其中内果皮的薄壁细胞部分已开始破裂,离层细胞及其下面的薄壁细胞完全解体,外部果瓣缘细胞内侧细胞壁破裂,但外侧异常加厚的细胞壁仍然保持完整并连接两个果瓣,使荚果不开裂;盛花后30~35d,内、中、外果皮的薄壁细胞完全失水,细胞壁皱缩在一起,同时外部果瓣缘细胞外侧细胞壁断裂成两部分,荚果的两个果瓣裂开。研究表明,盛花后25~30d荚果失去绿色变为浅棕色时是‘兰箭3号’的适宜收获时间,且离层和细胞失水产生的机械拉力是导致箭筈豌豆荚果开裂的主要原因,推测外部果瓣缘细胞外侧增厚融合的细胞壁很可能是‘兰箭3号’抵抗裂荚的关键结构。     

The effect of elevated temperatures and their duration on the incidence of hollow heart in pea seeds

Plants of Pisum sativum cv. Pania were exposed to high temperature (35 ^oC day/25 ^oC night) at three stages in development. These were: (i) at process crop maturity, when the seed moisture content (m.c.) was 80%, i.e. about 10 days prior to pod wrinkle, (ii) at a seed m.c. of 70 to 80%, about the onset of pod wrinkle, and (iii) when the seed m.c. was 50 to 60%, 10 days after the onset of pod wrinkle. At each stage of development the high temperature treatment was imposed for 2,4,6, 8 or 10 days. Plants exposed to high temperature at seed m.c. of 70 to 80% produced seed with the highest incidence of hollow heart and at all stages of development the incidence increased with the length of exposure to the high temperature. Where plants at the development stage of pod wrinkle (seed m.c. 70–80%) were exposed to a range of temperature regimes for 5 days, over 20% of the seeds had hollow heart when the mean day/night temperature was 25 ^oC. Above a mean temperature of 25 ^oC, the percentage of affected seed increased with increasing day or night temperature. Over 80% of the seeds had hollow heart symptoms after 5 days exposure to a daily mean temperature of 32-5 ^oC. The severity of symptom development was not related to the proportion of seeds with the disorder and, under laboratory conditions, seed germination was not affected.     

Seed Development in Phaseolus vulgaris L. cv Seminole: I. Developmental Independence of Seed Maturation

Phaseolus vulgaris L. cv Seminole pods removed from the plant continued their development when incubated in suitable conditions. Seeds continued to grow and develop and pods and seeds passed through an apparently normal developmental sequence to dryness. Seed growth was at the expense of pod dry weight (DW) reserves. Losses of pod DW paralleled DW gains by seeds in detached pods and in pod cylinders containing a seed. The transfer activity was apparent only within the period 10 to 30 days after anthesis (DAA) with maximal activity between 15 to 20 DAA. This period corresponds to maximum pod growth and the attainment of maximal DW. Seeds are in only the early phase of seed growth at this time. No DW transfer was observed at developmental stages beyond 30 to 35 DAA when normal senescence DW losses in pods became evident and seeds were in the later phase of seed fill. Pods or pod cylinders remained green and succulent over the transfer period, later passing through yellowing and drying phases characteristic of normal development. DW transfer was dependent on funicle integrity and was readily detectable in pod cylinders after 7 days incubation. The DW transfer activity may contribute to continuing nutrition of seeds under conditions where the normal assimilate supply to seeds becomes limiting. Defoliation and water stress treatments applied to Phaseolus plants reduced seed yields but allowed persistence of seed maturation processes such that all seeds developing to dryness were capable of germination.     

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.     

14C UPTAKE BY THE MARINE ANGIOSPERM PHYLLOSPADIX SCOULERP

Rhizomes and attached leaves of Phyllospadix scouleri Hook, were collected in the intertidal zone along the central California coast and exposed to a solution of NaH¹⁴CO₃ in seawater under controlled laboratory conditions. Over a 90-min period roots and rhizomes absorbed very little ¹⁴C compared to leaves. Translocation during that time was minor. Plants pretreated with the photosynthetic inhibitor DCMU showed no ¹⁴C uptake, indicating that under normal circumstances the carbon which is absorbed by leaves is fixed and accumulates as photosynthate. The rate of gross photosynthesis was about 13 mg CO₂ g dry wt⁻¹ hr⁻¹. Gross photosynthesis of wet leaves exposed to ¹⁴CO₂ in air was significantly less than leaves exposed to NaH¹⁴CO₃. The effect of a leaf-grazing limpet (Notoacmea paleacea) on leaf anatomy and ¹⁴C uptake is discussed.     

Distribution between parts of the main shoot and the tillers of photosynthate produced before and after anthesis in the top three leaves of main shoots of Hobbit and Maris Huntsman winter wheat

The top three leaves of main shoots in crops of Hobbit and Maris Huntsman winter wheat were exposed to ¹⁴CO₂ at 22 and 16 days before and at 10 days after anthesis in 1978. The distribution of the ¹⁴C recovered in whole plants at anthesis and at maturity was measured. There was negligible loss of ¹⁴C between these two times, but some redistribution. The percentage in the tillers was negligible when the flag leaf (leaf 1) was exposed to ¹⁴CO₂, and otherwise less than 12% except for ¹⁴C absorbed by the third leaf at 16 days before anthesis, when it averaged 26% but was very variable. When ¹⁴C was supplied before anthesis, about 20% reached the grain whichever leaf had been supplied. The ear structures contained about 10% of that absorbed by the third leaf and 35% of that absorbed by the flag or second leaf. When ¹⁴C was supplied after anthesis, the amounts reaching the grain from the different leaves were: flag 82%, second 68%, third 56%. Most of the remainder was in the stem. The exposed leaf never retained more than 6%. The amount of ¹⁴C that moved from the stem to the grain between anthesis and maturity was about 50% greater in the semi-dwarf variety Hobbit than in Maris Huntsman. There was no significant varietal difference in the percentage of post-anthesis ¹⁴C reaching the grain. The ear structures of Hobbit contained about a third more ¹⁴C than those of Maris Huntsman. An additional 90 kg N ha^-1, which increased grain yield by 46%, had negligible effects on the distribution of ¹⁴C.     

Differences in the responses of stem diameter and pod thickness to drought stress during the grain filling stage in soybean plants

This study investigated the factor of the physiological characteristics causing the reduction of yield of soybean plants (Glycine max (L.) Merr.) by drought stress, by monitoring changes in stem diameter and pod thickness, and photosynthetic activity, partitioning of ¹³C-labeled photosynthate. Drought stress reduced the whole plant dry weight due to the decrease in leaf and pod dry matter accumulation; however, this stress did not have a significant effect on stem growth. Leaf photosynthesis was also severely decreased by drought stress in the early stage of stress treatment as leaf water potential decreased. Imposition of stress decreased pod thickness, but stem diameter increased. The adverse effect of drought stress on pod thickness was more evident at night than during the day. The stem diameter also shrank during the day and expanded at night, but the nocturnal increase in stem diameter during drought stress treatment was greater for stressed plants compared with well-watered controls. Drought stress significantly promoted ¹³C partitioning from the fed leaf to other parts of the plant; the stem was the largest beneficiary. Soluble carbohydrates accumulated in various plant parts under the influence of the stress, but starch concentration declined in all organs except the stem. These results indicated that stem growth was promoted by drought stress compared to pod growth at the early grain-filling stage.     

Photosynthetic and translocation pattern in contrasting winter wheat varieties

The rates of photosynthesis and patterns of translocation of ¹⁴C from the flag leaves of tall and semi-dwarf wheat varieties, with and without awns, were compared in a field experiment. The rates of photosynthesis of the awnless varieties fell during the 14 days after anthesis but those of the awned varieties did not change. Wide differences in translocation pattern of ¹⁴C supplied at anthesis were also found between varieties, but the differences in distribution pattern of ¹⁴C supplied 14 days later were less. At anthesis, the high yielding semi-dwarf selection translocated most carbohydrate to the ears and awned selections least. It is suggested that varietal differences in translocation pattern are determined by differences in the capacity of the grain to store carbohydrate.     

LEAF DEVELOPMENT,PHOTOSYNTHESIS, AND C14 DISTRIBUTION IN POPULUS DELTOIDES SEEDLINGS

Rates of net photosynthesis and dark respiration and distribution of C¹⁴ from selected leaves were determined for young cottonwood (Populus deltoides) trees at different stages of development. Four series of five trees—one series for each of four treated leaf positions—were included in the study. Maximum C¹⁴ export occurred when a leaf had just attained maximum size. Lower stem leaves reached maturity quickly and began exporting photosynthate when demands of the young seedling were high. Leaves at higher stem positions matured more slowly, but senescence was also delayed so their effective export life was prolonged. Translocation from a newly exporting leaf was primarily upward to developing leaves and the apex. As a leaf at any one position aged, the translocation pattern gradually shifted from upward to bi-directional and finally to a predominantly downward direction. Photosynthate translocated downward was incorporated into stem wood and roots. Maximum photosynthetic efficiency coincided with the downward shift of C¹⁴ export. Thereafter, net photosynthesis began to decline, at first slowly and then more rapidly. The patterns of photosynthesis, respiration, and C¹⁴ export associated with leaf age all varied according to leaf position on the stem.     

Distribution of photosynthate produced before and after anthesis in tall and semi-dwarf winter wheat, as affected by nitrogen fertiliser

In 2 years the distribution of radioactivity recovered in entire shoots of field-grown winter wheat was determined at various times after exposing the top two leaves (flag leaf or second leaf) to ¹⁴CO₂ for 30 s. In 1976 when ¹⁴C was supplied to either leaf 14 days before anthesis, 30% was in the ear at anthesis. Less than 5% was in the leaf exposed to ^I4CO₂. The remainder was equally divided between the stem above and below the flag-leaf node when the flag leaf had been exposed, and was mainly in the lower part of the stem when the second leaf had been exposed. By maturity the proportion in the stem had decreased; 20% of the total activity was in the grain and 30% was still in the ear structures. When ¹⁴C was supplied 10 days after anthesis, the proportion in the ear 24 h later ranged from 42 to 69% of that in the whole shoot when the flag leaf was exposed, and from 6 to 28% when the second leaf was exposed. At maturity these proportions increased to 92 and 85% when the ¹⁴C had been supplied to flag leaves and second leaves respectively. When ¹⁴C was supplied 25 days after anthesis to either flag leaves or second leaves, more than 90% of the activity was in the mature ears. Less than 5% of the ¹⁴C remaining at maturity from any treatment was still in the leaf exposed to ¹⁴CO₂. Between 2 and 6% of ¹⁴C supplied after anthesis was in the non-grain parts of the ear. The proportion of the ¹⁴C in the ear was greatest for the semi-dwarf varieties Maris Fundin and Hobbit, less for Maris Huntsman, and least for Cappelle-Desprez. These varietal differences were large 24 h after exposure to ¹⁴CO₂, especially in 1976. They were small and rarely significant at maturity. Nitrogen fertiliser up to 210 kg N ha^-1 had negligible effects on the distribution of ¹⁴C, although it greatly increased growth and yield, especially in 1975.     

Quantitative Analysis of Photosynthate Unloading in Developing Seeds of Phaseolus vulgaris L. : I. The Use of Steady-State Labeling

The pathway and kinetics of photosynthate unloading in developing seeds of bean (Phaseolus vulgaris L.) were investigated using steady-state labeling with ¹⁴CO₂. The continuous assimilation of ¹⁴CO₂ at constant specific activity produced stable tracer fluxes that facilitated straightforward analyses of photosynthate import and unloading in developing seeds. The kinetics of tracer equilibration within intact seeds were compatible with a symplastic route of photosynthate unloading in the seed coat. The import and partitioning of tracer within seeds were partially disrupted by the surgical excision of the distal halves of seeds as practiced during the preparation of “empty” seed coats for perfusion.     

Distribution of imported 14C in developing leaves of eastern cottonwood according to phyllotaxy

Summary Individual leaves of eastern cottonwood (Populus deltoides Bartr.), representing an ontogenetic series from leaf plastochron index (LPI) 3.0 to 8.0, were fed ¹⁴CO₂ and harvested after 2–24 h. Importing leaves from LPI-1.0 through 8.0 on each plant were sectioned into 9 parts, and each part was quantitatively assayed for ¹⁴C activity. The highest level of ¹⁴C import was by leaves from LPI 1.0 to 3.0, irrespective of source-leaf age. ¹⁴C was translocated preferentially to either the right or left lamina-half depending on the position of the importing leaf in the phyllotactic sequence and its stage of development. For example, import was high when the importing leaf and the source leaf had two vascular bundles in common, moderately high with one bundle in common, and low with no bundles in common. The distribution of ¹⁴C within young importing leaves was highest in the lamina tip and decreased toward the base. With increasing leaf age, incorporation declined in the lamina tip and increased in the base.It may be concluded that each cottonwood leaf progresses through a continuum of importing and exporting stages as its lamina expands. The photosynthate imported by a given leaf is compartmentalized, with different exporting leaves supplying photosynthate to rather restricted regions of the lamina. Such localization within the importing leaf depends on its vascular connections with each of the exporting leaves, and these are predictable from a knowledge of the phyllotaxy.Plant Physiologists.     

Alteration of C-assimilate partitioning in leaves of soybeans having increased reproductive loads at one node

The objectives of this study were to determine if the partitioning of recently fixed carbon between starch and water-soluble compounds could be altered by increasing the pod load in the leaf axil, and if the presence of source leaves acropetal to such a node would influence the partitioning of carbon within the subtending leaf. Soybeans (Glycine max L. Merr. cv Hodgson 78) were grown to full-bloom in a controlled environment chamber, and then deflowered at all nodes except the eighth. This treatment resulted in an 83% increase in the number of pods at the eighth node. At 24 days after flowering, one-half of the treated plants were girdled above the untreated node. Forty-two hours later, the eighth trifoliolate was pulsed with ¹⁴CO₂ and sampled for radiolabeled starch and water-soluble compounds (WSC) at 0.5, 2, 4, 8, 12, and 24th after labeling.

When no girdling was applied above the increased pod load at the eighth node more label was accumulated by the pod walls (+6.9%) and seeds (+6.3%) when compared to the controls. Starch accumulation was not altered in the labeled leaf of the nongirdled plants. When the stem was girdled above the eighth node, significantly less starch was retained in the labeled leaf. Girdling also resulted in an increase in label accumulation by the pod walls (+5.4%) and seeds (+6.6%). These data suggest that the plant will change the distribution patterns of assimilate to supply added sink demand before altering the partitioning of recently fixed carbon in the subtending leaf.

     

Effect of Defoliation or Excision of Growing Axillary Shoots on the Composition of Labeled Products of Photosynthesis in the Leaves and Xylem Sap of Kidney Bean

The content of ¹⁴C in the products of photosynthesis of the source leaf and xylem sap was investigated in kidney bean (Phaseolus vulgaris L.) plants during the stage of mass tillering. ¹⁴C partition was measured a day after two-minute photoassimilation of ¹⁴CO₂ by an individual mature leaf located in the middle part of the shoot. The source-sink relations were disturbed by the excision of all mature leaves (except the source leaf) or all growing axillary shoots. The leaves or growing axillary shoots were excised 15 min after leaf feeding with ¹⁴C₂. A day later, in plants with excised growing axillary shoots, the content of ¹⁴C in the source leaf was by 18% higher and in those with removed leaves by 15% lower than in control plants. The next day after the excision of growing axillary shoots, radioactivity of the xylem sap increased; after defoliation, both the volume of the xylem sap and its specific radioactivity decreased. In the xylem sap of defoliated plants, the proportion of ¹⁴C in malate decreased more than six times, whereas the proportion of ¹⁴C in amino acids somewhat increased (1.5 times). In two days, the volume of the xylem sap exuded by treated plants became the same as in control plants and its radioactivity decreased almost by an order of magnitude but essentially did not differ in the both types of treatment. It is concluded that the processes occurring in the roots are governed by photosynthesis but its regulatory effect is limited by a photoperiod and largely depends on changes in the ratio between biosynthesis of amino acids in the roots and leaves.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 518–521.Original Russian Text Copyright © 2005 by Chikov, Bakirova, Batasheva, Sergeeva.     

Effects of Nitrogen Fertilizer on the Distribution of Photosynthate during Grain Growth of Spring Wheat

The distribution of photosynthate labelled with ¹⁴C was studiedin spring wheat grown with different amounts of nitrogen fertilizerin the three years 1972–4, after exposing the flag leafor the leaf below the flag leaf to ¹⁴CO₂ at 6–10 or 19–26days after anthesis. The movement of ¹⁴C to ears was unaffectedby nitrogen fertilizer except after early exposure in 1973,when nitrogen increased the retention of ¹⁴C in stems at maturity The concentration of sugar in the top part of the shoot at theend of the day was unaffected by nitrogen in 1973, but at 22days after anthesis in 1974 the concentration of sucrose inthe glumes and rachis, and in the flag leaf lamina was increasedby nitrogen. Loss of sugar by translocation and respirationduring the night may explain why this increase in concentrationwas not reflected in the ¹⁴C distribution 24 h after supplying¹⁴C. The proportion of the total ¹⁴C content of the shoot that wasin the ear at maturity ranged from 68 to 95 per cent dependingon when and to which leaf the ¹⁴CO₂ was supplied. Less than5 per cent remained in the leaf exposed to ¹⁴CO₂. The proportionof the final ear weight contributed by the leaf below the flagleaf was about half that contributed by the flag leaf. In 1974 about 24 per cent of the ¹⁴C absorbed by the flag leaf,and 56 per cent of that absorbed by the second leaf, was lostby maturity, presumably by respiration. Most loss occurred inthe first 24 h.