High-throughput characterization,correlation, and mapping of leaf photosynthetic and functional traits in the soybean (Glycine max) nested association mapping population

Photosynthesis is a key target to improve crop production in many species including soybean [Glycine max (L.) Merr.]. A challenge is that phenotyping photosynthetic traits by traditional approaches is slow and destructive. There is proof-of-concept for leaf hyperspectral reflectance as a rapid method to model photosynthetic traits. However, the crucial step of demonstrating that hyperspectral approaches can be used to advance understanding of the genetic architecture of photosynthetic traits is untested. To address this challenge, we used full-range (500–2,400 nm) leaf reflectance spectroscopy to build partial least squares regression models to estimate leaf traits, including the rate-limiting processes of photosynthesis, maximum Rubisco carboxylation rate, and maximum electron transport. In total, 11 models were produced from a diverse population of soybean sampled over multiple field seasons to estimate photosynthetic parameters, chlorophyll content, leaf carbon and leaf nitrogen percentage, and specific leaf area (with R² from 0.56 to 0.96 and root mean square error approximately <10% of the range of calibration data). We explore the utility of these models by applying them to the soybean nested association mapping population, which showed variability in photosynthetic and leaf traits. Genetic mapping provided insights into the underlying genetic architecture of photosynthetic traits and potential improvement in soybean. Notably, the maximum Rubisco carboxylation rate mapped to a region of chromosome 19 containing genes encoding multiple small subunits of Rubisco. We also mapped the maximum electron transport rate to a region of chromosome 10 containing a fructose 1,6-bisphosphatase gene, encoding an important enzyme in the regeneration of ribulose 1,5-bisphosphate and the sucrose biosynthetic pathway. The estimated rate-limiting steps of photosynthesis were low or negatively correlated with yield suggesting that these traits are not influenced by the same genetic mechanisms and are not limiting yield in the soybean NAM population. Leaf carbon percentage, leaf nitrogen percentage, and specific leaf area showed strong correlations with yield and may be of interest in breeding programs as a proxy for yield. This work is among the first to use hyperspectral reflectance to model and map the genetic architecture of the rate-limiting steps of photosynthesis.     

Has photosynthetic capacity increased with 80 years of soybean breeding? An examination of historical soybean cultivars

Crop biomass production is a function of the efficiencies with which sunlight can be intercepted by the canopy and then converted into biomass. Conversion efficiency has been identified as a target for improvement to enhance crop biomass and yield. Greater conversion efficiency in modern soybean [Glycine max (L.) Merr.] cultivars was documented in recent field trials, and this study explored the physiological basis for this observation. In replicated field trials conducted over three successive years, diurnal leaf gas exchange and photosynthetic CO₂ response curves were measured in 24 soybean cultivars with year of release dates (YOR) from 1923 to 2007. Maximum photosynthetic capacity, mesophyll conductance and nighttime respiration have not changed consistently with cultivar release date. However, daily carbon gain was periodically greater in more recently released cultivars compared with older cultivars. Our analysis suggests that this difference in daily carbon gain primarily occurred when stomatal conductance and soil water content were high. There was also evidence for greater chlorophyll content and greater sink capacity late in the growing season in more recently released soybean varieties. Better understanding of the mechanisms that have improved conversion efficiency in the past may help identify new, promising targets for the future.     

Physiological nitrogen efficiency in rice: Nitrogen utilization,photosynthesis, and yield potential

Characteristics of rice (Oryza sativa) as a crop plant are briefly introduced, and the relationship between formation of yield potential and nitrogen (N) nutrition is described on the basis of studies using 15N as a tracer. In addition, the relationship between the leaf photosynthetic capacity and leaf N, and the factors limiting leaf photosynthesis under different growth conditions are reviewed. Finally, targets for improving rice yield potential are discussed with a focus on the role of increased photosynthesis efficiency in relation to leaf N status and the photosynthetic components in the leaves.     

大豆生理生态特性与产量的关系

比较研究了1970s、1980s和1990s期间不同大豆 ( Glycine max (L.) Merr.)品种的生理生态特性:光合作用、蒸腾作用、气孔导度、水分利用效率、胞间CO2浓度和叶片水势,分析了各生理生态特性之间以及与产量在不同生育期的关系.研究表明光合速率和产量之间存在显著的相关关系,高产大豆同时具有高的气孔导度和水势,胞间CO2浓度则很低,特别是在鼓粒期关系更为显著.高产大豆的光合作用在鼓粒期达到最高值,光合作用日变化呈双峰曲线.在鼓粒期高产大豆的光合产物大量运往籽粒中,对于高产品种和高光效-"源"固然重要,而"流"-光合产物的合理运转和分配对产量则更为重要.     

Effect of soil moisture stress on legume-rhizobium symbiosis in soybeans

Summary In a pot culture experiment, the influence of soil moisture stress at different physiological stages of soybean, cv. Hark, on nodulation, symbiosis and nitrogen accumulation was studied. Moisture stress reduced leghemoglobin content of root nodules and nitrogen uptake by plants. It had no effect on number of bacteroids. Stress at mid bloom and rapid pod filling stages reduced yield and seed protein content. However, these parameters were not affected by stress at nodule initiation and early flowering stages, though, flower initiation and maturity of the plant were delayed. Moisture stress at any stage did not alter nitrogen status of roots.     

Limited carbohydrate availability as a potential cause of fruit abortion in Rubus chamaemorus

Fruit abortion can be caused by a range of abiotic and biotic factors. To gain a better understanding of the causes of the high fruit abortion frequency in cloudberry ( Rubus chamaemorus L.), we manipulated different sources of carbon, that is, leaves and rhizome. We also manipulated flower number to see if competition between floral ramets explained fruit abortion in cloudberry. Reducing the number of flowers had no impact on fruit abortion frequency. In fact, the species forms an extensive rhizome network with only a few ramets per clone and competition between floral ramets is unlikely. Ramet defoliation had limited impact on fruit abortion, but successful fruit development was affected by rhizome length. The longer the rhizome, the higher the chances to mature a fruit. These results suggest that current photoassimilate production by the reproductive ramet alone is insufficient to insure fruit development. Carbon can come from other ramets but distances are usually high between ramets. Fruit production might thus depend on the use of stored carbohydrates in the rhizome to balance insufficient photosynthetic contribution during fruit production.     

The influence of pre-floral growth and development on the pathway of floral development, dry matter distribution and seed yield in oilseed rape (Brassica napus L.)

Patterns of floral development, dry matter distribution and seed yield were examined in winter oilseed rape plants subjected to different pre-floral growth environments. The duration of pre-floral growth and plant size at flower initiation, measured in terms of total mainstem leaf number, were manipulated by varying the temperature between seedling emergence and flower initiation. Exposure of seedlings to low temperatures from cotyledon expansion onwards markedly reduced the duration of pre-floral growth and the number of leaves on the mainstem. The subsequent development pattern of plants was largely dependent on the date of flower initiation and therefore vernalisation requirement. Indeed, the period of growth from flower initiation to maturity, considered on the basis of thermal time, was directly related to the duration of pre-floral growth and mainstem leaf number. The thermal durations of the bud development phase and flowering period in plants exposed to different pre-floral cold treatments but with a common date of flower initiation were similarly linked to these two parameters. Plants exposed to prolonged periods of low temperature treatment from cotyledon expansion onwards initiated fewer mainstem leaves during a relatively short pre-floral growth phase and their yield potential was limited by a reduction in branch and flower numbers. Plants maintained at higher temperatures produced more mainstem leaves during an extended period of pre-floral growth and supported a greater number of branches and flowers. However, this additional yield potential was not realised due to a reduction in seed numbers and mean seed weight. It appeared that seed yield of these plants was limited by increased competition between an excessive number of lower branches and flowers, a problem apparently created by excessive pre-floral growth. Minimal competition for available assimilates between the limited number of branches of plants with a shorter pre-floral growth phase and fewer mainstem leaves, resulted in lower levels of pod abortion, greater seed production and ultimately increased seed yields.     

A high-grain protein content locus on barley (Hordeum vulgare) chromosome 6 is associated with increased flag leaf proteolysis and nitrogen remobilization

Leaf senescence and nitrogen remobilization from senescing tissues are two important factors determining grain protein content (GPC) in cereals. We compared near-isogenic barley ( Hordeum vulgare L.) germplasm varying in the allelic state of a major GPC quantitative trait locus on chromosome 6, delineated by molecular markers HVM74 and ABG458 and explaining approximately 46% of the variability in this trait. High GPC was consistently associated with earlier whole-plant senescence. SDS–PAGE and immunoblot analysis of flag leaf proteins indicated earlier leaf protein [including ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)] degradation in high-GPC germplasm. This was accompanied by enhanced availability of ammonium and glutamine in developing kernels, suggesting increased phloem retranslocation of nitrogen. Based on previous microarray analysis, we performed a detailed expression study of six leaf genes, tentatively involved in plastidial proteolysis, vacuolar proteolysis, intermediary N metabolism and N transport. All of these were upregulated in high-GPC barley, mostly around 21 to 28 days past anthesis, prior to or around the time demonstrating maximal differences in leaf protein (including Rubisco) levels. Therefore, these genes represent potential targets to manipulate grain protein accumulation. It appears likely that their functional analysis will enhance our understanding of whole-plant N recycling. Additionally, earlier leaf (photosynthetic) protein degradation may lead to reduced N carbon assimilation in high-GPC germplasm, explaining past studies demonstrating a negative correlation between GPC and yield.     

The challenge of remobilisation in plant nitrogen economy. A survey of physio-agronomic and molecular approaches

In this article, we discuss the ways in which our understanding of the controls of nitrogen remobilisation in model species and crop plants have been increased through classical physiological studies and the use of transgenic plants or mutants with modified capacities for nitrogen or carbon assimilation and recycling. An improved understanding of the transition between nitrogen assimilation and nitrogen recycling will be vital, if improvements in crop nitrogen use efficiency are to reduce the need for excessive input of fertilisers and improve or stabilise yield. In this review, we present an overall view of past work and more recent studies on this topic, using different plants systems and models depicting the biochemical and molecular events occurring during the transition between sink leaves and source leaves. These models may provide a way to identify the nature of the metabolic or developmental signals triggering in a coordinate manner nitrogen and carbon recycling during leaf senescence. Another way of developing crop varieties with improved nitrogen use efficiency, and identifying key elements controlling the process of nitrogen remobilisation, is the use of quantitative genetics. We present and discuss recent findings on the genetic variability and basis of nitrogen use efficiency in crops in general and in maize in particular. A genetic approach using maize recombinant inbred lines was undertaken allowing the detection of Quantitative Trait Loci (QTLs) for morphological traits, grain yield and its components under high nitrogen or low nitrogen input. Co‐mapping was observed between genes encoding enzymes involved in nitrogen assimilation (nitrate reductase, glutamine synthetase) and these Quantitative Trait Loci. All coincidences were consistent with the expected physiological function of the corresponding enzyme activities. This work strongly suggests that in maize, nitrogen use efficiency can be improved both by marker‐assisted selection and genetic engineering.     

Respiration and nitrogen assimilation: targeting mitochondria-associated metabolism as a means to enhance nitrogen use efficiency

Considerable advances in our understanding of the control of mitochondrial metabolism and its interactions with nitrogen metabolism and associated carbon/nitrogen interactions have occurred in recent years, particularly highlighting important roles in cellular redox homeostasis. The tricarboxylic acid (TCA) cycle is a central metabolic hub for the interacting pathways of respiration, nitrogen assimilation, and photorespiration, with components that show considerable flexibility in relation to adaptations to the different functions of mitochondria in photosynthetic and non-photosynthetic cells. By comparison, the operation of the oxidative pentose phosphate pathway appears to represent a significant limitation to nitrogen assimilation in non-photosynthetic tissues. Valuable new insights have been gained concerning the roles of the different enzymes involved in the production of 2-oxoglutarate (2-OG) for ammonia assimilation, yielding an improved understanding of the crucial role of cellular energy balance as a broker of co-ordinate regulation. Taken together with new information on the mechanisms that co-ordinate the expression of genes involved in organellar functions, including energy metabolism, and the potential for exploiting the existing flexibility for NAD(P)H utilization in the respiratory electron transport chain to drive nitrogen assimilation, the evidence that mitochondrial metabolism and machinery are potential novel targets for the enhancement of nitrogen use efficiency (NUE) is explored.     

Interrelationship Between Plant Developmental Stage, Plant Growth Rate, Nitrate Utilization and Nitrogen Fixation in Hydroponically Grown Soybean

The growth rate of the indeterminate soybean plant [Glycinemax (L.)Merr.] slows as it proceeds from vegetative phase intoreproductive growth. Yet, the well-nodulated plant acquiresmost of its nitrogen during reproductive growth. Thus, the interrelationshipbetween plant developmental stage and nitrogen fixation wasexamined. It is shown that, regardless of the age of the hydroponicallygrown soybean plant at the time of its inoculation with Bradyrhizobiumjaponicum, the highest rate of nitrogen fixation occurs duringthe pod-filling stage (R5). Nevertheless, maximum total nitrogenfixation is generally achieved when inoculation occurs at thefull-bloom stage (R2). It is shown, however, that flower budsand flowering are not responsible for the enhanced nodulationand nitrogen fixation. Rather, the data suggest that the onsetof rapid nodulation occurs soon after the initiation of thedevelopmentally programmed drop in foliar nitrate reductaseactivity. The ensuing increase in nitrogen fixation providesthe plant with much of its needed nitrogen and hence stimulatesplant mass accumulation during pod-fill. It is suggested thatnitrogen fixation enhances growth of the soybean plant by increasingits net photosynthetic efficiency during reproductive growthand by providing the needed nitrogen at the appropriate timefor maximum seed growth. Key words: Glycine max, nitrate, nitrogen fixation, nodulation     

苹果密植园与间伐园树冠层内叶片光合潜力比较

通过对成龄苹果密植园和间伐园树冠不同层次和部位叶片光合潜力及辐射通量密度、叶片N含量和比叶重等指标的比较分析,研究了苹果园改造前后辐射能和氮素利用效率差异及其与产量品质的关系.结果表明：间伐显著改善了冠层内的辐射环境,间伐园冠层内的辐射分布明显比密植园均匀,相对辐射通量密度小于30%的无效光区接近0,而密植园冠层内的最低相对辐射通量密度为17%,在相对高度03以下均为无效光区;间伐园内冠层叶片的光合效率显著提高,间伐园树冠中、下部叶片的光合速率比密植园分别提高了78%和102%;叶片的最大羧化速率和最大电子传递速率也有较大幅度的提升.苹果园冠层叶片的光合效率与叶片N含量存在显著的相关关系,而叶片N含量又与辐射通量密度存在显著的相关关系,因此,可根据冠层叶片相对N含量的垂直分布间接和定量地判断叶片的光合效率或相对辐射通量密度的空间分布.     

How is ozone pollution reducing our food supply?

Ground-level ozone pollution is already decreasing global crop yields (from ～2.2-5.5% for maize to 3.9-15% and 8.5-14% for wheat and soybean, respectively), to differing extents depending on genotype and environmental conditions, and this problem is predicted to escalate given climate change and increasing ozone precursor emissions in many areas. Here a summary is provided of how ozone pollution affects yield in a variety of crops, thus impacting global food security. Ozone causes visible injury symptoms to foliage; it induces early senescence and abscission of leaves; it can reduce stomatal aperture and thereby carbon uptake, and/or directly reduce photosynthetic carbon fixation; it can moderate biomass growth via carbon availability or more directly; it can decrease translocation of fixed carbon to edible plant parts (grains, fruits, pods, roots) due either to reduced availability at source, redirection to synthesis of chemical protectants, or reduced transport capabilities via phloem; decreased carbon transport to roots reduces nutrient and water uptake and affects anchorage; ozone can moderate or bring forward flowering and induce pollen sterility; it induces ovule and/or grain abortion; and finally it reduces the ability of some genotypes to withstand other stresses such as drought, high vapour pressure deficit, and high photon flux density via effects on stomatal control. This latter point is emphasized here, given predictions that atmospheric conditions conducive to drought formation that also give rise to intense precursor emission events will become more severe over the coming decades.     

Leaf physiological aspects of nitrogen-use efficiency in Brassica campestris L.: quantitative genetic variation across nutrient treatments

Summary Quantitative genetic parameters for leaf physiological and whole-plant aspects of nitrogen-use efficiency in Brassica camprestris L. were estimated in three nutrient treatments in the greenhouse. Narrow-sense heritabilities and genetic correlations varied across treatments for some traits. Sire effects were significant for leaf nitrogen content in near-optimal and super-optimal, but not in suboptimal nutrient treatments. Additive genetic variation for two estimates of leaf physiological nitrogen-use efficiency (nitrogen-based photosynthetic capacity and leaf carbon: nitrogen ratio) was significant only in the suboptimal nutrient treatment. Area-based photosynthetic capacity, on the other hand, exhibited no heritable variation in any nutrient treatment. Heritability estimates of aboveground biomass and flower production were greatest in sub- and super-optimal treatments, respectively. Negative genetic correlations between leaf nitrogen content and both estimates of leaf nitrogen-use efficiency were evident in the super-optimal treatment. Aboveground biomass and leaf nitrogen-use efficiency were positively correlated in the suboptimal treatment, suggesting that growth differences were due in part to the efficiency with which nitrogen was utilized in physiological processes. Although implications for breeding may differ for different sources of germ plasm or different measures of performance or yield, selection for improved whole-plant performance through increased nitrogen-use efficiency should proceed best in suboptimal nutrient treatments.     

Actual and potential nitrogen fixation ofAlbizia rhizobium symbioses as affected by nitrate

Actual nitrogen fixation of root nodules of differentAlbizia-rhizobium symbioses, was compared with the potential nitrogen fixation of isolated bacteroids. The potential nitrogen fixation exceeded actual nitrogen fixation in all symbionts. After addition of nitrate the actual nitrogen fixation decreased more than did potential nitrogen fixation in effective symbiosis, whereas in a less effective symbiosis, the actual and potential nitrogen fixation increased as a result of better photosynthate supply to the roots and nodules. As confirmed by correlation analysis, the nitrogen fixation and photosynthetic yield of suboptimum symbioses were relatively enhanced by dressing with inorganic nitrogen fertilizer.     

Cytokinin Regulation of Flower and Pod Set in Soybeans (Glycine max(L.) Merr.)

Exogenous application of cytokinin to raceme tissues of soybean(Glycine max(L.) Merr.) has been shown to stimulate flower productionand to prevent flower abortion. The effects of these hormoneapplications have been ascertained for treated tissues, butthe effects of cytokinins on total seed yields in treated plantshave not been evaluated. Our objectives were to examine theeffects of systemic cytokinin applications on soybean yieldsusing an experimental line of soybeans, SD-87001, that has beenshown to be highly sensitive to exogenous cytokinin application.Soybeans were grown hydroponically or in pots in the greenhouse,and 6-benzylaminopurine (BA) was introduced into the xylem streamthrough a cotton wick for 2 weeks during anthesis. After theplants had matured, the number of pods, seeds per pod, and thetotal seed weight per plant were measured. In the greenhouse,application of 3.4 x 10^-7 moles of BA resulted in a 79% increasein seed yield compared with controls. Results of field trialsshowed much greater variability within treatments, with consistent,but non-significant increases in seed number and total yieldsof about 3%. Data suggest that cytokinin levels play a significantrole in determining total yield in soybeans, and that increasingcytokinin concentrations in certain environments may resultin increased total seed production. Copyright 2001 Annals ofBotany Company Glycine max, soybean, flower abortion, cytokinin, 6-benzylaminopurine, hydroponic, seed yield, wicking     

Raising yield potential of wheat. III. Optimizing partitioning to grain while maintaining lodging resistance

A substantial increase in grain yield potential is required, along with better use of water and fertilizer, to ensure food security and environmental protection in future decades. For improvements in photosynthetic capacity to result in additional wheat yield, extra assimilates must be partitioned to developing spikes and grains and/or potential grain weight increased to accommodate the extra assimilates. At the same time, improvement in dry matter partitioning to spikes should ensure that it does not increase stem or root lodging. It is therefore crucial that improvements in structural and reproductive aspects of growth accompany increases in photosynthesis to enhance the net agronomic benefits of genetic modifications. In this article, six complementary approaches are proposed, namely: (i) optimizing developmental pattern to maximize spike fertility and grain number, (ii) optimizing spike growth to maximize grain number and dry matter harvest index, (iii) improving spike fertility through desensitizing floret abortion to environmental cues, (iv) improving potential grain size and grain filling, and (v) improving lodging resistance. Since many of the traits tackled in these approaches interact strongly, an integrative modelling approach is also proposed, to (vi) identify any trade-offs between key traits, hence to define target ideotypes in quantitative terms. The potential for genetic dissection of key traits via quantitative trait loci analysis is discussed for the efficient deployment of existing variation in breeding programmes. These proposals should maximize returns in food production from investments in increased crop biomass by increasing spike fertility, grain number per unit area and harvest index whilst optimizing the trade-offs with potential grain weight and lodging resistance.     

抗旱性不同品种的小麦叶片中光合电子传递和分配对氮素水平的响应

在大田控制条件下,以植物叶片气体交换和叶绿素荧光测定相结合的方法,研究抗旱性不同品种冬小麦拔节期叶片的光合电子传递及激发能利用分配对氮素响应的结果表明,施氮可提高抗旱性不同品种小麦叶片天线色素吸收光能的能力,虽然氮素不能改变激发能在光合碳还原（PCR）和光合碳氧化（PCO）之间的分配比例,但可提高PSⅡ总电子传递速率（JF）和Pn。低氮下不同品种小麦叶片的热耗散比例有差异,但中高氛下叶片之间无显著差异。旱地品种的鼻值随氮素水平的提高而先增加后下降,而水地品种则表现为持续升高：2个小麦品种的叶片J0值随氮素水平的提高而呈持续升高的变化趋势。氮素对叶片PSⅡ反应中心活性有影响．而不同抗旱性品种之间亦有差别,说明施氮可改善小麦叶片热耗散和光化学反应对激发能的竞争关系,从而增强光合机构的自我保护能力。     

Naturally occurring variation in high temperature induced floral bud abortion across Arabidopsis thaliana accessions

A system to study the basis of high temperature-induced floral bud abortion using naturally occurring variation for heat-tolerance of floral development among Arabidopsis thaliana (L.) Heynh. wild-collected accessions is described. High temperature-induced floral bud abortion was dependent on both temperature and duration of exposure. Normalizing high temperature exposures to degree-hours (°C-h) above 33 °C indicated that abortion of flower buds increased as exposure increased between 200 and 300 °C-h above 33 °C and exposures > 300 °C-h above 33 °C resulted in abortion of the entire primary inflorescence. Thirteen wild-collected Arabidopsis accessions representing a latitudinal gradient were screened for variation in high temperature-induced floral bud abortion, and Col-0 and No-0 were selected as models for heat-tolerance and -sensitivity for flower development, respectively. No-0 flower buds were heat-sensitive across a wider range of developmental stages (stages 9–12, compared to stage 12 for Col-0 flower buds). Exposing the inflorescence alone to high temperature was sufficient to induce floral bud abortion, and Col-0 and No-0 photosynthetic rates were similar during high temperature exposure and recovery, indicating that high temperature induced floral abortion is not simply due to reductions in carbon assimilation under high temperatures. Determining that exposing floral buds alone to high temperature is sufficient to induce abortion and identifying the stages of floral development sensitive to high temperature-induced abortion will aid in identifying the developmental events subject to disruption under high temperatures.