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.     

Effects of nitrogen deficiency on photosynthetic traits of maize hybrids released in different years

BACKGROUND AND AIMS: New maize (Zea mays) hybrids outperformed old ones even at reduced N rates. Understanding the mechanisms of the differences in performance between newer and older hybrids under N deficiency could provide avenues for breeding maize cultivars with large yield under N deficiency, and reduce environmental pollution caused by N fertilizers. METHODS: N deficiency effects on grain weight, plant weight, harvest index, leaf area and photosynthetic traits were studied in the field for six maize hybrids released during the past 50 years to compare their tolerance and to explore their physiological mechanisms. KEY RESULTS: N deficiency decreased grain yield and plant weight in all hybrids, especially in the older hybrids. However, there was no significant difference in harvest index, rate of light-saturated photosynthesis (Psat) 20 d before flowering, leaf area or plant weight at flowering between the N-deficient and control plants of all hybrids. Dry matter production after flowering of the N-deficient plants was significantly lower than that of the control plants in all hybrids, especially in the older hybrids, and was mostly due to differences in the rate of decrease in photosynthetic capacity during this stage. The lower Psat of the older hybrids was not due to stomatal limitation, as there was no significant difference in stomatal conductance (gs) and intercellular CO2 concentration (Ci) between the hybrids. N deficiency accelerated senescence, i.e. decreased chlorophyll and soluble protein contents, after anthesis more for the earlier released hybrids than for the later ones. N deficiency decreased phosphoenolpyruvate carboxylase (PEPCase) activity significantly more in older hybrids than newer hybrids, and affected the maximal efficiency of PSII photochemistry (Fv/Fm) only in the old hybrids and at the late stage. CONCLUSIONS: Compared with older (earlier released) hybrids, newer (later released) hybrids maintained greater plant and grain weight under N deficiency because their photosynthetic capacity decreased more slowly after anthesis, associated with smaller non-stomatal limitations due to maintenance of PEPCase activity, and chlorophyll and soluble protein content.     

Photosynthetic capacity of field-grown durum wheat under different N availabilities: A comparative study from leaf to canopy

The effect of N availability on photosynthetic capacity, growth parameters and yield was studied in field-grown durum-wheat plants at both the leaf and canopy levels. Two contrasting nitrogen levels (120 and 0 kg ha^?1) were assayed in a randomised block design with nine replicates each. Total biomass was measured at anthesis and yield and its agronomical components at maturity. Photosynthetic measurements were performed 2 weeks after anthesis in two plots of each N treatment. Flag leaves were measured, using a LI-COR 6400 combined with the chlorophyll fluorescence meter, and the whole canopy by measuring CO₂ and H₂O fluxes in an innovative canopy-chamber system. We showed a clear increase in photosynthetic gas exchange and chlorophyll contents with N fertilisation at both canopy and leaf levels. As a consequence the increase in yield as response to N fertilisation seems the result of a larger green leaf area combined with a higher photosynthetic capacity of the leaves attributable to an increase in the maximum carboxylation velocity of Rubisco. Moreover gas-exchange measurements of the flag leaf during grain filling seem to provide a realistic characterisation, not just of the photosynthetic performance of the crop, but also about the impact of N availability on yield. Thus, measurements performed on the flag leaf matched those at the canopy level, with proportional increases in terms of gas exchange and chlorophyll content, providing a fast, cheap and reliable estimation of canopy photosynthesis and the grain yield attained by the crop.     

Ozone tolerant maize hybrids maintain Rubisco content and activity during long-term exposure in the field

Ozone pollution is a damaging air pollutant that reduces maize yields equivalently to nutrient deficiency, heat, and aridity stress. Therefore, understanding the physiological and biochemical responses of maize to ozone pollution and identifying traits predictive of ozone tolerance is important. In this study, we examined the physiological, biochemical and yield responses of six maize hybrids to elevated ozone in the field using Free Air Ozone Enrichment. Elevated ozone stress reduced photosynthetic capacity, in vivo and in vitro, decreasing Rubisco content, but not activation state. Contrary to our hypotheses, variation in maize hybrid responses to ozone was not associated with stomatal limitation or antioxidant pools in maize. Rather, tolerance to ozone stress in the hybrid B73 × Mo17 was correlated with maintenance of leaf N content. Sensitive lines showed greater ozone-induced senescence and loss of photosynthetic capacity compared to the tolerant line.     

Comparison Between Growth Chamber and Field-Grown Zea mays Plants for Photosynthetic Carboxylase Activities and .Other Physiological Characteristics with Respect to Leaf Position

Baldy, P. 1986. Comparison between growth chamber and field-grownZea mays plants for photosynthetic carboxylase activities andother physiological characteristics with respect to leaf position.—J.exp. Bot. 37: 309–314. The lengths and fresh weights of leaves, and the amounts ofchlorophyll and protein per leaf, were higher in maize grownin a controlled environment than for field-grown maize harvestedat a similar stage of growth with the seventh leaf just emerging.However, the fresh weight and chlorophyll per unit leaf areawere not different and more protein was present per Unit leafarea in maize cultivated in the field, particularly in the 4thleaf. Except in the seventh leaf the youngest present, the PEP carboxylaseactivity was 2-fold higher in field-grown maize than in plantsfrom the controlled environment. The RuBP carboxylase activitywas not significantly affected by growth conditions. The maximumactivities of the carboxylases were found in the 5th leaf formaize grown in the controlled environment and the 4th leaf forfield maize; in these two leaves the ratios of the PEP: RuBPcarboxylasc activities were 3·0 and 4·4 respectively. The results are used to justify the choice of the fully expanded4th leaf of l9-d-old plants grown in a controlled environmentfor studies of the enzymes involved in the photosynthetic carbonmetabolism of this C₄ plant. Key words: Growth conditions, PEP carboxylase/RuBP carboxylase activity ratio, Zea mays leaf position     

Carbon dioxide exchange rates, ribulose bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase activities, and kernel growth characteristics of maize

Four high-yield-potential maize hybrids (FS854, CB596 x LH38, B73 x LH38, and B73 x Mo17) and four inbred lines (LH38, CB59G, Mo17, and B73) were grown in the field to study traits associated with leaf area duration (LAD) and the relationship between LAD and kernel growth characters. Based on decline in chlorophyll, leaf N concentration, CO(2) exchange rate, and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) activities, the hybrid B73 x Mo17 had a significantly shorter LAD than the other three hybrids. The shorter LAD was not due to maturity because B73 x Mo17 is in a maturity class similar to the other hybrids except CB59G x LH38, which is approximately 1 week earlier. At the time of grain maturity, leaves of B73 x Mo17 had lost all chlorophyll and CO(2) exchange and carboxylase activities. The other three hybrids, however, retained green leaves which still had 20% of the maximum CO(2) exchange rate. In addition, B73 x Mo17 remobilized leaf N more extensively. For all hybrids, declines in CO(2) exchange were closely correlated with declines in PEPCase activity, whereas the relationship between CO(2) exchange and Rubisco activity was weak. Responses of the inbred lines predicted, to some extent, physiological characteristics of the hybrids. CB59G and LH38 both had a longer LAD than either B73 or Mo17 as judged by decline in chlorophyll, leaf N, CO(2) exchange rate, and Rubisco and PEPCase activities. With the exception of B73 x LH38, kernel growth characteristics of the hybrids were related to LAD. Effective filling period (EFP) measured in days was 32.9, 31.5, 30.8, and 30.4 for FS854, CB59G x LH38, B73 x LH38, and B73 x Mo17, respectively. For FS854 and CB59G x LH38, the longer EFP was associated with a larger kernel weight. These data suggested that late season photoassimilate resulting from longer LAD could be utilized by the kernels of these two hybrids. For B73 x Mo17, the shorter LAD and EFP was associated with a kernel dry matter accumulation rate (10.1 milligrams per kernel per day) which was significantly higher than for the other three hybrids. Thus, the more rapid leaf senescence of B73 x Mo17 appeared to be coordinated with efficient leaf N remobilization and a relatively short grain-filling period characterized by rapid kernel dry matter accumulation.     

Inhibition and Acclimation of Photosynthesis to Heat Stress Is Closely Correlated with Activation of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

Increasing the leaf temperature of intact cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) plants caused a progressive decline in the light-saturated CO₂-exchange rate (CER). CER was more sensitive to increased leaf temperature in wheat than in cotton, and both species demonstrated photosynthetic acclimation when leaf temperature was increased gradually. Inhibition of CER was not a consequence of stomatal closure, as indicated by a positive relationship between leaf temperature and transpiration. The activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is regulated by Rubisco activase, was closely correlated with temperature-induced changes in CER. Nonphotochemical chlorophyll fluorescence quenching increased with leaf temperature in a manner consistent with inhibited CER and Rubisco activation. Both nonphotochemical fluorescence quenching and Rubisco activation were more sensitive to heat stress than the maximum quantum yield of photochemistry of photosystem II. Heat stress led to decreased 3-phosphoglyceric acid content and increased ribulose-1,5-bisphosphate content, which is indicative of inhibited metabolite flow through Rubisco. We conclude that heat stress inhibited CER primarily by decreasing the activation state of Rubisco via inhibition of Rubisco activase. Although Rubisco activation was more closely correlated with CER than the maximum quantum yield of photochemistry of photosystem II, both processes could be acclimated to heat stress by gradually increasing the leaf temperature.     

Importance of Rubisco activase in maize productivity based on mass selection procedure

The original maize (Zea mays L.) var. Zacatecas 58 (Z0) and five composites of cycles 5, 10, 15, 20, and 23 of stratified mass selection (Z5, Z10, Z15, Z20, and Z23) for improved productivity applied to the original variety were used as the model system. Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) (Rubisco) activity and the rates of net photosynthesis in the leaves above the ear were compared in all the composites during the grain filling period. Leaves were sampled weekly from anthesis to physiological maturity. Results showed a significant and gradual increase in Rubisco activity for the improved populations. In vivo photosynthesis measured by IRGA during the same period also showed increased levels associated with increases of grain yield. The highest Rubisco activity, photosynthetic rate and grain yield were found in the Z23 population. Western blot analysis for Rubisco protein did not show significant differences either during the filling period or between populations. The same analysis, however, for Rubisco activase protein showed increasing contents in the improved populations. These data confirm and complement previous findings, indicating that the stratified visual mass selection procedure applied to maize plants is associated with leaf content of Rubisco activase protein. The possible regulatory role of Rubisco activase during grain filling is discussed.     

Genetic modification of photosynthesis with E. coli genes for trehalose synthesis

Improvement in photosynthesis per unit leaf area has been difficult to alter by breeding or genetic modification. We report large changes in photosynthesis in Nicotiana tabacum transformed with E. coli genes for the trehalose pathway. Significantly, photosynthetic capacity (CO2 assimilation at varying light and CO2, and quantum yield of PSII electron transport) per unit leaf area and per leaf dry weight were increased in lines of N. tabacum transformed with the E. coli gene otsA, which encodes trehalose phosphate synthase. In contrast, transformation with otsB, which encodes trehalose phosphate phosphatase or Trec, encoding trehalose phosphate hydrolase, produced the opposite effect. Changes in CO2 assimilation per unit leaf area were closely related to the amount and activity of Rubisco, but not to the maximum activities of other Calvin cycle enzymes. Alterations in photosynthesis were associated with trehalose 6-phosphate content rather than trehalose. When growth parameters were determined, a greater photosynthetic capacity did not translate into greater relative growth rate or biomass. This was because photosynthetic capacity was negatively related to leaf area and leaf area ratio. In contrast, relative growth rate and biomass were positively related to leaf area. These results demonstrate a novel means of modifying Rubisco content and photosynthesis, and the complexities of regulation of photosynthesis at the whole plant level, with potential benefits to biomass production through improved leaf area.     

Grain yield and kernel weight of two maize genotypes differing in nitrogen use efficiency at various levels of nitrogen and carbohydrate availability during flowering and grain filling

Grain yield per plant (GYP) and mean kernel weight (KW) of maize (Zea mays L.) are sensitive to changes in the environment during the lag phase of kernel growth (the time after pollination in which the potential kernel size is determined), and during the phase of linear kernel growth. The aim of this study was to assess genotypic differences in the response to environmental stresses associated with N and/or carbohydrate shortage at different phases during plant development. The rate and timing of N and carbohydrate supply were modified by application of fertilizer, shading, and varying the plant density at sowing, at silking or at 14 d after silking. The effects of these treatments on the photosynthetic capacity, grain yield and mean kernel weight were investigated in two hybrids differing in N use efficiency. The total above-ground biomass and grain yield per plant of the efficient hybrid responded little to altered environmental conditions such as suboptimal N supply, enhanced inter-plant competition, and shading for 14 d during flowering, when compared to the less efficient genotype. We conclude that grain yields in the efficient genotype are less sensitive not only to N stress, but also to carbohydrate shortage before grain filling. Shading of N deficient plants from 14 d after silking to maturity did not significantly reduce grain yield in the non-efficient genotype, indicating complete sink limitation of grain yield during grain filling. In the efficient genotype, in contrast, grain yield of N-deficient plants was significantly reduced by shading during grain filling. The rate of photosynthesis declined with decreasing foliar N content. No genotypic differences in photosynthesis were observed at high or low foliar N contents. However, at high plant density and low N supply, the leaf chlorophyll content after flowering in the efficient genotype was higher than that in the non-efficient genotype. Obviously, the higher source capacity of the efficient genotype was not due to higher photosynthetic N use efficiency but due to maintenance of high chlorophyll contents under stressful conditions. In the efficient genotype, the harvest index was not significantly affected by N fertilization, plant density, or shading before the grain filling period. In contrast, in the non-efficient genotype the harvest index was diminished by N deficiency and shading during flowering. We conclude that the high yielding ability of the efficient genotype under stressful conditions was associated with formation of a high sink capacity of the grains under conditions of low carbohydrate and N availability during flowering and with maintenance of high source strength during grain filling under conditions of high plant density and low N availability.     

Does down‐regulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?

Dactylis glomerata was grown hydroponically in a controlled environment at ambient (360 μl l⁻¹) or elevated (680 μl l⁻¹) CO₂ and four concentrations of nitrogen (0.15, 0.6, 1.5 and 6.0 m M NO₃⁻), to test the hypothesis that reduction of photosynthetic capacity at elevated [CO₂] is dependent on N availability and mediated by a build-up of non-structural carbohydrates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO₂-enriched plants at low, but not high N supply and so the stimulation of net photosynthesis by CO₂ enhancement was less at low than at high N supply. CO₂ enrichment resulted in a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content on a leaf area basis at 0.6 and 1.5 m M NO₃⁻, but not at 0.15 and 6.0 m M NO₃⁻, and had no effect on the total N content of the leaf on an area basis. However, decreases in Rubisco content could be primarily accounted for by a decrease in total N content of leaves, independent of [CO₂]. A doubling of the Rubisco content by increasing the N supply beyond 0.6 m M had only a marginal effect on the maximum carboxylation velocity in vivo, suggesting that the fraction of inactive Rubisco increased with increasing N supply. Although CO₂-enriched plants accumulated more non-structural carbohydrates in the leaf, the reduction of photosynthetic capacity at low N supply was not mediated simply by a build-up of carbohydrates. In D . glomerata , the photosynthetic capacity was mainly determined by the total N content of the leaf.     

The Physiology of Heterosis in Sorghum with Respect to Environmental Stress

The hypothesis that heterosis in biomass production of sorghum(Sorghum bicolor L. Moench) may be ascribed to stability incarbon exchange rate (CER) over a wide range of environmentalconditions was evaluated. This hypothesis was based on previousresults from detached leaves that hybrids sustained greaterCER over a wider temperture range than their parents. Two grain sorghum hybrids (ATx378/RTx430 and ATx378/RTx434)and their parental lines were grown in the greenhouse in a gradientof ambient temperatures under two water regimes (well-irrigatedand drought up to heading). Plant water-use (estimated by weighingpots), leaf area, leaf gas exchange, grain yield, and above-groundbiomass were determined. Significant heterosis was found for biomass, grain yield perplant, and grain number per panicle. No heterosis occurred forharvest index, indicating that heterosis in grain yield wasdue to heterosis in biomass. Neither growth duration nor leafarea could explain heterosis in biomass. CER and stomatal conductancefor hybrid ATx378/RTx430 in the controls were greater than forboth its parents at leaf temperatures above 38 °C. This,however, was not observed in the other hybrid which was lessheterotic for biomass and grain yield in the controls. WhenCER data were subjected to a stability analysis by joint linearregression, the two hybrids had greater CER than their respectiveparents especially under conditions favouring high CER. Whenextreme stress conditions developed, the hybrid's performancedepended on its genetic background more than on heterosis. Sorghum, Sorghum bicolor L. Moench., heterosis, hybrids, photosynthesis, transpiration, stomata, drought, heat, temperature     

CO(2) Assimilation and Activities of Photosynthetic Enzymes in High Chlorophyll Fluorescence Mutants of Maize Having Low Levels of Ribulose 1,5-Bisphosphate Carboxylase

Photosynthetic properties were examined in several hcf (high chlorophyll fluorescence 11, 21, 42 and 45) nuclear recessive mutants of maize which were previously found to have normal photochemistry and low CO₂ fixation. Mutants usually either died after depletion of seed reserves (about 18 days after planting), or survived with slow growth up to 7 or 8 weeks. Both the activity and quantity of ribulose 1,5-bisphosphate carboxylase (Rubisco) were low in the mutants (5-25% of the normal siblings on a leaf area basis) and the loss of Rubisco tended to parallel the reduction in photosynthetic capacity. The Rubisco content in the mutants was often marginal for photosynthetic carbon gain, with some leaves and positions along a leaf having no net photosynthesis, while other leaves had a low carbon gain. Conversely, the activities of C₄ cycle enzymes, phosphoenolpyruvate carboxylase, pyruvate, Pi dikinase, NADP-malate dehydrogenase, and NADP-malic enzyme, were the same or only slightly reduced compared to the normal siblings. The mutants had about half as much chlorophyll content per leaf area as the normal green plants. However, the Rubisco activity in the mutants was low on both a leaf area and chlorophyll basis. Low Rubisco activity and lower chlorophyll content may both contribute to the low rates of photosynthesis in the mutants on a leaf area basis.     

Photosynthesis,photosynthetic enzymes and leaf area development in relation to hybrid vigour in Sorghum vulgare L.

Heterotic hybrids of sorghum produced more dry matter than their respective parents. Therefore, an analysis of leaf are development, rate of photosynthesis and activities of RuBP carboxylase and PEP carboxylase was made to determine whether the superior dry matter production in the hybrids could be attributed to any of these characteristics. Heterosis in leaf area was maintained at all stages in plant growth. Heterosis in photosynthesis was observed only during grain development in certain hybrids. At all other stages, the photosynthesis rate in hybrids were either intermediate or similar to one of the parents. No heterotic effect was observed in enzyme activity at any stage of growth. It is suggested that a multiplicative interaction between the heterotic leaf area and photosynthesis rate could possibly explain heterosis in dry matter production in heterotic hybrids.     

Trends in leaf photosynthesis in historical rice varieties developed in the Philippines since 1966

Crop improvement in terms of yield is rarely linked to leaf photosynthesis. However, in certain crop plants such as rice, it is predicted that an increase in photosynthetic rate will be required to support future grain yield potential. In order to understand the relationships between yield improvement and leaf photosynthesis, controlled environment conditions were used to grow 10 varieties which were released from the International Rice Research Institute (IRRI) between 1966 and 1995 and one newly developed line. Two growth light intensities were used: high light (1500 micromol m(-2) s(-1)) and low light (300 micromol m(-2) s(-1)). Gas exchange, leaf protein, chlorophyll, and leaf morphology were measured in the ninth leaf on the main stem. A high level of variation was observed among high light-grown plants for light-saturated photosynthetic rate per unit leaf area (P(max)), stomatal conductance (g), content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), and total leaf protein content. Notably, between 1966 and 1980 there was a decline in P(max), g, leaf protein, chlorophyll, and Rubisco content. Values recovered in those varieties released after 1980. This striking trend coincides with a previous published observation that grain yield in IRRI varieties released prior to 1980 correlated with harvest index whereas that for those released after 1980 correlated with biomass. P(max) showed significant correlations with both g and Rubisco content. Large differences were observed between high light- and low light-grown plants (photoacclimation). The photoacclimation 'range' for P(max) correlated with P(max) in high light-grown plants. It is concluded that (i) leaf photosynthesis may be systematically affected by breeding strategy; (ii) P(max) is a useful target for yield improvements where yield is limited by biomass production rather than partitioning; and (iii) the capacity for photoacclimation is related to high P(max) values.     

Functional analysis of corn husk photosynthesis

The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C(3) photosynthetic pathway, in contrast to its leaves, which use the C(4) photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO(2) compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO(2) assimilation rate in the husk was less than that in the leaves and did not saturate at high CO(2), indicating CO(2) diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO(2) compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C(4)-like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C(4) photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.     

大田增温对夏玉米光合特性的影响

在大田条件下研究了增温对两个夏玉米品种农大108(ND108)和掖单13号(YD13)光合特性及产量的影响.结果表明:大喇叭口期到成熟期增温显著降低了夏玉米籽粒产量,ND108和YD13的籽粒产量分别较对照降低了46.6%和45.1%;叶面积指数平均值分别降低了15.4%和11.5%;穗位叶叶片光合速率平均值分别降低了22.85%和18.14%;两个玉米品种叶片的叶绿素a和叶绿素b含量显著降低,但对叶绿素a的影响更显著.增温后玉米叶片的磷酸烯醇式丙酮酸羧化酶(PEPCase)和核酮糖二磷酸羧化酶(RuBPCase)活性都显著降低,ND108、YD13叶片的PEPCase和RuBPCase活性分别较对照降低了51.1%、32.4%和29.5%、7.7%.     

Use of Transgenic Plants with Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Antisense DNA to Evaluate the Rate Limitation of Photosynthesis under Water Stress

The biochemical lesion that causes impaired chloroplast metabolism (and, hence, photosynthetic capacity) in plants exposed to water deficits is still a subject of controversy. In this study we used tobacco (Nicotiana tabacum L.) transformed with "antisense" ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) DNA sequences to evaluate whether Rubisco or some other enzymic step in the photosynthetic carbon reduction cycle pathway rate limits photosynthesis at low leaf water potential ([psi]w). These transformants, along with the wild-type material, provided a novel model system allowing for an evaluation of photosynthetic response to water stress in near-isogenic plants with widely varying levels of functional Rubisco. It was determined that impaired chloroplast metabolism (rather than decreased leaf conductance to CO2) was the major cause of photosynthetic inhibition as leaf [psi]w declined. Significantly, the extent of photosynthetic inhibition at low [psi]w was identical in wild-type and transformed plants. Decreasing Rubisco activity by 68% did not sensitize photosynthetic capacity to water stress. It was hypothesized that, if water stress effects on Rubisco caused photosynthetic inhibition under stress, an increase in the steady-state level of the substrate for this enzyme, ribulose 1,5-bisphosphate (RuBP), would be associated with stress-induced photosynthetic inhibition. Steady-state levels of RuBP were reduced as leaf [psi]w declined, even in transformed plants with low levels of Rubisco. Based on the similarity in photosynthetic response to water stress in wild-type and transformed plants, the reduction in RuBP as stress developed, and studies that demonstrated that ATP supply did not rate limit photosynthesis under stress, we concluded that stress effects on an enzymic step involved in RuBP regeneration caused impaired chloroplast metabolism and photosynthetic inhibition in plants exposed to water deficits.