Future CO2 concentrations, though not warmer temperatures, enhance wheat photosynthesis temperature responses

The temperature dependence of C3 photosynthesis is known to vary according to the growth environment. Atmospheric CO2 concentration and temperature are predicted to increase with climate change. To test whether long-term growth in elevated CO2 and temperature modifies photosynthesis temperature response, wheat (Triticum aestivum L.) was grown in ambient CO2 (370 micromol mol(-1)) and elevated CO2 (700 micromol mol(-1)) combined with ambient temperatures and 4 degrees C warmer ones, using temperature gradient chambers in the field. Flag leaf photosynthesis was measured at temperatures ranging from 20 to 35 degrees C and varying CO2 concentrations between ear emergence and anthesis. The maximum rate of carboxylation was determined in vitro in the first year of the experiment and from the photosynthesis-intercellular CO2 response in the second year. With measurement CO2 concentrations of 330 micromol mol(-1) or lower, growth temperature had no effect on flag leaf photosynthesis in plants grown in ambient CO2, while it increased photosynthesis in elevated growth CO2. However, warmer growth temperatures did not modify the response of photosynthesis to measurement temperatures from 20 to 35 degrees C. A central finding of this study was that the increase with temperature in photosynthesis and the photosynthesis temperature optimum were significantly higher in plants grown in elevated rather than ambient CO2. In association with this, growth in elevated CO2 increased the temperature response (activation energy) of the maximum rate of carboxylation. The results provide field evidence that growth under CO2 enrichment enhances the response of Rubisco activity to temperature in wheat.     

The role of sugars in integrating environmental signals during the regulation of leaf senescence

Although leaf senescence results in a loss of photosynthetic carbon fixation, the senescence-dependent release of nutrients, especially of nitrogen, is important for the growth of young leaves and for reproduction. Environmental regulation of senescence is therefore a vital factor in the carbon and nitrogen economy of plants. Leaf senescence is a highly plastic trait that is affected by a range of different environmental factors including light, nutrient supply, CO2 concentration, and abiotic and biotic stress. In this review, the focus is on the impact of environmental conditions on sugar accumulation and sugar signalling during senescence. By signalling a high availability of carbon relative to nitrogen in the old leaves, sugar accumulation can trigger leaf senescence. Sugar-induced senescence is therefore particularly important under low nitrogen availability and may also play a role in light signalling. Whether or not sugars are involved in regulating the senescence response of plants to elevated CO2 remains unresolved. Senescence can be delayed or accelerated in elevated CO2 and no clear relationship between sugar accumulation and senescence has been found. Plasticity in the response to environmental factors, such as daylength and sugar accumulation, varies between different Arabidopsis accessions. This natural variation can be exploited to analyse the genetic basis of the regulation of senescence and the consequences for growth and fecundity. Different evolutionary strategies, i.e. early senescence combined with a high reproductive effort or late senescence combined with a low reproductive effort, may be an important adaptation of Arabidopsis accessions to their natural habitat.     

Growth under elevated atmospheric CO(2) concentration accelerates leaf senescence in sunflower (Helianthus annuus L.) plants

Some morphogenetic and metabolic processes were sensitive to a high atmospheric CO(2) concentration during sunflower primary leaf ontogeny. Young leaves of sunflower plants growing under elevated CO(2) concentration exhibited increased growth, as reflected by the high specific leaf mass referred to as dry weight in young leaves (16days). The content of photosynthetic pigments decreased with leaf development, especially in plants grown under elevated CO(2) concentrations, suggesting that high CO(2) accelerates chlorophyll degradation, and also possibly leaf senescence. Elevated CO(2) concentration increased the oxidative stress in sunflower plants by increasing H(2)O(2) levels and decreasing activity of antioxidant enzymes such as catalase and ascorbate peroxidase. The loss of plant defenses probably increases the concentration of reactive oxygen species in the chloroplast, decreasing the photosynthetic pigment content as a result. Elevated CO(2) concentration was found to boost photosynthetic CO(2) fixation, especially in young leaves. High CO(2) also increased the starch and soluble sugar contents (glucose and fructose) and the C/N ratio during sunflower primary leaf development. At the beginning of senescence, we observed a strong increase in the hexoses to sucrose ratio that was especially marked at high CO(2) concentration. These results indicate that elevated CO(2) concentration could promote leaf senescence in sunflower plants by affecting the soluble sugar levels, the C/N ratio and the oxidative status during leaf ontogeny. It is likely that systemic signals produced in plants grown with elevated CO(2), lead to early senescence and a higher oxidation state of the cells of these plant leaves.     

Direct and Indirect Effects of Atmospheric Carbon Dioxide Enrichment on Leaf Respiration of Glycine max (L.) Merr

Long-term and short-term effects of CO2 enrichment on dark respiration were investigated using soybean (Glycine max [L.] Merr.) plants grown at either 35.5 or 71.0 Pa CO2. Indirect effects, or effects of growth in elevated CO2, were examined using a functional model that partitioned respiration into growth and maintenance components. Direct effects, or immediate effects of a short-term change in CO2, were examined by measuring dark respiration, first, at the CO2 partial pressure at which plants were grown, and second, after equilibration in the reciprocal CO2 partial pressure. The functional component model indicated that the maintenance coefficient of respiration increased 34% with elevated CO2, whereas the growth coefficient was not significantly affected. Changes in maintenance respiration were correlated with a 33% increase in leaf total nonstructural carbohydrate concentration, but leaf nitrogen content of soybean leaves was not affected by CO2 enrichment. Thus, increased maintenance respiration may be a consequence of increased nonstructural carbohydrate accumulation. When whole soybean plants were switched from low CO2 to high CO2 for a brief period, leaf respiration was always reduced. However, this direct effect of CO2 partial pressure was approximately 50% less in plants grown in elevated CO2. We conclude from this study that there are potentially important effects of CO2 enrichment on plant respiration but that the effects are different for plants given a short-term increase in CO2 partial pressure versus plants grown in elevated CO2.     

Elevated CO(2) induces biochemical and ultrastructural changes in leaves of the C(4) cereal sorghum

We analyzed the impact of growth at either 350 (ambient) or 700 (elevated) microL L(-1) CO(2) on key elements of the C(4) pathway (photosynthesis, carbon isotope discrimination, and leaf anatomy) using the C(4) cereal sorghum (Sorghum bicolor L. Moench.). Gas-exchange analysis of the CO(2) response of photosynthesis indicated that both carboxylation efficiency and the CO(2) saturated rate of photosynthesis were lower in plants grown at elevated relative to ambient CO(2). This was accompanied by a 49% reduction in the phosphoenolpyruvate carboxylase content of leaves (area basis) in the elevated CO(2)-grown plants, but no change in Rubisco content. Despite the lower phosphoenolpyruvate carboxylase content, there was a 3-fold increase in C isotope discrimination in leaves of plants grown at elevated CO(2) and bundle sheath leakiness was estimated to be 24% and 33%, respectively, for the ambient and elevated CO(2)-grown plants. However, we could detect no difference in quantum yield. The ratio of quantum yield of CO(2) fixation to PSII efficiency was lower in plants grown at elevated CO(2), but only when leaf internal was below 50 microL L(-1). This suggests a reduction in the efficiency of the C(4) cycle when [CO(2)] is low, and also implies increased electron transport to acceptors other than CO(2). Analysis of leaf sections using a transmission electron microscope indicated a 2-fold decrease in the thickness of the bundle sheath cell walls in plants grown at elevated relative to ambient CO(2). These results suggest that significant acclimation to increased CO(2) concentrations occurs in sorghum.     

Mobilization of metabolites from leaves to grains as the cause of monocarpic senescence in rice

The pattern of senescence was studied by following the changes in chlorophyll and protein in the leaves and by measuring ³²P retention and export from source to sink during development of the rice plant (Oryza sativa L. cv. Jaya) subjected to different manipulative treatments. With the advance of reproductive development, the chronological sequence of leaf senescence was changed, so that the flag and the third leaf senesced earlier than did the second leaf. In presence of the daughter shoot of defruited plants, senescence was delayed in all three leaves of the mother plant, as compared to the same leaves of intact plants. Senescence of all three leaves was further delayed when both panicle and daughter shoots were removed from the plant. The above manipulative treatments caused the initial sequential pattern of senescence of leaves to persist. Removal of both panicle and daughter shoots caused little export of ³²P between leaves. In the presence of daughter shoots of defruited plants, export of ³²P was maximum from leaves of the mother plant to the nearest daughter shoots. This led to earlier senescence of such mother plant leaves than that of plants from which both panicle and daughter shoots were removed. The pattern of senescence and export of ³²P in the flag and the second leaf of the daughter shoot was essentially the same as that of the intact plant. Based on these findings, it was concluded that mobilization of metabolites from source to sink is the primary cause of monocarpic senescence in rice.     

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

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

Growth, photosynthesis, nitrogen partitioning and responses to CO2 enrichment in a barley mutant lacking NADH-dependent nitrate reductase activity

Plant growth, photosynthesis and leaf constituents were examined in the wild-type (WT) and mutant nar1 of barley (Hordeum vulgare L. cv. Steptoe) that contains a defective structural gene encoding NADH-dependent nitrate reductase (NADH-NAR). In controlled environment experiments, total biomass, rates of photosynthesis, stomatal conductance, intercellular CO(2) concentrations and foliar non-structural carbohydrate levels were unchanged or differed slightly in the mutant compared with the WT. Both genotypes displayed accelerated plant growth rates when the CO(2) partial pressure was increased from 36 to 98 Pa. Total NADH-NAR activity was 90% lower in the mutant than in the WT, and this was further decreased by CO(2) enrichment in both genotypes. Inorganic nitrate was greater in the mutant than in the WT, whereas in situ nitrate assimilation by excised leaves was two-fold greater for the WT than for the mutant. Foliar ammonia was 50% lower in the mutant than in the WT under ambient CO(2). Ammonia levels in the WT were decreased by about one-half by CO(2) enrichment, whereas ammonia was unaffected by elevated CO(2) in mutant leaves. Total soluble amino acid concentrations in WT and mutant plants grown in the ambient CO(2) treatment were 30.1 and 28.4 micromol g(-1) FW, respectively, when measured at the onset of the light period. Seven of the twelve individual amino acids reported here increased during the first 12 h of light in the ambient CO(2) treatment, leading to a doubling of total soluble amino acids in the WT. The most striking effect of the mutation was to eliminate increases of glutamine, aspartate and alanine during the latter half of the photoperiod in the ambient CO(2) treatment. Growth in elevated CO(2) decreased levels of total soluble amino acids on a diurnal basis in the WT but not in mutant barley leaves. The above results indicated that a defect in NADH-NAR primarily affected nitrogenous leaf constituents in barley. Also, we did not observe synergistic effects of CO(2) enrichment and decreased foliar NADH-NAR activity on most N-containing compounds.     

Subtilisin-like serine proteases involved in N remobilization during grain filling in wheat

The induction of two subtilisin-like proteases (P1 and P2) associated with stress-induced senescence in young plants was investigated in adult wheat plants during the grain-filling period. Western blot analysis of flag leaf extracts showed that P1 was induced very late in the life cycle of the plants (9 days post-anthesis) and that 7 days later it reached a 2.5-fold increase with respect to the initial value at anthesis. On the other hand, the P2 signal was already detected previous to anthesis and increased soon after anthesis, reaching a fourfold increase by the end of the grain-filling period. The induction of P1 and P2 temporally correlates with the degradation of the Rubisco small and large subunits in the flag leaf, as well as with nitrogen (N) accumulation in the ears. At the same time, a decrease in the endogenous concentration of the cytokinins isopentenyladenine and isopentenyladenosine (iP + iPA) in the leaves was observed. In detached leaves senescing in the dark, the levels of both proteases were affected by 6-benzylaminopurine application: the induction of P1 was completely prevented, whereas the induction of P2 was reduced. Our findings demonstrate that both P1 and P2 are expressed in leaves of adult plants and are induced during natural senescence. These results enable us to postulate their participation in N remobilization to developing grains during monocarpic senescence and their regulation by a cytokinin-mediated mechanism.     

Characterisation of grain yield and nitrogen level in relation to flag leaf senescence of wheat varieties

Three wheat cultivars ( Triticum aestivum L.), Splendeur, Hobbit and Maris Huntsman grown in pots were compared. Especially when compared to Splendeur, the flag leaf senesced most rapidly in Maris Huntsman, which presented the most rapid loss of moisture, chlorophyll and nitrogen. The uptake of exogenous nitrogen during the post-anthesis period was lower in the rapidly than in the slowly senescing variety. A higher concentration of free amino nitrogen in the flag leaf at a given sampling date was associated with a lower percentage decrease of soluble proteins at the following date. Acid proteinase activity in the flag leaf was inversely related to moisture percentage and free amino nitrogen level, but unrelated to the nitrogen loss of the flag leaf. Acid proteinase activity in the flag leaf was directly related to grain nitrogen percentage, but inversely related to grain yield. Grain yield was also directly related to the mean soluble protein content of the flag leaf through senescence.     

Regulation of Leaf Senescence by Reproductive Sink Intensity in Cowpea (Vigna unguiculata L. Walp)

The relationship between seed number per pod and senescenceof the leaf in its axil was examined in a determinate cowpea(Vigna unguiculata L. Walp) variety C.779. The seed number perpod was reduced at all fruiting nodes by surgical excision ofpart of the 4-d-old pod. Leaf senescence as measured by lossof leaf area, chlorophyll content and soluble protein was sloweddown in leaves supporting the development of an artificiallyreduced number of seeds. Diminished nitrogen mobilization fromthe leaf could not account for the reduced rate of leaf senescence.The result suggests the involvement of a senescence signal fromthe developing seeds to the leaf in its axil. Development ofthe basal half of the excised pod in the cowpea provides a uniquesystem for manipulating seed number per pod. Senescence, monocarpic, chlorophyll, protein, Vigna unguiculata, cowpea     

Elevated CO2 reduces leaf damage by insect herbivores in a forest community

By altering foliage quality, exposure to elevated levels of atmospheric CO(2) potentially affects the amount of herbivore damage experienced by plants. Here, we quantified foliar carbon (C) and nitrogen (N) content, C : N ratio, phenolic levels, specific leaf area (SLA) and the amount of leaf tissue damaged by chewing insects for 12 hardwood tree species grown in plots exposed to elevated CO(2) (ambient plus 200 microl l(-1)) using free-air CO(2) enrichment (FACE) over 3 yr. The effects of elevated CO(2) varied considerably by year and across species. Elevated CO(2) decreased herbivore damage across 12 species in 1 yr but had no detectable effect in others. Decreased damage may have been related to lower average foliar N concentration and SLA and increased C : N ratio and phenolic content for some species under elevated compared with ambient CO(2). It remains unclear how these changes in leaf properties affect herbivory. Damage to the leaves of hardwood trees by herbivorous insects may be reduced in the future as the concentration of CO(2) continues to increase, perhaps altering the trophic structure of forest ecosystems.     

Consequences of elevated CO2, augmented nitrogen-deposition and soil type on the soluble nitrogen and sulphur in the phloem of beech (Fagus sylvatica) and spruce (Picea abies) in a competitive situation

Mixed spruce-beech plantations grown in large open-top chambers (OTC) were used to study consequences of elevated CO2, nitrogen-deposition and soil type on plant internal nitrogen and sulphur cycling of juvenile beech (Fagus sylvatica L.) and spruce (Picea abies Karst.) in a competitive situation. Processes of re-cycling as a consequence of protein turnover during leaf senescence in autumn were of further interest. For this purpose, phloem sap was collected in September 1998 and analysed for the composition and concentrations of organic and inorganic nitrogen and sulphur compounds. The phloem exudate of spruce showed higher total soluble non-protein nitrogen (TSNN) concentration on calcareous soil than on acidic soil, independent of the treatment. N-fertilization increased the N-concentration of phloem exudate significantly on both soil types, mainly by an increase of Arg and Gln concentrations. Elevated CO2 slightly increased TSNN on calcareous, but not on acidic soil. The combination of elevated CO2 and augmented N-deposition induced a further increase of TSNN on calcareous soil, but caused a lower N-effect on TSNN on acidic soil. Arg, the main TSNN component in phloem exudate, mediated this effect. Since Arg is considered to be a major nitrogen storage compound, it is concluded that in autumn elevated CO2 and augmented N-deposition, influence storage of N rather than N-supply of spruce. An effect of elevated CO2 and augmented N-deposition on GSH and sulphate concentrations in phloem exudate of spruce was not observed on acidic soil. On calcareous soil augmented N-deposition enhanced, elevated CO2 decreased phloem exudate GSH contents. In combination, elevated CO2 compensated the positive effect of N-deposition. The effects of elevated CO2 and augmented N-deposition on phloem sap N- and S-contents described above were not observed for beech trees. Apparently, elevated CO2 and augmented N-deposition did not affect plants internal S and N cycling of beech grown in spruce-beech plantations.     

Senescence mechanisms

Senescence in plants is usually viewed as an internally programmed degeneration leading to death. It is a developmental process that occurs in many different tissues and serves different purposes. Generally, apoptosis refers to programmed death of small numbers of animal cells, and it shows some special features at the cell level. Some senescing plant cells show some symptoms typical of apoptosis, while others do not. This review will focus primarily on leaf senescence with ultimate aim of explaining whole plant senescence (i.e., monocarpic senescence). Traditionally, the ideas on senescence mechanisms fall into two major groupings, nutrient deficiencies (e.g., starvation) and genetic programming (i.e., senescence-promoting and senescence-inhibiting genes). Considerable evidence indicates that nutrient deficiencies are not central senescence program components, while increasing evidence supports genetic programming. Because chlorophyll (Chl) and chloroplast (CP) breakdown are so prominent, leaf senescence is generally measured in terms of Chl loss. Although CP breakdown may not be the proximate cause of leaf cell death, it certainly is important as a source of nutrients for use elsewhere, e.g., for developing reproductive structures in monocarpic plants, and this loss limits assimilatory capacity. The CP is dismantled in an orderly sequence. Individual protein complexes seem to be taken out all at once, not one subunit at a time. Removal of any component, e.g., Chl, seems to destabilize the whole complex. It is of special interest that senescing CPs secrete Chl-containing globules indicating that some CP components are broken down outside the CP. Senescence appears to be imposed on the CP by the nucleus, and all the known senescence-altering genes except one, cytG in soybean, are nuclear. Only the d₁d₂ mutation(s) in soybean prevents a broad range of leaf senescence processes. Exactly, what causes cell death is unclear; however, the selective thiol protease inhibitor, E-64, does delay death, and this suggests that proteases play a key role.     

施氮和大气CO2浓度升高对小麦旗叶光合电子传递和分配的影响

采用开顶式气室盆栽培养小麦,设计2个大气CO2浓度(正常:400 μmol.mol-1;高:760 μmol·mol-1)、2个氮素水平(0和200 mg·kg-1土)的组合处理,通过测定小麦抽穗期旗叶氮素和叶绿素浓度、光合速率(Pn)-胞间CO2浓度(C1)响应曲线及荧光动力学参数,来测算小麦叶片光合电子传递速率等,研究了高大气CO2浓度下施氮对小麦旗叶光合能量分配的影响.结果表明:与正常大气CO2浓度相比,高大气CO2浓度下小麦叶片氮浓度和叶绿素浓度降低,高氮处理的小麦叶片叶绿素a/b升高.施氮后小麦叶片PSⅡ最大光化学效率(Fv/Fm)、PSⅡ反应中心最大量子产额(Fv'/Fm')、PSⅡ反应中心的开放比例(qr)和PSⅡ反应中心实际光化学效率(φPSⅡ)在大气CO2浓度升高后无明显变化,虽然叶片非光化学猝灭系数(NPQ)显著降低,但PSⅡ总电子传递速率(JF)无明显增加;不施氮处理的Fv'/Fm'、φPSⅡ和NPQ在高大气CO2浓度下显著降低,尽管Fv/Fm和qp无明显变化,JF仍显著下降.施氮后小麦叶片JF增加,参与光化学反应的非环式电子流传递速率(Jc)明显升高.大气CO2浓度升高使参与光呼吸的非环式电子流传递速率(J0)、Rubisco氧化速率(V0)、光合电子的光呼吸/光化学传递速率比(J0/Jc)和Rubisco氧化/羧化比(V0/Vc)降低,但使Jc和Rubisco羧化速率(Vc)增加.因此,高大气CO2浓度下小麦叶片氮浓度和叶绿素浓度降低,而增施氮素使通过PSⅡ反应中心的电子流速率显著增加,促进了光合电子流向光化学方向的传递,使更多的电子进入Rubisco羧化过程,Pn显著升高.     

Photosynthesis, growth and nutrient changes in non-nodulated Phaseolus vulgaris grown under atmospheric and elevated carbon dioxide conditions

The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol^-1, respectively) CO₂ concentrations ([CO₂]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO₂ had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO₂-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO₂], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO₂ enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO₂ occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO₂-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO₂ plants. Under enhanced CO₂, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO₂ grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.