Photosynthetic acclimation to elevated CO2 in wheat cultivars

Wheat (T. aestivum) cvs. Kalyansona and Kundan grown under atmospheric (CA) and elevated CO₂ concentrations (650±50 cm³ m^-3 - CE) in open top chambers were examined for net photosynthetic rate (PN), stomatal limitation (l _s) of P _N, ribulose-1,5-bisphosphate carboxylase (RuBPC) activity, and saccharide content of the leaves. The P _N values of both CA- and CE-grown plants compared at the same CO₂ concentration showed a down regulation under CE at the post-anthesis stage. The negative acclimation of P _N appeared to be due to both stomatal and mesophyll components, and the RuBPC activity got also adjusted. There was a decrease in activation state of RuBPC under CE. In connection with this, an increased accumulation of saccharides in wheat leaf under CE was observed. Kalyansona, owing to its larger sink potential in terms of the number of grains, showed a greater enhancement under CE in both post-ear emergence dry matter production and grain yield. Under CE, this cultivar also showed a lower down regulation of P _N than Kundan. This revised version was published online in June 2006 with corrections to the Cover Date.     

Optimal stomatal conductance in relation to photosynthesis in climatically contrasting Eucalyptus species under drought

Models of stomatal conductance (g_s) are based on coupling between g_s and CO₂ assimilation (A_net), and it is often assumed that the slope of this relationship (‘g₁’) is constant across species. However, if different plant species have adapted to different access costs of water, then there will be differences in g₁ among species. We hypothesized that g₁ should vary among species adapted to different climates, and tested the theory and its linkage to plant hydraulics using four Eucalyptus species from different climatic origins in a common garden. Optimal stomatal theory predicts that species from sub‐humid zones have a lower marginal water cost of C gain, hence lower g₁ than humid‐zone species. In agreement with the theory that g₁ is related to tissue carbon costs for water supply, we found a relationship between wood density and g₁ across Eucalyptus species of contrasting climatic origins. There were significant reductions in the parameter g₁ during drought in humid but not sub‐humid species, with the latter group maintaining g₁ in drought. There are strong differences in stomatal behaviour among related tree species in agreement with optimal stomatal theory, and these differences are consistent with the economics involved in water uptake and transport for carbon gain.     

Theory and practice of a portable photosynthesis instrument

Abstract. Theory and practice of non-steady-state portable photosynthesis instruments (LI-6000 and 6200, LI-COR Inc., Nebraska, U.S.A.) are presented. Mass balance equations for the time dependence of H₂O and CO₂ mol fractions within the leaf chamber were used to describe instrument function. Measurements for each run were fitted to an exponential function to estimate average rates of CO₂ assimilation and transpiration during the measurement period. Stomatal conductances and intercellular CO₂ mol fractions were also computed. Linear data analysis used in the LI-6200 produced similar results for assimilation rates, stomatal conductances and intercellular CO₂ concentrations compared to a more rigorous nonlinear analysis, provided humidity within the chamber was kept constant during the measurement period. Instrument performance for CO₂ fluxes was confirmed by injecting pure CO₂ at steady rates from a microsyringe into the chamber. Miniature evaporimeters were designed to check H₂O flux measurements. Significant discrepancies were observed between LI-6200 estimates of H₂O fluxes and direct measurement and errors were attributed to adsorption desorption of water vapour on chamber walls or to leaks. The leaf chamber should be stored at humidities and temperatures similar to those during measurement conditions for maximum reliability of results.     

Role of silicon in enhancing resistance to freezing stress in two contrasting winter wheat cultivars

The main objective of this study was to elucidate the roles of silicon (Si) in enhancing tolerance to freezing stress (?5 °C) in two contrasting wheat (Triticum aestivum L.) cultivars: i.e. cv. Yangmai No. 5, a freezing-susceptible cultivar and cv. Linmai No. 2, a freezing-tolerant cultivar. Shoot dry weight of the freezing-susceptible wheat was significantly lower under freezing stress than in controls, but increased significantly with Si amendment. The freezing treatment did not affect shoot dry weight of the freezing-tolerant cultivar. The leaf water content was considerably decreased by freezing stress in the freezing-susceptible cultivar, but was significantly increased by Si amendment. In contrast, freezing treatment did not significantly reduce leaf water content in the freezing-tolerant cultivar and Si played no role in water retention in this cultivar. The concentrations of H₂O₂ and free proline along with malondialdehyde (MDA) were progressively enhanced by freezing stress in the two wheat cultivars used, but were significantly suppressed by amendment with Si. The major antioxidant enzyme activities and non-enzymatic antioxidants (i.e. glutathione and ascorbic acid) in the leaves of freezing-stressed plants were decreased, but were stimulated significantly by the exogenous Si. The possible mechanisms for Si-enhanced freezing stress may be attributed to the higher antioxidant defense activity and lower lipid peroxidation through water retention in leaf tissues.     

Physiological framework for adaptation of stomata to CO2 from glacial to future concentrations

In response to short-term fluctuations in atmospheric CO(2) concentration, c(a), plants adjust leaf diffusive conductance to CO(2), g(c), via feedback regulation of stomatal aperture as part of a mechanism for optimizing CO(2) uptake with respect to water loss. The operational range of this elaborate control mechanism is determined by the maximum diffusive conductance to CO(2), g(c(max)), which is set by the size (S) and density (number per unit area, D) of stomata on the leaf surface. Here, we show that, in response to long-term exposure to elevated or subambient c(a), plants alter g(c(max)) in the direction of the short-term feedback response of g(c) to c(a) via adjustment of S and D. This adaptive feedback response to c(a), consistent with long-term optimization of leaf gas exchange, was observed in four species spanning a diverse taxonomic range (the lycophyte Selaginella uncinata, the fern Osmunda regalis and the angiosperms Commelina communis and Vicia faba). Furthermore, using direct observation as well as flow cytometry, we observed correlated increases in S, guard cell nucleus size and average apparent 1C DNA amount in epidermal cell nuclei with increasing c(a), suggesting that stomatal and leaf adaptation to c(a) is linked to genome scaling.     

Photosynthetic limitations in olive cultivars with different sensitivity to salt stress

Olive (Olea europea L) is one of the most valuable and widespread fruit trees in the Mediterranean area. To breed olive for resistance to salinity, an environmental constraint typical of the Mediterranean, is an important goal. The photosynthetic limitations associated with salt stress caused by irrigation with saline (200 mm ) water were assessed with simultaneous gas‐exchange and fluorescence field measurements in six olive cultivars. Cultivars were found to possess inherently different photosynthesis when non‐stressed. When exposed to salt stress, cultivars with inherently high photosynthesis showed the highest photosynthetic reductions. There was no relationship between salt accumulation and photosynthesis reduction in either young or old leaves. Thus photosynthetic sensitivity to salt did not depend on salt exclusion or compartmentalization in the old leaves of the olive cultivars investigated. Salt reduced the photochemical efficiency, but this reduction was also not associated with photosynthesis reduction. Salt caused a reduction of stomatal and mesophyll conductance, especially in cultivars with inherently high photosynthesis. Mesophyll conductance was generally strongly associated with photosynthesis, but not in salt‐stressed leaves with a mesophyll conductance higher than 50 mmol m^?2 s^?1. The combined reduction of stomatal and mesophyll conductances in salt‐stressed leaves increased the CO₂ draw‐down between ambient air and the chloroplasts. The CO₂ draw‐down was strongly associated with photosynthesis reduction of salt‐stressed leaves but also with the variable photosynthesis of controls. The relationship between photosynthesis and CO₂ draw‐down remained unchanged in most of the cultivars, suggesting no or small changes in Rubisco activity of salt‐stressed leaves. The present results indicate that the low chloroplast CO₂ concentration set by both low stomatal and mesophyll conductances were the main limitations of photosynthesis in salt‐stressed olive as well as in cultivars with inherently low photosynthesis. It is consequently suggested that, independently of the apparent sensitivity of photosynthesis to salt, this effect may be relieved if conductances to CO₂ diffusion are restored.     

The wheat durable,multipathogen resistance gene Lr34 confers partial blast resistance in rice

The wheat gene Lr34 confers durable and partial field resistance against the obligate biotrophic, pathogenic rust fungi and powdery mildew in adult wheat plants. The resistant Lr34 allele evolved after wheat domestication through two gain‐of‐function mutations in an ATP‐binding cassette transporter gene. An Lr34‐like fungal disease resistance with a similar broad‐spectrum specificity and durability has not been described in other cereals. Here, we transformed the resistant Lr34 allele into the japonica rice cultivar Nipponbare. Transgenic rice plants expressing Lr34 showed increased resistance against multiple isolates of the hemibiotrophic pathogen Magnaporthe oryzae, the causal agent of rice blast disease. Host cell invasion during the biotrophic growth phase of rice blast was delayed in Lr34‐expressing rice plants, resulting in smaller necrotic lesions on leaves. Lines with Lr34 also developed a typical, senescence‐based leaf tip necrosis (LTN) phenotype. Development of LTN during early seedling growth had a negative impact on formation of axillary shoots and spikelets in some transgenic lines. One transgenic line developed LTN only at adult plant stage which was correlated with lower Lr34 expression levels at seedling stage. This line showed normal tiller formation and more importantly, disease resistance in this particular line was not compromised. Interestingly, Lr34 in rice is effective against a hemibiotrophic pathogen with a lifestyle and infection strategy that is different from obligate biotrophic rusts and mildew fungi. Lr34 might therefore be used as a source in rice breeding to improve broad‐spectrum disease resistance against the most devastating fungal disease of rice.     

Stomata response to changes in temperature and humidity in wheat cultivars grown under contrasting climatic conditions

Stomatal response to changes in temperature and humidity was studied in wheat (Triticum aestivum L.) cv. Iren’ cultivated under conditions of high water supply and cv. Kazakhstanskaya 10, which is relatively drought tolerant. Experiments were performed under both laboratory and field conditions. It was demonstrated that stomata of cv. Kazakhstanskaya 10 plants closed rapidly with reducing humidity (the response of the first type), whereas, in cv. Iren’, this response was less expressed and, under conditions of a high water content in soil, stomatal conductance could increase in response to reduced humidity (the response of the second type). At an increased stomatal conductance and transpiration, water content in cv. Iren’ plants was maintained due to the increase in hydraulic conductance and water inflow from the roots. A possible role of the first-type response (rapid stomata closure) for growth maintenance under drought and of the second-type response (a parallel increase in the stomatal and hydraulic conductance) for providing of rapid growth and high productivity under sufficient water supply is discussed. A possibility to use the type of stomata behavior for cultivar assessment is considered.     

河南省16个主栽小麦品种抗叶锈基因分析

为了明确河南省小麦品种的抗叶锈性及抗叶锈基因的分布,为小麦品种推广与合理布局、叶锈病防治及抗病育种提供依据,本研究利用2015年采自河南省的5个小麦叶锈菌流行小种混合菌株,对近几年河南省16个主栽小麦品种进行了苗期抗性鉴定,然后选用12个小麦叶锈菌生理小种对这些品种进行苗期基因推导,同时利用与24个小麦抗叶锈基因紧密连锁(或共分离)的30个分子标记对该16个品种进行了抗叶锈基因分子检测。结果显示,供试品种苗期对小麦叶锈菌混合流行小种均表现高度感病;基因推导与分子检测结果表明,供试品种可能含有Lr1、Lr16、Lr26和Lr30这4个抗叶锈基因,其中先麦8号含有Lr1和Lr26;郑麦366和郑麦9023含有Lr1;西农979和怀川916含有Lr16;中麦895、偃展4110、郑麦7698、平安8号、众麦1号、周麦16、衡观35和矮抗58含有Lr26;周麦22中含有Lr26,还可能含有Lr1和Lr30;豫麦49-198和洛麦23可能含有本研究中检测以外的其他抗叶锈基因。因此,河南省主栽小麦品种的抗叶锈基因丰富度较低,今后育种工作应注重引入其他抗叶锈性基因,提高抗叶锈性,有效控制小麦叶锈病。     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

An in vivo analysis of photosynthesis during short-term O3 exposure in three contrasting species

The depressions of photosynthetic CO₂ uptake following O₃ exposures of 200 and 400 nmol mol^-1 for between 4 and 16 h were compared between Pisum sativum, Quercus robur and Triticum aestivum, and the potential causes of change identified in vivo. Photosynthetic change was examined by analysis of CO₂, O₂, O₃ and water vapour exchanges together with chlorophyll fluorescence in controlled environments. Under identical fumigation conditions, each species showed very similar rates of O₃ consumption. The light-saturated rate of CO₂ uptake showed a statistically significant decrease in each species with increasing O₃ dose. Although stomatal conductance declined in parallel with CO₂ uptake this did not account for the observed decrease in photosynthesis. The decrease in mesophyll conductance resulted primarily from a decrease in the apparent carboxylation capacity, implying in decreased activity of ribulose 1,5-bisphosphate carboxylase/oxygenase. The maximum capacity of carboxylation was consequently reduced by over 30% and 50% after 16 h fumigation with 200 and 400 nmol mol^-1 O₃ respectively. Additionally, in Q. robur, a statistically significant inhibition of the CO₂ saturated rate of photosynthesis occurred after 16 h with 400 nmol mol^-1 O₃, suggesting that the ability to regenerate ribulose 1,5-bisphosphate was also impaired. None of the species showed any significant decrease in the efficiency of light-limited photosynthesis following fumigation at 200 nmol mol^-1 O₃, but effects were apparent at 400 nmol mol^-1 O₃. The common feature in all three species was a decline in carboxylation capacity which preceded any other change in the photosynthetic apparatus.Abbreviations A_sat net CO₂ uptake rate per unit leaf area at light saturation - A net CO₂ uptake rate per unit leaf area - A_max net CO₂ uptake rate per unit leaf area at CO₂ and light saturation - c_i mole fraction of CO₂ in the intercellular air space - g_s stomatal conductance to CO₂ - F_m maximum chlorophyll fluorescence - F_v variable chlorophyll fluorescence - _c quantum yield of CO₂ uptake for absorbed light - ₀ quantum yield of oxygen evolution for incident light - PPFD photosynthetically active radiation - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate - Vc_max maximum rate of carboxylation     

Photosynthetic and yield responses of an old and a modern winter wheat cultivars to short-term ozone exposure

In order to study the responses of winter wheat cultivars released in different years to short-term high O₃ exposure, an old cultivar (‘Nongda 311’, released in 1960s) and a modern one (‘Yannong 19’, released in 1990s) were treated with an O₃ exposure (145 ± 12 mm³ m⁻³, 4 h d⁻¹ for 3 d) shortly after anthesis stage (> 50 % main stems blossomed). During the O₃ exposure, light-saturated photosynthetic rate (P _N) and stomatal conductance (g _s) of both cultivars decreased considerably. Elevated O₃ did not decrease dark-adapted maximum photochemical efficiency, but induced significant reduction in actual photochemical efficiency and thereby considerably increase in non-photochemical quenching. P _N, g _s of the modern cultivar ‘Yannong 19’ decreased more than the older one ‘Nongda 311’, indicating the former exhibited higher sensitivity to O₃ than the latter. After O₃ exposure, P _N, g _s and chlorophyll (Chl) content in flag leaf decreased more quickly than control, indicating induction of faster premature leaf senescence. As a result, the short-term O₃ exposure caused substantial yield loss, with larger reduction in ‘Yannong 19’ (−19.2 %) than in ‘Nongda 311’ (−8.4 %). Our results indicated that high O₃ exposure at grain filling stage would have greater negative impacts on the high yielding modern cultivar relative to the old one with lower yield.     

Boron accumulation and partitioning in wheat cultivars with contrasting tolerance to boron deficiency

Two pot experiments at the Plant Environment Laboratory (PEL), Reading, UK investigated sterility, boron (B) accumulation and B partitioning of wheat cultivars grown with limited B in the growing medium. The first experiment evaluated nine cultivars of spring wheat with diverse field responses to low available soil B, supplied with or without 20 μM B. A second experiment examined the response of a susceptible (SW-41) and a tolerant (Fang-60) cultivar to B-deficiency. These cultivars were supplied with either 20 μM B from sowing to flag leaf emergence and no added B thereafter, or 20 μM B from sowing to maturity. When B was not supplied in the nutrient solution, the number of grains ranged from 4 per ear (cv. BL-1135) to 32 per ear (cv. BL-1249) and sterility of competent florets ranged from 39% to 93%. Boron concentration in the flag leaf at anthesis did not differ greatly when the growing medium contained limited B, but differences between cultivars were evident when B was unlimited. Tolerance of B-deficiency was not related to the B concentration in the flag leaf. Some cultivars produced viable pollen and set grains while others failed to do so at similar B concentrations in the flag leaf. The two contrasting cultivars did not differ much in their pattern of B partitioning when B supply was restricted from flag leaf emergence onwards. Similarly, little evidence was found that the tolerant cultivars translocated B from their leaves, roots or stems when the supply in the growing medium was restricted. The proportion of total B partitioned in different organs was the same irrespective of B supply and cultivar. On average, leaves contained 68% of the total B content in the whole plant compared to 16% in the roots, 10% in the ears and only 6% in the stems. Tolerant or susceptible cultivars of wheat could not be distinguished based on the B concentration and B content of the flag leaf. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic Response of Wheat to Stress Induced by Puccinia Recondita and Post-Infection Drought

To quantify photosynthetic response of wheat to the combination of a fungal brown rust infection and a post-infection drought, four treatments were compared: no stress (control), fungal stress (FS), water stress (WS), and twofold stress (WS×FS). Predawn leaf water potential (_wp) was similar in FS and WS treatments over a 3-week period. In the WS treatment, net photosynthetic rate (P _N) and stomata CO₂ conductance (g _s) diminished concomitantly with a constant intercellular CO₂ concentration (C _i) close to 200 µmol mol^–1. In the FS treatment, a reduction of P _N occurred with an increase in respiration rate (doubling of the CO₂ compensation concentration) and in C _i but with no water loss modification. Healthy leaves of infected plants (FS) showed a reduction of P _N as well, with constant g _s and increased C _i. In the twofold stress treatment (WS×FS), leaves showed reduced P _N in relation to the lower _wp. Deleterious effects of both drought and fungal infection on the final area of leaves and dry matter were additive.     

Intrinsic morphology and spatial distribution of non-structural carbohydrates contribute to drought resistance of two mulberry cultivars

• Drought is one of the most adverse environmental stresses limiting plant growth and productivity. However, the underlying mechanisms regarding metabolism of non-structural carbohydrates (NSC) in source and sink organs are still not fully elucidated in woody trees.
• Saplings of mulberry cv Zhongshen1 and Wubu were subjected to a 15-day progressive drought stress. NSC levels and gene expression involved in NSC metabolism were investigated in roots and leaves. Growth performance and photosynthesis, leaf stomatal morphology, and other physiological parameters were also analysed.
• Under well-watered conditions, Wubu had a higher R/S, with higher NSC in leaves than in roots; Zhongshen1 had a lower R/S with higher NSC in roots than leaves. Under drought stress, Zhongshen1 showed decreased productivity and increased proline, abscisic acid, ROS content and activity of antioxidant enzymes, while Wubu sustained comparable productivity and photosynthesis. Interestingly, drought resulted in decreased starch and slightly increased soluble sugars in leaves of Wubu, accompanied by notable downregulation of starch-synthesizing genes and upregulation of starch-degrading genes. Similar patterns in NSC levels and relevant gene expression were also observed in roots of Zhongshen1. Concurrently, soluble sugars decreased and starch was unchanged in roots of Wubu and leaves of Zhongshen1. However, gene expression of starch metabolism in roots of Wubu was unaltered, but in leaves of Zhongshen1 starch metabolism was more activated.
• These findings revealed that intrinsic R/S and spatial distribution of NSC in roots and leaves concomitantly contribute to drought resistance in mulberry.

     

Metabolic changes associated with cold-acclimation in contrasting cultivars of barley

Cereal plants become more resistant to freezing when first exposed to a period of cold-acclimation. Many physiological and molecular changes have been shown to occur at low temperatures, but the role and the contribution of each to frost resistance is still poorly understood. Two cultivars of barley ( Hordeum vulgare L.), the winter barley Onice and the spring barley Gitane, were acclimated under controlled conditions under an 8-h photoperiod at 4°C (light) and 2°C (dark) for 21 days. Changes in free proline, ABA, water-soluble carbohydrates and free fatty acids were measured to assess their involvement in cold-acclimation and to explain the different frost-resistant capacities of the two cultivars. Exposure of barley plants to low temperature resulted in an equal increase in proline in both cultivars. During the first days of cold acclimation, ABA levels showed a peak in the frost-resistant cultivar, lasting about 24 h, followed by a decrease. The water soluble carbohydrates reached their highest content after 3 days of hardening, although after 14 to 21 days of acclimation the carbohydrate content was similar to that of unhardened plants. The frost-resistant Onice had a much higher free fatty acid content than the frost-sensitive Gitane. Furthermore in Onice 86% of free farty acids was represented by unsaturated molecular species. Inolenic acid alone being 71%. In contrast, in the frost-sensitive cultivar only 31% of free fatty acids was unsaturated and a large amount of 9-oxo-nonanoic acid, a product present in the linolenic acid cascade, was also detected.
The ABA content after 2 days of hardening and the free fatty acid composition were clearly different between the two cultivars and may explain, at least in part, the different frost-resistant capacities of Onice and Gitane.     

Diurnal changes in leaf gas exchange and validation of a mathematical model for coffee (<Emphasis Type="Italic">Coffea arabica</Emphasis> L.) canopy photosynthesis

Diurnal variations in net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), internal CO₂ concentration (C _i), and water use efficiency (WUE) were studied on individual leaves of coffee plants to determine the effect of climatic factors on photosynthetic capacity. P _N and E showed bimodal behaviour with the maximum values of P _N at mid-morning. At noon, under saturating photosynthetically active radiation (PAR) and high leaf temperature (T _l), P _N declined. In the afternoon (14:00), P _N slightly recovered in association with a decrease in Tl and in leaf-to-air vapour pressure deficit (VPD). Reductions in E during the morning were associated with decreases in g _s. Higher WUE in the morning was related to higher P _N and lower E. The reverse occurred in the afternoon. Goudriaan's simulation model, adapted for coffee canopy photosynthesis, was tested at the level of whole plant (P _pl). Three methods were used: (a) Whole plant net photosynthesis (P _pl) under semi-controlled conditions in a chamber. (b) P _pl estimation following Goudriaan's method (Gaussian integration) of instantaneous P _N in single leaves at three canopy depths and at three different hours assuming a photosynthesis unimodal behaviour. (c) P _pl using Goudriaan's method but at five different hours according to the bimodal behaviour reported above. Results of P _pl estimates using Goudriaan's model adapted for coffee canopy confirm the observed P _pl bimodal behaviour with high fitness degree of the measured whole plant photosynthesis. The high fitness found among observed and simulated data indicates that the modified model may be used as a subroutine for the general simulation model of coffee crop growth.