Rafael Martínez-Carrasco

Does ear C sink strength contribute to overcoming photosynthetic acclimation of wheat plants exposed to elevated CO2

Wheat plants (Triticum durum Desf., cv. Regallo) were grown in the field to study the effects of contrasting [CO2] conditions (700 versus 370 μmol mol−1) on growth, photosynthetic performance, and C management during the post-anthesis period. The aim was to test whether a restricted capacity of sink organs to utilize photosynthates drives a loss of photosynthetic capacity in elevated CO2. The ambient 13C/12C isotopic composition (δ13C) of air CO2 was changed from –10.2‰ in ambient [CO2] to –23.6‰ under elevated [CO2] between the 7th and the 14th days after anthesis in order to study C assimilation and partitioning between leaves and ears. Elevated [CO2] had no significant effect on biomass production and grain filling, and caused an accumulation of C compounds in leaves. This was accompanied by up-regulation of phosphoglycerate mutase and ATP synthase protein content, together with down-regulation of adenosine diphosphate glucose pyrophosphatase protein. Growth in elevated [CO2] negatively affected Rubisco and Rubisco activase protein content and induced photosynthetic down-regulation. CO2 enrichment caused a specific decrease in Rubisco content, together with decreases in the amino acid and total N content of leaves. The C labelling revealed that in flag leaves, part of the C fixed during grain filling was stored as starch and structural C compounds whereas the rest of the labelled C (mainly in the form of soluble sugars) was completely respired 48 h after the end of labelling. Although labelled C was not detected in the δ13C of ear total organic matter and respired CO2, soluble sugar δ13C revealed that a small amount of labelled C reached the ear. The 12CO2 labelling suggests that during the beginning of post-anthesis the ear did not contribute towards overcoming flag leaf carbohydrate accumulation, and this had a consequent effect on protein expression and photosynthetic acclimation.

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.

Growth in elevated CO2 enhances temperature response of photosynthesis in wheat

The temperature dependence of C(3) photosynthesis may be altered by the growth environment. The effects of long-term growth in elevated CO(2) on photosynthesis temperature response have been investigated in wheat (Triticum aestivum L.) grown in controlled chambers with 370 or 700 mumol mol(-1) CO(2) from sowing through to anthesis. Gas exchange was measured in flag leaves at ear emergence, and the parameters of a biochemical photosynthesis model were determined along with their temperature responses. Elevated CO(2) slightly decreased the CO(2) compensation point and increased the rate of respiration in the light and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) V(cmax), although the latter effect was reversed at 15 degrees C. With elevated CO(2), J(max) decreased in the 15-25 degrees C temperature range and increased at 30 and 35 degrees C. The temperature response (activation energy) of V(cmax) and J(max) increased with growth in elevated CO(2). CO(2) enrichment decreased the ribulose 1,5-bisphosphate (RuBP)-limited photosynthesis rates at lower temperatures and increased Rubisco- and RuBP-limited rates at higher temperatures. The results show that the photosynthesis temperature response is enhanced by growth in elevated CO(2). We conclude that if temperature acclimation and factors such as nutrients or water availability do not modify or negate this enhancement, the effects of future increases in air CO(2) on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming.

Elevated CO2 and temperature differentially affect photosynthesis and resource allocation in flag and penultimate leaves of wheat

Differences in acclimation to elevated growth CO2 (700 µmol mol−1, EC) and elevated temperature (ambient +4 °C, ET) in successive leaves of wheat were investigated in field chambers. At a common measurement CO2, EC increased photosynthesis and the quantum yield of electron transport (Φ) early on in the growth of penultimate leaves, and later decreased them. In contrast, EC did not change photosynthesis, and increased Φ at later growth stages in the flag leaf. Contents of chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and total soluble protein were initially higher and subsequently lower in penultimate than flag leaves. EC decreased RuBPCO protein content relative to soluble protein and Chl contents throughout the development of penultimate leaves. On the other hand, EC initially increased the RuBPCO:Chl and Chl a/b ratios, but later decreased them in flag leaves. In the flag leaves but not in the penultimate leaves, ET initially decreased initial and specific RuBPCO activities at ambient CO2 (AC) and increased them at EC. Late in leaf growth, ET decreased Chl contents under AC in both kinds of leaves, and had no effect or a positive one under EC. Thus the differences between the two kinds of leaves were due to resource availability, and to EC-increased allocation of resources to photon harvesting in the penultimate leaves, but to increased allocation to carboxylation early on in growth, and to light harvesting subsequently, in the flag leaves.

Photosynthetic electron transport and carbon-reduction-cycle enzyme activities under long-term drought stress in Casuarina equisetifolia Forst. & Forst.

The inhibition of photosynthetic electron transport and the activity of photosynthetic carbon reduction cycle (PCR) enzymes under long-term water stress after slow dehydration was studied in non-nodulated Casuarina equisetifolia Forst. & Forst. plants. Initially, drought increased the fraction of closed Photosystem II (PS II) reaction centres (lowered qP) and decreased the quantum yield of PS II electron transport (ΦPSII) with no enhancement of non-radiative dissipation of light energy (qN) because it increased the efficiency of electron capture by open PS II centres (F′v/F′m). As drought progressed, F′v/F′m fell and the decrease in ΦPSII was associated with an increased qN. The kinetics of dark relaxation of fluorescence quenching pointed to an increase in a slowly-relaxing component under drought, in association with increased contents of zeaxanthin and antheraxanthin. Total NADP-dependent malate dehydrogenase activity increased and total stromal fructose-1,6-bisphosphatase activity decreased under drought, while the activation state of these enzymes remained unchanged. Water stress did not alter the activity and the activation state of ribulose bisphosphate carboxylase oxygenase.

Short-term feedback inhibition of photosynthesis in wheat leaves supplied with sucrose and glycerol at two temperatures

AbstractThe inhibition of photosynthesis by reduced sink demand or low rates of end product synthesis was investigated by supplying detached wheat (Triticum aestivum L. cv. Tauro) leaves with 50 mM sucrose, 50 mM glycerol or water through the transpiration stream for 2 h, either at 23 or 12 °C. Lowering the temperature and sucrose and glycerol feeding decreased photosynthetic oxygen evolution at high irradiance and saturating CO2. The decrease in temperature reduced the pools of sucrose and starch, and the ratio glucose 6-phosphate (G6P)/fructose 6-phosphate (F6P), while it increased the concentrations of G6P and F6P (hexose phosphates). Sucrose feeding, in contrast to glycerol feeding, increased sucrose, glucose and fructose contents and the G6P/F6P ratio. Sucrose and glycerol incubations at 23 °C, as well as decreasing the temperature in leaves incubated in water, increased the concentration of triose-phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP) and decreased the glycerate 3-phosphate (PGA) content, thus increasing the TP/PGA ratio; they also tended to increase the ribulose 1,5-bisphosphate (RuBP) content and the RuBP/PGA ratio. Sucrose and glycerol feeding at 12 °C and the decrease in temperature of leaves incubated in these solutions decreased TP and RuBP contents and the TP/PGA and RuBP/PGA ratios. The results suggest that the phosphate limitation caused by accumulation of end products, restriction of their synthesis and sequestration of cytosolic phosphate can inhibit photosynthesis through decreased carboxylation of RuBP or, with increased phosphate limitation, through lowered supply of ATP.

Quantitative RT–PCR Platform to Measure Transcript Levels of C and N Metabolism-Related Genes in Durum Wheat: Transcript Profiles in Elevated [CO2] and High Temperature at Different Levels of N Supply

Only limited public transcriptomics resources are available for durum wheat and its responses to environmental changes. We developed a quantitative reverse transcription-PCR (qRT-PCR) platform for analysing the expression of primary C and N metabolism genes in durum wheat in leaves (125 genes) and roots (38 genes), based on available bread wheat genes and the identification of orthologs of known genes in other species. We also assessed the expression stability of seven reference genes for qRT-PCR under varying environments. We therefore present a functional qRT-PCR platform for gene expression analysis in durum wheat, and suggest using the ADP-ribosylation factor as a reference gene for qRT-PCR normalization. We investigated the effects of elevated [CO(2)] and temperature at two levels of N supply on C and N metabolism by combining gene expression analysis, using our qRT-PCR platform, with biochemical and physiological parameters in durum wheat grown in field chambers. Elevated CO(2) down-regulated the photosynthetic capacity and led to the loss of N compounds, including Rubisco; this effect was exacerbated at low N. Mechanistically, the reduction in photosynthesis and N levels could be associated with a decreased transcription of the genes involved in photosynthesis and N assimilation. High temperatures increased stomatal conductance, and thus did not inhibit photosynthesis, even though Rubisco protein and activity, soluble protein, leaf N, and gene expression for C fixation and N assimilation were down-regulated. Under a future scenario of climate change, the extent to which C fixation capacity and N assimilation are down-regulated will depend upon the N supply.

Nitrate supply and plant development influence nitrogen uptake and allocation under elevated CO2 in durum wheat grown hydroponically

Growth in elevated CO2 often leads to decreased plant nitrogen contents and down-regulation of photosynthetic capacity. Here, we investigated whether elevated CO2 limits nitrogen uptake when nutrient movement to roots is unrestricted, and the dependence of this limitation on nitrogen supply and plant development in durum wheat (Triticum durum Desf.). Plants were grown hydroponically at two N supplies and ambient and elevated CO2 concentrations. Elevated CO2 decreased nitrate uptake per unit root mass with low N supply at early grain filling, but not at anthesis. This decrease was not associated with higher nitrate or amino acid, or lower non-structural carbohydrate contents in roots. At anthesis, elevated CO2 decreased the nitrogen content of roots with both levels of N and that of aboveground organs with high N. With low N, elevated CO2 increased N allocation to aboveground plant organs and nitrogen concentration per unit flag leaf area at anthesis, and per unit aboveground dry mass at both growth stages. The results from the hydroponic experiment suggest that elevated CO2 restricts nitrate uptake late in development, high N supply overriding this restriction. Increased nitrogen allocation to young leaves at low N supply could alleviate photosynthetic acclimation to elevated CO2.

Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat

Acclimation of photosynthetic capacity to elevated CO₂ involves a decrease of the leaf Rubisco content. In the present study, it was hypothesized that nitrogen uptake and partitioning within the leaf and among different aboveground organs affects the down-regulation of Rubisco. Given the interdependence of nitrogen and cytokinin signals at the whole plant level, it is also proposed that cytokinins affect the nitrogen economy of plants under elevated CO₂, and therefore the acclimatory responses. Spring wheat received varying levels of nitrogen and cytokinin in field chambers with ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂. Gas exchange, Rubisco, soluble protein and nitrogen contents were determined in the top three leaves in the canopy, together with total nitrogen contents per shoot. Growth in elevated CO₂ induced decreases in photosynthetic capacity only when nitrogen supply was low. However, the leaf contents of Rubisco, soluble protein and total nitrogen on an area basis declined in elevated CO₂ regardless of nitrogen supply. Total nitrogen in the shoot was no lower in elevated than ambient CO₂, but the fraction of this nitrogen located in flag and penultimate leaves was lower in elevated CO₂. Decreased Rubisco: chlorophyll ratios accompanied losses of leaf Rubisco with CO₂ enrichment. Cytokinin applications increased nitrogen content in all leaves and nitrogen allocation to senescing leaves, but decreased Rubisco contents in flag leaves at anthesis and in all leaves 20 days later, together with the amount of Rubisco relative to soluble protein in all leaves at both growth stages. The results suggest that down regulation of Rubisco in leaves at elevated CO₂ is linked with decreased allocation of nitrogen to the younger leaves and that cytokinins cause a fractional decrease of Rubisco and therefore do not alleviate acclimation to elevated CO₂.

Restoration of photosystem II photochemistry and carbon assimilation and related changes in chlorophyll and protein contents during the rehydration of desiccated Xerophyta scabrida leaves

Recovery of photosynthesis in rehydrating desiccated leaves of the poikilochlorophyllous desiccation-tolerant plant Xerophyta scabrida was investigated. Detached leaves were remoistened under 12 h light/dark cycles for 96 h. Water, chlorophyll (Chl), and protein contents, Chl fluorescence, photosynthesis–CO2 concentration response, and the amount and activity of Rubisco were measured at intervals during the rehydration period. Leaf relative water contents reached 87% in 12 h and full turgor in 96 h. Chl synthesis was slower before than after 24 h, and Chla:Chlb ratios changed from 0.13 to 2.6 in 48 h. The maximum quantum efficiency recovered faster during rehydration than the photosystem II operating efficiency and the efficiency factor, which is known to depend mainly on the use of the electron transport chain products. From 24 h to 96 h of rehydration, net carbon fixation was Rubisco limited, rather than electron transport limited. Total Rubisco activity increased during rehydration more than the Rubisco protein content. Desiccated leaves contained, in a close to functional state, more than half the amount of the Rubisco protein present in rehydrated leaves. The results suggest that in X. scabrida leaves Rubisco adopts a special, protective conformation and recovers its activity during rehydration through modifications in redox status.