Joseph C.V. Vu

Leaf ultrastructure, carbohydrates and protein of soybeans grown under CO2 enrichment

Abstract Soybeans (Glycine max [L.] Merr. cv Bragg) grown under 800 μl CO2 per liter showed increases of 37, 205, 108, 33, 22 and 31% in specific leaf weight, starch, sucrose, reducing sugars, chlorophyll and soluble protein, respectively, over control plants at 330 μl CO2 per liter. CO2 enrichment caused an increase in leaf thickness, due to an increased number of palisade cells, but no major alteration in chloroplast ultrastructure. The extra starch formed under CO2 enrichment was distributed as slightly larger starch grains among more and larger chloroplasts and more mesophyll cells, rather than as more starch grains per chloroplast. The Km(CO2) of ribulose biphosphate carboxylase was unchanged by growth at high CO2, similarly there was little effect on the activity of this enzyme. The increase in starch storage sites, together with the maintenance of ribulose bisphosphate carboxylase activity, enable soybeans to continue to exhibit high photosynthetic rates throughout the growth period under high CO2 levels.

Growth at elevated CO2 delays the adverse effects of drought stress on leaf photosynthesis of the C4 sugarcane.

Sugarcane (Saccharum officinarum L. cv. CP72-2086) was grown in sunlit greenhouses at daytime [CO(2)] of 360 (ambient) and 720 (elevated)mumolmol(-1). Drought stress was imposed for 13d when plants were 4 months old, and various photosynthetic parameters and levels of nonstructural carbohydrates were determined for uppermost fully expanded leaves of well-watered (control) and drought stress plants. Control plants at elevated [CO(2)] were 34% and 25% lower in leaf stomatal conductance (g(s)) and transpiration rate (E) and 35% greater in leaf water-use efficiency (WUE) than their counterparts at ambient [CO(2)]. Leaf CO(2) exchange rate (CER) and activities of Rubisco, NADP-malate dehydrogenase, NADP-malic enzyme and pyruvate P(i) dikinase were marginally affected by elevated [CO(2)], but were reduced by drought, whereas activity of PEP carboxylase was reduced by elevated [CO(2)], but not by drought. At severe drought developed at day 12, leaf g(s) and WUE of ambient-[CO(2)] stress plants declined to 5% and 7%, while elevated-[CO(2)] stress plants still maintained g(s) and WUE at 20% and 74% of their controls. In control plants, elevated [CO(2)] did not enhance the midday levels of starch, sucrose, or reducing sugars. For both ambient- and elevated-[CO(2)] stress plants, severe drought did not affect the midday level of sucrose but substantially reduced that of starch. Nighttime starch decomposition in control plants was 55% for ambient [CO(2)] and 59% for elevated [CO(2)], but was negligible for stress plants of both [CO(2)] treatments. For both ambient-[CO(2)] control and stress plants, midday sucrose level at day 12 was similar to the predawn value at day 13. In contrast, sucrose levels of elevated-[CO(2)] control and stress plants at predawn of day 13 were 61-65% of the midday values of day 12. Levels of reducing sugars were much greater for both ambient- and elevated-[CO(2)] stress plants, implying an adaptation to drought stress. Sugarcane grown at elevated [CO(2)] had lower leaf g(s) and E and greater leaf WUE, which helped to delay the adverse effects of drought and, thus, allowed the stress plants to continue photosynthesis for at least an extra day during episodic drought cycles.

Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum.

Maize and grain sorghum seeds were sown in pots and grown for 39 days in sunlit controlled-environment chambers at 360 (ambient) and 720 (double-ambient, elevated)μmol mol(-1) carbon dioxide concentrations [CO(2)]. Canopy net photosynthesis (PS) and evapotranspiration (TR) was measured throughout and summarized daily from 08:00 to 17:00h Eastern Standard Time. Irrigation was withheld from matched pairs of treatments starting on 26 days after sowing (DAS). By 35 DAS, cumulative PS of drought-stress maize, compared to well-watered plants, was 41% lower under ambient [CO(2)] but only 13% lower under elevated [CO(2)]. In contrast, by 35 DAS, cumulative PS of drought-stress grain sorghum, compared to well-watered plants, was only 9% lower under ambient [CO(2)] and 7% lower under elevated [CO(2)]. During the 27-35 DAS drought period, water use efficiency (WUE, mol CO(2)Kmol(-1)H(2)O), was 3.99, 3.88, 5.50, and 8.65 for maize and 3.75, 4.43, 5.26, and 9.94 for grain sorghum, for ambient-[CO(2)] well-watered, ambient-[CO(2)] stressed, elevated-[CO(2)] well-watered and elevated-[CO(2)] stressed plants, respectively. Young plants of maize and sorghum used water more efficiently at elevated [CO(2)] than at ambient [CO(2)], especially under drought. Reductions in biomass by drought for young maize and grain sorghum plants were 42 and 36% at ambient [CO(2)], compared to 18 and 14% at elevated [CO(2)], respectively. Results of our water stress experiment demonstrated that maintenance of relatively high canopy photosynthetic rates in the face of decreased transpiration rates enhanced WUE in plants grown at elevated [CO(2)]. This confirms experimental evidence and conceptual models that suggest that an increase of intercellular [CO(2)] (or a sustained intercellular [CO(2)]) in the face of decreased stomatal conductance results in relative increases of growth of C(4) plants. In short, drought stress in C(4) crop plants can be ameliorated at elevated [CO(2)] as a result of lower stomatal conductance and sustaining intercellular [CO(2)]. Furthermore, less water might be required for C(4) crops in future higher CO(2) atmospheres, assuming weather and climate similar to present conditions.

Soybean photosynthesis, Rubisco, and carbohydrate enzymes function at supraoptimal temperatures in elevated CO2

Summary Soybean (Glycine max L. Merr. cv. Bragg) was grown season-long in eight sunlit, controlled–environment chambers at two daytime [CO2] of 350 (ambient) and 700 (elevated) µmol mol –1 . Dry bulb day/night maximum/minimum air temperatures, which followed a continuously and diurnally varying, near sine-wave control set point that operated between maximum (daytime, at 1500 EST) and minimum (nighttime, at 0700 EST) values, were controlled at 28/18 and 40/30 uC for the ambient-CO2 plants, and at 28/18, 32/22, 36/26, 40/30, 44/34 and 48/38 uC for the elevated-CO2 plants. The objective was to assess the upper threshold tolerance of photosynthesis and carbohydrate metabolism with increasing temperatures at elevated [CO2], as it is predicted that air temperatures could rise as much as 4–6 uC within the 21st century with a doubling of atmospheric [CO2]. Leaf photosynthesis measured at growth [CO2] and temperature was greater for elevated-CO2 plants and was highest at 32/22 uC, but markedly declined at temperatures above 40/30 uC. Growth temperatures from 28/18 to 40/30 uC had little effect on midday total activity and protein content of Rubisco, while higher temperatures substantially reduced them. Conversely, midday Rubisco rbcS transcript abundance declined with increasing temperatures from 28/18 to 48/38 uC. Elevated-CO2 plants exceeded the ambientCO2 plants in most aspects of carbohydrate metabolism. Under elevated [CO2], midday activities of ADPG pyrophosphorylase and sucrose-P synthase and invertase paralleled net increases in starch and sucrose contents, respectively. They were highest at 36/26–40/30 uC, but declined at higher or lower growth temperatures. Thus, in the absence of other climatic stresses, soybean photosynthesis and carbohydrate metabolism would perform well under rising atmospheric [CO2] and temperature predicted for the 21st century.

Elevated growth CO2 delays drought stress and accelerates recovery of rice leaf photosynthesis

Rising atmospheric carbon dioxide concentration ([CO2]) and predicted changes in rainfall frequency and intensity could have considerable impact on crop growth and yield. Our objective was to assess rice leaf photosynthesis and carbohydrate metabolism in response to decreased soil water availability at elevated growth [CO2]. Rice (Oryza sativa [L.] cv. IR-72) was grown season-long in eight sunlit, controlled-environment chambers at two daytime [CO2] of 350 μmol mol−1 (ambient) and 700 μmol mol−1 (elevated). Drought stress was imposed during panicle initiation and anthesis growth phases. At elevated [CO2], midday leaf photosynthetic CO2 exchange rates (CER) and concentrations of chlorophyll (Chl) were higher at most sampling dates, whereas total soluble protein (TSP) decreased on several sampling dates, compared with plants at ambient [CO2]. Furthermore, elevated [CO2] increased midday leaf sucrose-phosphate synthase (SPS) activity throughout the season, and enhanced midday leaf sucrose and starch accumulation during early reproductive phases, but not during later reproductive phases. Near the end of drought periods, water deficit caused substantial decreases in midday leaf CER and concentrations of Chl and TSP, with concomitant reductions in photosynthetic primary products and SPS activity. These drought-induced effects were more severe for plants grown at ambient than at elevated [CO2]. Plants grown under elevated [CO2] were able to maintain midday leaf photosynthesis, and to some extent other photosynthetic-related parameters, longer into the drought period than plants grown at ambient [CO2]. In addition, midday leaf CER recovered from water deficit more rapidly in the elevated [CO2] treatment. Thus, in the absence of other potential climate stresses, rice grown under future increases in atmospheric [CO2] may be better able to tolerate drought situations.

Photosynthetic acclimation of young sweet orange trees to elevated growth CO2 and temperature

Summary Two-year old trees of ‘Ambersweet’ orange, a hybrid of ‘Clementine’ tangerine (Citrus reticulata Blanco) and ‘Orlando’ tangelo (C. paradisi Macf. × C. reticulata), were grown for twenty-nine months under two daytime [CO2] of 360 (ambient) and 720 (elevated) μmol mol−1, and at two temperatures of 1.5 and 6.0 °C above ambient temperature. The objectives were to characterize the physiology and biochemistry of citrus photosynthesis in response to both elevated [CO2] and temperature, and to test if the photosynthetic capacity of sweet orange, in terms of rubisco activity and protein concentration, was down-regulated under long-term elevated growth [CO2]. Both mature (old) and expanding (new) leaves of trees grown under elevated [CO2] had higher photosynthetic rates, lower transpiration and conductance, and higher water-use efficiency (WUE), compared to those grown under ambient [CO2]. Although leaf WUE was reduced by high temperature, elevated [CO2] compensated for adverse effect of high temperature on leaf WUE. Activity and protein concentration of rubisco were down-regulated in both new and old leaves at elevated [CO2]. In contrast, the amount of total leaf soluble protein was not affected by elevated [CO2] and high temperature. Down-regulation of photosynthetic capacity was greater for the old leaves, although activity and protein concentration of rubisco in the new leaves were higher. Contents of soluble sugars and starch in all leaves sampled, which were higher under elevated [CO2], were generally not affected by high temperature. Within each specific CO2-temperature treatment and leaf type, total soluble sugars remained relatively unchanged throughout the day, as did the starch content of early morning and midday samples, and only a moderate increase in starch for the old leaves at late afternoon sampling was observed. In contrast, starch content in the new leaves increased substantially at late afternoon. Activities of sucrose-P synthase and adenosine 5′-diphosphoglucose pyrophosphorylase were reduced at elevated [CO2] in the old leaves, but not in the new leaves. The photosynthetic acclimation of Ambersweet orange leaves at elevated [CO2] allowed an optimization of nitrogen use by reallocation/redistribution of the nitrogen resources away from rubisco. Thus, in the absence of other environmental stresses, citrus photosynthesis would perform well under rising atmospheric [CO2] and temperature as predicted for this century.

C02 Enrichment Delays a Rapid, Drought-Induced Decrease in Rubisco Small Subunit Transcript Abundance

Summary Rice ( Oryza sativa L. cv. IR-72) was grown in sunlit chambers at 350 and 700 gmol CO 2 mol -1 under conditions of continuous flooding (control) or drought which was imposed at panicle initiation, to evaluate the effects of C0 2 enrichment and soil water deficit on photosynthesis and Rubisco gene expression. Leaf and canopy photosynthetic rates were enhanced by high [CO 2 ] but reduced by drought. High [CO 2 ] and severe drought both reduced rbcS transcript abundance, along with the activity, activation and protein content of Rubisco, but the K m (CO 2 ) was not affected. The transition from moderate to severe drought caused a rapid decline, within 24 h, in the rbcS transcript abundance. High [CO 2 ], however, delayed the adverse effects of severe drought on rbcS transcript abundance and activities of Rubisco, and permitted photosynthesis to continue for an extra day during the drought-stress cycle.

Glycerol stimulation of chlorophyll synthesis, embryogenesis, and carboxylation and sucrose metabolism enzymes in nucellar callus of Hamlin' sweet orange

Phosphoenolpyruvate carboxylase (PEPCase) and sucrose phosphate synthase (SPS) were active in nucellar calli of ‘Hamlin’ sweet orange,Citrus sinensis (L.) Osbeck, grown on media containing either 5% sucrose or 2% glycerol as the primary carbon source. Activities of the enzymes, however, were much higher in the glycerol-grown tissues than those grown in sucrose. Glycerol, in addition, stimulated embryogenesis and chlorophyll biosynthesis and initiated the onset of ribulose bisphosphate carboxylase-oxygenase (Rubisco) activity. These stimulatory effects, which were not observed in sucrose-grown calli, became more obvious when calli grown on glycerol-containing, agar-solidified medium were transferred and grown in liquid suspension medium containing 2% glycerol. Starch levels in both sucrose- and glycerol-grown tissues were as high as 30% of tissue dry weight. There were, however, higher concentrations of soluble sugars in sucrose-grown calli than those grown on glycerol. Stimulation of embryogenesis, chlorophyll synthesis, Rubisco onset, and activities of PEPCase and SPS by glycerol offered potential prospects in using this compound in citrus tissue cultures.

Stem juice production of the C4 sugarcane (Saccharum officinarum) is enhanced by growth at double-ambient CO2 and high temperature

Two cultivars of sugarcane (Saccharum officinarum cv. CP73-1547 and CP88-1508) were grown for 3 months in paired-companion, temperature-gradient, sunlit greenhouses under daytime [CO2] of 360 (ambient) and 720 (double ambient) micromol mol(-1) and at temperatures of 1.5 degrees C (near ambient) and 6.0 degrees C higher than outside ambient temperature. Leaf area and biomass, stem biomass and juice and CO2 exchange rate (CER) and activities of ribulose bisphosphate carboxylase-oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) of fully developed leaves were measured at harvest. On a main stem basis, leaf area, leaf dry weight, stem dry weight and stem juice volume were increased by growth at doubled [CO2] or high temperature. Such increases were even greater under combination of doubled [CO2]/high temperature. Plants grown at doubled [CO2]/high temperature combination averaged 50%, 26%, 84% and 124% greater in leaf area, leaf dry weight, stem dry weight and stem juice volume, respectively, compared with plants grown at ambient [CO2]/near-ambient temperature combination. In addition, plants grown at doubled [CO2]/high temperature combination were 2-3-fold higher in stem soluble solids than those at ambient [CO2]/near-ambient temperature combination. Although midday CER of fully developed leaves was not affected by doubled [CO2] or high temperature, plants grown at doubled [CO2] were 41-43% less in leaf stomatal conductance and 69-79% greater in leaf water-use efficiency, compared with plants grown at ambient [CO2]. Activity of PEPC was down-regulated 23-32% at doubled [CO2], while high temperature did not have a significant impact on this enzyme. Activity of Rubisco was not affected by growth at doubled [CO2], but was reduced 15-28% at high temperature. The increases in stem juice production and stem juice soluble solids concentration for sugarcane grown at doubled [CO2] or high temperature, or at doubled [CO2]/high temperature combination, were partially the outcome of an increase in whole plant leaf area. Such increase would enhance the ongoing and cumulative photosynthetic capability of the whole plant. The results indicate that a doubling of [CO2] would benefit sugarcane production more than the anticipated 10-15% increase for a C4 species.

Enhancement in leaf photosynthesis and upregulation of Rubisco in the C4 sorghum plant at elevated growth carbon dioxide and temperature occur at early stages of leaf ontogeny.

Rising atmospheric carbon dioxide (CO2) concentration and temperature will influence photosynthesis, growth and yield of agronomic crops. To investigate effects of elevated CO2 and high temperature on leaf gas exchanges, activities of Rubisco and phosphoenolpyruvate carboxylase (PEPC) and growth of grain sorghum (Sorghum bicolor L. Moench), plants were grown in controlled environments at day-time maximum/night-time minimum temperatures of 30/20°C or 36/26°C at ambient (350 µmol mol-1) or elevated (700 µmol mol-1) CO2. Gas-exchange rates, activities of Rubisco and PEPC and growth parameters (leaf, stem and total dry weights) were determined at different stages of leaf development. Between 6 and 25 days after leaf tip emergence, leaf carbon exchange rate (CER) of elevated CO2 plants was greater at 30/20°C and 36/26°C than that of ambient CO2 plants at the same temperatures. The positive response of CER to elevated CO2 was greater in young leaves than in old leaves. In young leaves, elevated CO2 enhanced Rubisco activity at 30/20°C and 36/26°C, whereas PEPC activity was not affected by elevated CO2 at 30/20°C but was marginally enhanced at 36/26°C. At 30/20°C, growth parameters were not affected by elevated CO2 until 50 days after sowing (DAS); at 36/26°C, they were progressively enhanced by elevated CO2 to as high as 49 to 62% by 50 DAS. Leaf CER and Rubisco activity were enhanced by elevated CO2 at early stages of leaf ontogeny for the C4 grain sorghum. Such enhancement should have a significant role in dry matter production under elevated CO2.