Photosynthetic CO2 Response Curves Research Articles

A module involving HIGH LEAF TEMPERATURE1 controls instantaneous water use efficiency.

Drought stress inhibits plant growth and agricultural production. Improving plant instantaneous water use efficiency (iWUE), which is strictly regulated by stomata, is an effective way to cope with drought stress. However, the mechanisms of iWUE regulation are poorly understood. Through genetic screening for suppressors of mpk12-4, an Arabidopsis (Arabidopsis thaliana) mutant with a major iWUE quantitative trait locus gene MITOGEN-ACTIVATED PROTEIN KINASE12 deleted, we identified HIGH LEAF TEMPERATURE1 (HT1). Genetic interaction and physiological analyses showed that MPK12 controls iWUE through multiple modules in a high CO2-induced stomatal closing pathway that regulate SLOW ANION CHANNEL-ASSOCIATED1 (SLAC1) activity. HT1 acts downstream of MPK12, whereas OPEN STOMATA1 (OST1) and GUARD CELL HYDROGEN PEROXIDE-RESISTANT1 (GHR1) function downstream of HT1 by activating SLAC1 in iWUE. Photosynthetic-CO2 response curves and biomass analyses under different water-supply conditions showed that HT1 dysfunction improved iWUE and also increased plant growth capacity, and products of HT1 putative orthologs from Brassica (Brassica napus) and rice (Oryza sativa) exhibited functions similar to that of Arabidopsis HT1 in iWUE and the CO2-signaling pathway. Our study revealed the mechanism of MPK12-mediated iWUE regulation in Arabidopsis and provided insight into the internal relationship between iWUE and CO2 signaling in guard cells and a potential target for improving crop iWUE and drought tolerance.

The Dynamic Assimilation Technique measures photosynthetic CO2 response curves with similar fidelity to steady-state approaches in half the time.

The net CO2 assimilation (A) response to intercellular CO2 concentration (Ci) is a fundamental measurement in photosynthesis and plant physiology research. The conventional A/Ci protocols rely on steady-state measurements and take 15-40 min per measurement, limiting data resolution or biological replication. Additionally, there are several CO2 protocols employed across the literature, without clear consensus as to the optimal protocol or systematic biases in their estimations. We compared the non-steady-state Dynamic Assimilation Technique (DAT) protocol and the three most used CO2 protocols in steady-state measurements, and tested whether different CO2 protocols lead to systematic differences in estimations of the biochemical limitations to photosynthesis. The DAT protocol reduced the measurement time by almost half without compromising estimation accuracy or precision. The monotonic protocol was the fastest steady-state method. Estimations of biochemical limitations to photosynthesis were very consistent across all CO2 protocols, with slight differences in Rubisco carboxylation limitation. The A/Ci curves were not affected by the direction of the change of CO2 concentration but rather the time spent under triose phosphate utilization (TPU)-limited conditions. Our results suggest that the maximum rate of Rubisco carboxylation (Vcmax), linear electron flow for NADPH supply (J), and TPU measured using different protocols within the literature are comparable, or at least not systematically different based on the measurement protocol used.

Comparative flooding tolerance of Typha latifolia and Phalaris arundinacea in wetland restoration: Insights from photosynthetic CO2 response curves, photobiology and biomass allocation

Tall helophytes such as Typha latifolia and Phalaris arundinacea often rapidly colonise after rewetting of former agricultural soil and are therefore often the first plants to contribute to the soil carbon pool. In this study we carried out a mesocosm experiment where these two species grew at three different water levels relative to the soil surface (−15 cm, 0 cm, +15 cm). After eight weeks' growth, measurements of photosynthetic CO2-response curves, stomatal conductance and chlorophyll fluorescence of photosystem II were carried out to detect flooding stress. After 10 weeks’ growth, the plants were harvested and biomass production, biomass allocation and specific leaf area were determined. T. latifolia had a higher and more stable photosynthetic performance across all water level treatments, which resulted in an overall higher aboveground and belowground production than P. arundinacea. In contrast, Vcmax and Jmax decreased by 41 % and 42 %, respectively from drained to flooded conditions with signs of flooding stress as impairment of the photosynthetic apparatus. Moreover, increasing water level resulted in maintenance of aboveground organs for P. arundinacea but a decrease in allocation to belowground organs. P. arundinacea did not invest in a higher specific leaf area to counter the decreased photosynthesis under flooding. From −15 cm to 0 cm water levels, P. arundinacea showed a 68 % reduction in belowground biomass, which has negative implication for carbon retention immediately after rewetting. In contrast, recolonization of T. latifolia is likely to be a suitable contributor to the soil carbon pool due to its stable physiology and high above- and belowground biomass production at all water depths, and also likely under natural water level fluctuations. We showed that even though both species are generally considered wetland plants, they are likely to support considerably different photosynthetic carbon assimilation and soil carbon sequestration rates.

Vertical profile of photosynthetic CO<sub>2</sub> response within rice canopy

Leaf-specific Farquhar-von Caemmerer-Berry (FvCB) model was fitted to characterize the vertical profile of photosynthetic CO2 response within rice canopy. Leaf-position-specific and canopy average FvCB models were fitted to study a suitable leaf representing photosynthetic parameters at the canopy scale. The results showed that leaf photosynthesis was limited by Rubisco activity or ribulose-1,5-bisphosphate regeneration under field conditions. The maximum rate of carboxylation, maximum rate of electron transport, rate of triose phosphates utilization, and light respiration rate in the FvCB model reached the highest values for the top second leaf and then decreased, while the mesophyll diffusion conductance kept decreased in downward leaves. The integrated photosynthetic CO2-response curves for the top fourth and fifth leaves were appropriate for estimating parameters in the FvCB model at the canopy scale.

Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms.

Red and blue light are traditionally believed to have a higher quantum yield of CO2 assimilation (QY, moles of CO2 assimilated per mole of photons) than green light, because green light is absorbed less efficiently. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. We hypothesized that, at high photosynthetic photon flux density (PPFD), green light may achieve higher QY and net CO2 assimilation rate (An) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of PPFD and light spectrum on photosynthesis, we measured leaf An of “Green Tower” lettuce (Lactuca sativa) under red, blue, and green light, and combinations of those at PPFDs from 30 to 1,300 μmol⋅m–2⋅s–1. The electron transport rates (J) and the maximum Rubisco carboxylation rate (Vc,max) at low (200 μmol⋅m–2⋅s–1) and high PPFD (1,000 μmol⋅m–2⋅s–1) were estimated from photosynthetic CO2 response curves. Both QYm,inc (maximum QY on incident PPFD basis) and J at low PPFD were higher under red light than under blue and green light. Factoring in light absorption, QYm,abs (the maximum QY on absorbed PPFD basis) under green and red light were both higher than under blue light, indicating that the low QYm,inc under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high PPFD, the QYinc [gross CO2 assimilation (Ag)/incident PPFD] and J under red and green light were similar, and higher than under blue light, confirming our hypothesis. Vc,max may not limit photosynthesis at a PPFD of 200 μmol m–2 s–1 and was largely unaffected by light spectrum at 1,000 μmol⋅m–2⋅s–1. Ag and J under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and PPFD on photosynthesis was due to effects on J. No interaction between the three colors of light was detected. In summary, at low PPFD, green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high PPFD, QYinc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.

Evaluating Growth and Photosynthesis of Kimchi Cabbage According to Extreme Weather Conditions

The aim of this study was to develop and validate growth and photosynthetic models of Kimchi cabbages under extreme temperature conditions at different growth stages. Kimchi cabbage plants were subjected to low and high air temperatures 7–10 days after transplanting (DAT) and 40–43 DAT using extreme weather simulators. Except during these periods, the air temperature, relative humidity, solar radiation, and precipitation were set according to previous meteorological data. The experiments were performed over two years: in the first year, data were used to develop the models; the second-year experimental data were used for validation. The growth parameters and relative growth rate of Kimchi cabbage decreased due to low and high air temperature treatments. Photosynthetic CO2 response curves, which were measured using a portable gas exchange system, were used to calculate three biochemical parameters from measured data: photochemical efficiency, carboxylation conductance, and dark respiration. These parameters were used to develop the photosynthetic models (modified Thornley’s models) representing predictions of net photosynthetic rate by CO2 concentration and growth stage. The simulated photosynthetic rate with extreme high temperature treatment (35/31 °C) was 19.7 μmol m−2 s−1 which was evaluated approximately 3% deduction compared with control. Results of this study indicate that the growth and photosynthetic models developed here could be applied to evaluate retarded growth and net photosynthetic rate under extreme temperature conditions.

No evidence for triose phosphate limitation of light-saturated leaf photosynthesis under current atmospheric CO2 concentration.

The triose phosphate utilization (TPU) rate has been identified as one of the processes that can limit terrestrial plant photosynthesis. However, we lack a robust quantitative assessment of TPU limitation of photosynthesis at the global scale. As a result, TPU, and its potential limitation of photosynthesis, is poorly represented in terrestrial biosphere models (TBMs). In this study, we utilized a global data set of photosynthetic CO2 response curves representing 141 species from tropical rainforests to Arctic tundra. We quantified TPU by fitting the standard biochemical model of C3 photosynthesis to measured photosynthetic CO2 response curves and characterized its instantaneous temperature response. Our results demonstrate that TPU does not limit leaf photosynthesis at the current ambient atmospheric CO2 concentration. Furthermore, our results showed that the light-saturated photosynthetic rates of plants growing in cold environments are not more often limited by TPU than those of plants growing in warmer environments. In addition, our study showed that the instantaneous temperature response of TPU is distinct from temperature response of the maximum rate of Rubisco carboxylation. The new formulations of the temperature response of TPU derived in this study may prove useful in quantifying the biochemical limits to terrestrial plant photosynthesis and improve the representation of plant photosynthesis in TBMs.

Net photosynthetic CO2 assimilation: more than just CO2 and O2 reduction cycles.

Net photosynthetic assimilation in C3 plants is mostly viewed as a simple balance between CO2 fixation by Rubisco-catalyzed carboxylation and CO2 production by photorespiration (and to a lower extent, by day respiration) that can be easily manipulated during gas exchange experiments using the CO2 :O2 ratio of the environment. However, it now becomes clear that it is not so simple, because the photosynthetic response to gaseous conditions involves 'ancillary' metabolisms, even in the short-term. That is, carbon and nitrogen utilization by pathways other than the Calvin cycle and the photorespiratory cycle, as well as rapid signaling events, can influence the observed rate of net photosynthesis. The potential impact of such ancillary metabolisms is assessed as well as how it must be taken into account to avoid misinterpretation of photosynthetic CO2 response curves or low O2 effects in C3 leaves.

Acclimation and adaptation components of the temperature dependence of plant photosynthesis at the global scale.

The temperature response of photosynthesis is one of the key factors determining predicted responses to warming in global vegetation models (GVMs). The response may vary geographically, owing to genetic adaptation to climate, and temporally, as a result of acclimation to changes in ambient temperature. Our goal was to develop a robust quantitative global model representing acclimation and adaptation of photosynthetic temperature responses. We quantified and modelled key mechanisms responsible for photosynthetic temperature acclimation and adaptation using a global dataset of photosynthetic CO2 response curves, including data from 141 C3 species from tropical rainforest to Arctic tundra. We separated temperature acclimation and adaptation processes by considering seasonal and common-garden datasets, respectively. The observed global variation in the temperature optimum of photosynthesis was primarily explained by biochemical limitations to photosynthesis, rather than stomatal conductance or respiration. We found acclimation to growth temperature to be a stronger driver of this variation than adaptation to temperature at climate of origin. We developed a summary model to represent photosynthetic temperature responses and showed that it predicted the observed global variation in optimal temperatures with high accuracy. This novel algorithm should enable improved prediction of the function of global ecosystems in a warming climate.

Simulation on photosynthetic-CO2 response of Quercus variabilis and Robinia pseudoacacia in the southern foot of the Taihang Mountain, China.

In this study, leaf photosynthetic CO2-response curves of Quercus variabilis and Robinia pseudoacacia were measured using a Li-6400XT photosynthetic measurement system in the southern foot of the Taihang Mountain, China. The rectangular hyperbola model (RH), nonrectangular hyperbola model (NRH) andYe model (YZP) were used to fit photosynthetic-CO2 response curves and compare photosynthetic parameters, including the maximum net photosynthetic rate (Amax), the initial carboxylation rate (η), light respiration rate (Rp), CO2 compensation point (CCP) and CO2 saturation point (CSP). Compared with the NRH and YZP models, Amax, η, Rp and CCP obtained by the RH model were higher, and were 59.8%, 128.6%, 133.4% and 19.8% higher than the measured values. The accuracy of the RH model was lower and its relative error was higher than that of the NRH and YZP models.Compared with the RH and YZP models, Amax fitted by the NRH model was higher, and was 11.1% higher than the measured value. η, Rp and CCP fitted by the NRH model were closer to the measured values. CO2 saturation phenomenon of photosynthesis could be simulated by the YZP model, and Amax and CSP were fitted well. Amax, Rp and CCP in the shaded leaves of Q. variabilis were 31.3%, 5.2% and 14.3% lower than those in the sunlit leaves. Amax, Rp and CCP in shaded leaves of R. pseudoacacia were 23.5%, 11.0% and 5.4% more than those in the sunlit leaves. η in the shaded leaves of Q. variabilis and R. pseudoacacia were 6.9% and 7.0% higher than those in the sunlit leaves, respectively. Rp and CCP of R. pseudoacacia leaves had linear relationships with temperature (T) and photosynthetic active radiation (PAR), and η had a significant relationship with stomatal conductance (gs). η of Q. variabilis leaves was linearly correlated with PAR and gs, and CCP was affected by T and relative humidity. Amax of Q. variabilis leaves had significant positive linear relationships with RH and gs.

Terrestrial biosphere models underestimate photosynthetic capacity and CO2 assimilation in the Arctic.

Terrestrial biosphere models (TBMs) are highly sensitive to model representation of photosynthesis, in particular the parameters maximum carboxylation rate and maximum electron transport rate at 25°C (Vc,max.25 and Jmax.25 , respectively). Many TBMs do not include representation of Arctic plants, and those that do rely on understanding and parameterization from temperate species. We measured photosynthetic CO2 response curves and leaf nitrogen (N) content in species representing the dominant vascular plant functional types found on the coastal tundra near Barrow, Alaska. The activation energies associated with the temperature response functions of Vc,max and Jmax were 17% lower than commonly used values. When scaled to 25°C, Vc,max.25 and Jmax.25 were two- to five-fold higher than the values used to parameterize current TBMs. This high photosynthetic capacity was attributable to a high leaf N content and the high fraction of N invested in Rubisco. Leaf-level modeling demonstrated that current parameterization of TBMs resulted in a two-fold underestimation of the capacity for leaf-level CO2 assimilation in Arctic vegetation. This study highlights the poor representation of Arctic photosynthesis in TBMs, and provides the critical data necessary to improve our ability to project the response of the Arctic to global environmental change.

Has photosynthetic capacity increased with 80 years of soybean breeding? An examination of historical soybean cultivars.

Crop biomass production is a function of the efficiencies with which sunlight can be intercepted by the canopy and then converted into biomass. Conversion efficiency has been identified as a target for improvement to enhance crop biomass and yield. Greater conversion efficiency in modern soybean [Glycine max (L.) Merr.] cultivars was documented in recent field trials, and this study explored the physiological basis for this observation. In replicated field trials conducted over three successive years, diurnal leaf gas exchange and photosynthetic CO2 response curves were measured in 24 soybean cultivars with year of release dates (YOR) from 1923 to 2007. Maximum photosynthetic capacity, mesophyll conductance and nighttime respiration have not changed consistently with cultivar release date. However, daily carbon gain was periodically greater in more recently released cultivars compared with older cultivars. Our analysis suggests that this difference in daily carbon gain primarily occurred when stomatal conductance and soil water content were high. There was also evidence for greater chlorophyll content and greater sink capacity late in the growing season in more recently released soybean varieties. Better understanding of the mechanisms that have improved conversion efficiency in the past may help identify new, promising targets for the future.

Expression of functional traits during seedling establishment in two populations of Pinus ponderosa from contrasting climates.

First-year tree seedlings represent a particularly vulnerable life stage and successful seedling establishment is crucial for forest regeneration. We investigated the extent to which Pinus ponderosa P. & C. Lawson populations from different climate zones exhibit differential expression of functional traits that may facilitate their establishment. Seeds from two populations from sites with contrasting precipitation and temperature regimes east (PIPO dry) and west (PIPO mesic) of the Oregon Cascade mountains were sown in a common garden experiment and grown under two water availability treatments (control and drought). Aboveground biomass accumulation, vegetative phenology, xylem anatomy, plant hydraulic architecture, foliar stable carbon isotope ratios (δ(13)C), gas exchange and leaf water relations characteristics were measured. No treatment or population-related differences in leaf water potential were detected. At the end of the first growing season, aboveground biomass was 74 and 44% greater in PIPO mesic in the control and drought treatments, respectively. By early October, 73% of PIPO dry seedlings had formed dormant buds compared with only 15% of PIPO mesic seedlings. Stem theoretical specific conductivity, calculated from tracheid dimensions and packing density, declined from June through September and was nearly twice as high in PIPO mesic during most of the growing season, consistent with measured values of specific conductivity. Intrinsic water-use efficiency based on δ(13)C values was higher in PIPO dry seedlings for both treatments across all sampling dates. There was a negative relationship between values of δ(13)C and leaf-specific hydraulic conductivity across populations and treatments, consistent with greater stomatal constraints on gas exchange with declining seedling hydraulic capacity. Integrated growing season assimilation and stomatal conductance estimated from foliar δ(13)C values and photosynthetic CO2-response curves were 6 and 28% lower, respectively, in PIPO dry seedlings. Leaf water potential at the turgor loss point was 0.33 MPa more negative in PIPO dry, independent of treatment. Overall, PIPO dry seedlings exhibited more conservative behavior, suggesting reduced growth is traded off for increased resistance to drought and extreme temperatures.

Carbon dioxide stimulation of photosynthesis in Liquidambar styraciflua is not sustained during a 12-year field experiment.

Elevated atmospheric CO2 (eCO2) often increases photosynthetic CO2 assimilation (A) in field studies of temperate tree species. However, there is evidence that A may decline through time due to biochemical and morphological acclimation, and environmental constraints. Indeed, at the free-air CO2 enrichment (FACE) study in Oak Ridge, Tennessee, A was increased in 12-year-old sweetgum trees following 2 years of ∼40 % enhancement of CO2. A was re-assessed a decade later to determine if the initial enhancement of photosynthesis by eCO2 was sustained through time. Measurements were conducted at prevailing CO2 and temperature on detached, re-hydrated branches using a portable gas exchange system. Photosynthetic CO2 response curves (A versus the CO2 concentration in the intercellular air space (Ci); or A-Ci curves) were contrasted with earlier measurements using leaf photosynthesis model equations. Relationships between light-saturated photosynthesis (Asat), maximum electron transport rate (Jmax), maximum Rubisco activity (Vcmax), chlorophyll content and foliar nitrogen (N) were assessed. In 1999, Asat for eCO2 treatments was 15.4 ± 0.8 μmol m(-2) s(-1), 22 % higher than aCO2 treatments (P < 0.01). By 2009, Asat declined to <50 % of 1999 values, and there was no longer a significant effect of eCO2 (Asat = 6.9 or 5.7 ± 0.7 μmol m(-2) s(-1) for eCO2 or aCO2, respectively). In 1999, there was no treatment effect on area-based foliar N; however, by 2008, N content in eCO2 foliage was 17 % less than that in aCO2 foliage. Photosynthetic N-use efficiency (Asat : N) was greater in eCO2 in 1999 resulting in greater Asat despite similar N content, but the enhanced efficiency in eCO2 trees was lost as foliar N declined to sub-optimal levels. There was no treatment difference in the declining linear relationships between Jmax or Vcmax with declining N, or in the ratio of Jmax : Vcmax through time. Results suggest that the initial enhancement of photosynthesis to elevated CO2 will not be sustained through time if N becomes limited.

Effects of maize-peanut intercropping and phosphate fertilizer on photosynthetic characteristics and yield of intercropped peanut plants

Aims Our objective was to determine the enhanced utilization of weak light in intercropped peanut plants(Arachis hypogaea) and the regulatory effects of phosphate(P) fertilizer in maize-peanut intercropping system, which is of great significance to elucidate photosynthetic adaptation mechanism of intercropped peanut plants and to increase the yield. Methods Chlorophyll(Chl) content and composition, photosynthetic light response curves, photosynthetic CO2 response curves, and fluorescence parameters in the functional leaves of intercropped peanut plants were studied at the experimental farm of Henan University of Science and Technology in 2011 and 2012. Important findings The Chl b content was higher, and the Chl a/b ratio was lower, in the functional leaves of intercropped peanut plants than in those of single cropped peanut plants with or without P fertilizer application. There were higher values of maximum photochemical efficiency(Fv /Fm), actual photochemical efficiency(ΦPSII), photochemical quenching coefficient(qP) of photosystem II, apparent quantum yield(AQY), and photosynthetic rate, but lower values of stomatal conductance, RuBP carboxylation rate(Vcmax), electron transport rate(Jmax), and triose-phosphate utilization rate(TPU) in the functional leaves of intercropped peanut plants than in those of single cropped peanut plants. Application of P fertilizer was beneficial to improving chlorophyll content, ΦPSII and qP of photosystem II, Vcmax, Jmax, and TPU in the functional leaves of intercropped peanut plants. It suggested that intercropped peanut plants could make more efficient use of weak light because of the higher Fv /Fm, ΦPSII and qP of photosystem II resulting from higher Chl b content and changes chlorophyll composition of functional leaves, which strengthened the capacity of light absorption and transformation, but not because of the higher CO2 rate of fixation and improved carboxylation efficiency. Application of P fertilizer was beneficial to improving the utilization of weak light and to increasing the yield of intercropped peanut, resulting in an increase in the land equivalentratio of the maize-peanut intercropping system by 6.2%–9.3%.

Applicability of modified exponential model in photosynthetic-CO2 response curve of barley

Applicability of modified exponential model in photosynthetic-CO2 response curve of barley

Effect of overnight temperature on leaf photosynthesis in seedlings ofSwietenia macrophylla King

After exposure of one-year old seedlings ofSwietenia macrophylla to an overnight temperature (13°C, 19°C, 25°C, 31°C or 35°C), the leaf net photosynthetic rate (P n) was researched through measuring photosynthetic light-response curves at 360 μmol·mol−1 CO2, and photosynthetic CO2-response curves at light-saturated intensity (1500 μmol·m−2·s−1). The optimal temperature for photosynthesis measured at 360 μmol·mol−1 CO2 was from 25°C to 31°C, but which was from 31°C to 35°C at saturating CO2 concentration. At temperature of below 25°C, the decline inP n was mainly due to the drop in carboxylation efficiency (C e), while as temperature was over 31°C, the reduction inP n resulted from both decrease inC e and increase in leaf respiration. The CO2-induced stimulation of photosynthesis was strongly inhibited at temperatures below 13°C. The results showed that, the leaf photosynthesis of tropical evergreen plants should not be accelerated at low temperature in winter season with elevated CO2 concentration in the future.

Response to elevated CO2 from a natural spring in a C4-dominated grassland depends on seasonal phenology

A South African C4-dominated grassland was exposed to twice-ambient atmospheric CO2 concentration using gas emitted by a natural CO2 spring and distributed over a 7m x 7m plot. A similar control plot was established 20m away at near-ambient CO2 concentrations. Photosynthetic CO2 response curves were performed on three C4 and one C3 grass species under both treatments, in spring (post-fire), mid-summer and autumn. Photosynthetic efficiency of the post-fire dominants, Alloteropsis semialata subsp. eckloniana (C3) and Andropogon appendiculatus (C4) was significantly enhanced in high CO2 only during the early season, when photosynthetic capacity was high. Thereafter, photosynthetic capacity decreased with advancing season in both species, and positive responses to high CO2 were lost or reduced. In the mid- to late-season dominant Themeda triandra (C4), photosynthetic capacity was maintained in elevated CO2 during the mid- to late-season, while decreasing in ambient CO2 relative to high CO2. Eragrostis racemosa (C4), a slow grower, showed no significant responses to high CO2 during the late season. CO2-responsiveness in the field generally followed the seasonal phenologies of each species, being greatest during the most active growth period.

Effects of Growth Temperature on the Responses of Ribulose-1,5-Biphosphate Carboxylase, Electron Transport Components, and Sucrose Synthesis Enzymes to Leaf Nitrogen in Rice, and Their Relationships to Photosynthesis.

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under day/night temperature regimes of 18/15[deg]C, 23/18[deg]C, and 30/23[deg]C and all photosynthetic measurements were made at a leaf temperature of 25[deg]C and an irradiance of 1800 [mu]mol quanta m-2 s-1. Growth temperature affected the photosynthetic CO2 response curve. The relative ratio of the initial slope to the CO2-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO2-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO2-limited photosynthesis with rising growth temperature was the result of an increase in the CO2 transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18[deg]C and those grown at 30/23[deg]C. Similarly, CO2-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15[deg]C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO2-saturated photosynthesis.

Influence of the tobacco mosaic virus 30-kDa movement protein on carbon metabolism and photosynthate partitioning in transgenic tobacco plants

Transgenic tobacco (Nicotiana tabacum L.) plants expressing the 30-kDa movement protein of tobacco mosaic virus (TMV-MP) were employed to investigate the influence of a localized change in mesophyll-bundle sheath plasmodesmal size exclusion limit on photosynthetic performance and on carbon metabolism and allocation. Under conditions of saturating irradiance, tobacco plants expressing the TMV-MP were found to have higher photosynthetic CO2-response curves compared with vector control plants. However, this difference was significant only in the presence of elevated CO2 levels. Photosynthetic measurements made in the green-house, under endogenous growth conditions, revealed that there was little difference between TMV-MP-expressing and control tobacco plants. However, analysis of carbon metabolites within source leaves where a TMV-MP-induced increase in plasmodesmal size exclusion limit had recently taken place established that the levels of sucrose, glucose, fructose and starch were considerably elevated above those present in equivalent control leaves. Although expression of the TMV-MP did not alter total plant biomass, it reduced carbon allocation to the lower region of the stem and roots. This difference in biomass distribution was clearly evident in the lower root-to-shoot ratios for the TMV-MP transgenic plants. Microinjection (dye-coupling) studies established that the TMV-MP-associated reduction in photosynthate delivery (allocation) to the roots was not due to a direct effect on root cortical plasmodesmata. Rather, this change appeared to result from an alteration in phloem transport from young source leaves in which the TMV-MP had yet to exert its influence over plasmodesmal size exclusion limits. These results are discussed in terms of the rate-limiting steps involved in sucrose movement into the phloem.