Effects of elevated ozone and carbon dioxide on the dynamic photosynthesis of Fagus crenata seedlings under variable light conditions

Ozone (O₃) is an air pollutant that is toxic to trees. O₃ reduces steady-state net photosynthetic rate (A), and the adverse effects of O₃ are mitigated under elevated CO₂ condition. However, the combined effects of O₃ and elevated CO₂ on dynamic photosynthesis under variable light conditions have not yet been clarified. In this study, we investigated the effects of O₃ and elevated CO₂ on dynamic photosynthesis in the leaves of Fagus crenata seedlings under variable light conditions. The seedlings were grown under four gas treatments comprising two levels of O₃ concentration (lower and two times higher than the ambient O₃ concentration) and two levels of CO₂ concentration (ambient and 700 ppm). Although O₃ significantly decreased steady-state An under ambient CO₂ concentrations, no significant decrease was observed under elevated CO₂ concentrations, indicating the mitigating effect of elevated CO₂ on O₃-induced adverse effects on steady-state A. During photosynthetic induction, the response of A to the change in photosynthetic photon flux density (PPFD) from 50 (low light) to 1000 μmol m^-2 s^-1 (high light) showed that the increase in A was slowed by O₃ and accelerated by elevated CO₂. Under fluctuating light condition of repeating low light for 4 min and high light for 1 min, A at end of each high light period gradually decreased in all treatments, and O₃ and elevated CO₂ accelerated the reduction of A. In contrast to steady-state A, no mitigating effect of elevated CO₂ was observed for any parameters related to dynamic photosynthesis. We conclude that the combined effects of O₃ and elevated CO₂ on A of F. crenata are different under steady-state and variable light conditions, and the O₃-induced decrease in leaf A may not be mitigated by elevated CO₂ in the field under variable light conditions.

Towards improved dynamic photosynthesis in C3 crops by utilizing natural genetic variation

Under field environments, fluctuating light conditions induce dynamic photosynthesis, which affects carbon gain by crop plants. Elucidating the natural genetic variations among untapped germplasm resources and their underlying mechanisms can provide an effective strategy to improve dynamic photosynthesis and, ultimately, improve crop yields through molecular breeding approaches. In this review, we first overview two processes affecting dynamic photosynthesis, namely (i) biochemical processes associated with CO2 fixation and photoprotection and (ii) gas diffusion processes from the atmosphere to the chloroplast stroma. Next, we review the intra- and interspecific variations in dynamic photosynthesis in relation to each of these two processes. It is suggested that plant adaptations to different hydrological environments underlie natural genetic variation explained by gas diffusion through stomata. This emphasizes the importance of the coordination of photosynthetic and stomatal dynamics to optimize the balance between carbon gain and water use efficiency under field environments. Finally, we discuss future challenges in improving dynamic photosynthesis by utilizing natural genetic variation. The forward genetic approach supported by high-throughput phenotyping should be introduced to evaluate the effects of genetic and environmental factors and their interactions on the natural variation in dynamic photosynthesis.

Variation in photosynthetic induction between super hybrid rice and inbred super rice

It is well documented that yield superiority of super hybrid rice is linked with its improved photosynthetic capacity and/or efficiency. In natural environments, the amounts of CO₂ assimilated by plants was also impacted by the rapidity of leaf photosynthesis response to fluctuations of light. However, it remains unknow whether the high yield of super hybrid rice was associated with photosynthetic traits under dynamic state. Here, photosynthetic traits under steady-and dynamic state in two super hybrid rice varieties (Ylinagyou 3218 and Yliangyou 5867) with high yield and two inbred super rice varieties (Zhendao 11 and Nanjing 9108) with lower yield. Under steady state, the net photosynthetic rate (A*) in super hybrid rice was 25.3% larger compared with inbred super rice. During photosynthetic induction, there was no obvious association of the rapidity of net photosynthesis rate (A) to sunflecks with rice subpopulations. Stomatal conductance (g_s) of super hybrid rice increased slower than that of inbred super rice. The cumulative CO₂ fixation (CCF) during photosynthetic induction was 25.2% larger in super hybrid rice than that in inbred super rice. The primary limitation during induction was biochemical limitation rather than stomatal limitation. There was a significantly positive relationship between A* and CCF, while A* was not related with the induction response rate of A. Overall, A* and CCF in super hybrid rice have been improved together, which contributed to its yield superiority, whereas its yield potential still can be improved by increasing induction rate of A under fluctuations of irradiance.

A low-cost automated growth chamber system for continuous measurements of gas exchange at canopy scale in dynamic conditions

BACKGROUND

Obtaining instantaneous gas exchanges data is fundamental to gain information on photosynthesis. Leaf level data are reliable, but their scaling up to canopy scale is difficult as they are acquired in standard and/or controlled conditions, while natural environments are extremely dynamic. Responses to dynamic environmental conditions need to be considered, as measurements at steady state and their related models may overestimate total carbon (C) plant uptake.

RESULTS

In this paper, we describe an automatic, low-cost measuring system composed of 12 open chambers (60 × 60 × 150 cm; around 400 euros per chamber) able to measure instantaneous CO₂ and H₂O gas exchanges, as well as environmental parameters, at canopy level. We tested the system's performance by simulating different CO₂ uptake and respiration levels using a tube filled with soda lime or pure CO₂, respectively, and quantified its response time and measurement accuracy. We have been also able to evaluate the delayed response due to the dimension of the chambers, proposing a method to correct the data by taking into account the response time ([Formula: see text]) and the residence time (τ). Finally, we tested the system by growing a commercial soybean variety in fluctuating and non-fluctuating light, showing the system to be fast enough to capture fast dynamic conditions. At the end of the experiment, we compared cumulative fluxes with total plant dry biomass.

CONCLUSIONS

The system slightly over-estimated (+ 7.6%) the total C uptake, even though not significantly, confirming its ability in measuring the overall CO₂ fluxes at canopy scale. Furthermore, the system resulted to be accurate and stable, allowing to estimate the response time and to determine steady state fluxes from unsteady state measured values. Thanks to the flexibility in the software and to the dimensions of the chambers, even if only tested in dynamic light conditions, the system is thought to be used for several applications and with different plant canopies by mimicking different environmental conditions.

Photosynthetic induction and its diffusional, carboxylation and electron transport processes as affected by CO2 partial pressure, temperature, air humidity and blue irradiance

BACKGROUND AND AIMS

Plants depend on photosynthesis for growth. In nature, factors such as temperature, humidity, CO₂ partial pressure, and spectrum and intensity of irradiance often fluctuate. Whereas irradiance intensity is most influential and has been studied in detail, understanding of interactions with other factors is lacking.

METHODS

We tested how photosynthetic induction after dark-light transitions was affected by CO₂ partial pressure (20, 40, 80 Pa), leaf temperatures (15·5, 22·8, 30·5 °C), leaf-to-air vapour pressure deficits (VPD_leaf-air; 0·5, 0·8, 1·6, 2·3 kPa) and blue irradiance (0-20 %) in tomato leaves (Solanum lycopersicum).

KEY RESULTS

Rates of photosynthetic induction strongly increased with CO₂ partial pressure, due to increased apparent Rubisco activation rates and reduced diffusional limitations. High leaf temperature produced slightly higher induction rates, and increased intrinsic water use efficiency and diffusional limitation. High VPD_leaf-air slowed down induction rates and apparent Rubisco activation and (at 2·3 kPa) induced damped stomatal oscillations. Blue irradiance had no effect. Slower apparent Rubisco activation in elevated VPD_leaf-air may be explained by low leaf internal CO₂ partial pressure at the beginning of induction.

CONCLUSIONS

The environmental factors CO₂ partial pressure, temperature and VPD_leaf-air had significant impacts on rates of photosynthetic induction, as well as on underlying diffusional, carboxylation and electron transport processes. Furthermore, maximizing Rubisco activation rates would increase photosynthesis by at most 6-8 % in ambient CO₂ partial pressure (across temperatures and humidities), while maximizing rates of stomatal opening would increase photosynthesis by at most 1-3 %.

Photosynthetic induction and its diffusional, carboxylation and electron transport processes as affected by CO2 partial pressure, temperature, air humidity and blue irradiance

BACKGROUND AND AIMS

Plants depend on photosynthesis for growth. In nature, factors such as temperature, humidity, CO₂ partial pressure, and spectrum and intensity of irradiance often fluctuate. Whereas irradiance intensity is most influential and has been studied in detail, understanding of interactions with other factors is lacking.

METHODS

We tested how photosynthetic induction after dark-light transitions was affected by CO₂ partial pressure (20, 40, 80 Pa), leaf temperatures (15·5, 22·8, 30·5 °C), leaf-to-air vapour pressure deficits (VPD_leaf-air; 0·5, 0·8, 1·6, 2·3 kPa) and blue irradiance (0-20 %) in tomato leaves (Solanum lycopersicum).

KEY RESULTS

Rates of photosynthetic induction strongly increased with CO₂ partial pressure, due to increased apparent Rubisco activation rates and reduced diffusional limitations. High leaf temperature produced slightly higher induction rates, and increased intrinsic water use efficiency and diffusional limitation. High VPD_leaf-air slowed down induction rates and apparent Rubisco activation and (at 2·3 kPa) induced damped stomatal oscillations. Blue irradiance had no effect. Slower apparent Rubisco activation in elevated VPD_leaf-air may be explained by low leaf internal CO₂ partial pressure at the beginning of induction.

CONCLUSIONS

The environmental factors CO₂ partial pressure, temperature and VPD_leaf-air had significant impacts on rates of photosynthetic induction, as well as on underlying diffusional, carboxylation and electron transport processes. Furthermore, maximizing Rubisco activation rates would increase photosynthesis by at most 6-8 % in ambient CO₂ partial pressure (across temperatures and humidities), while maximizing rates of stomatal opening would increase photosynthesis by at most 1-3 %.

Effects of high CO2 levels on dynamic photosynthesis: carbon gain, mechanisms, and environmental interactions

Understanding the photosynthetic responses of terrestrial plants to environments with high levels of CO2 is essential to address the ecological effects of elevated atmospheric CO2. Most photosynthetic models used for global carbon issues are based on steady-state photosynthesis, whereby photosynthesis is measured under constant environmental conditions; however, terrestrial plant photosynthesis under natural conditions is highly dynamic, and photosynthetic rates change in response to rapid changes in environmental factors. To predict future contributions of photosynthesis to the global carbon cycle, it is necessary to understand the dynamic nature of photosynthesis in relation to high CO2 levels. In this review, we summarize the current body of knowledge on the photosynthetic response to changes in light intensity under experimentally elevated CO2 conditions. We found that short-term exposure to high CO2 enhances photosynthetic rate, reduces photosynthetic induction time, and reduces post-illumination CO2 burst, resulting in increased leaf carbon gain during dynamic photosynthesis. However, long-term exposure to high CO2 during plant growth has varying effects on dynamic photosynthesis. High levels of CO2 increase the carbon gain in photosynthetic induction in some species, but have no significant effects in other species. Some studies have shown that high CO2 levels reduce the biochemical limitation on RuBP regeneration and Rubisco activation during photosynthetic induction, whereas the effects of high levels of CO2 on stomatal conductance differ among species. Few studies have examined the influence of environmental factors on effects of high levels of CO2 on dynamic photosynthesis. We identified several knowledge gaps that should be addressed to aid future predictions of photosynthesis in high-CO2 environments.

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 μmol photons m^-2 s^-1) and light compensation (70 to 100 μmol m^-2 s^-1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO₂ fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.