Far-Red Chlorophyll Fluorescence Radiance Tracks Photosynthetic Carbon Assimilation Efficiency of Dark Reactions

Solar-induced chlorophyll fluorescence (SIF) observations from space have shown close relationships with terrestrial photosynthesis rates. SIF originates from the light reactions of photosynthesis, whereas carbon fixation takes place during the dark reactions of photosynthesis. Questions remain regarding whether SIF is able to track changes in the efficiency of the dark reactions in photosynthesis. Using concurrent measurements of leaf-scale gas exchange, pulse amplitude-modulated (PAM) fluorescence, and fluorescence spectral radiances, we found that both far-red fluorescence radiances and PAM fluorescence yields responded rapidly to changes in photosynthetic carbon assimilation due to changes in environmental factors or induced stomatal closure under constant light conditions. Uncertainties in outgoing and incoming irradiance mismatch for SIF measurements may very likely obscure the contributions of the dark reactions, thereby causing the inconsistent findings previously reported, which were no change in far-red SIF and PAM fluorescence yields after clear reductions in the photosynthetic carbon assimilation efficiency of dark reactions. Our results confirm that high-quality SIF measurements have the potential to provide insights into the dark reactions of photosynthesis. This study is particularly relevant for better interpreting satellite SIF observations that are obtained under roughly constant overpass times and relatively stable light intensities.

Physiological and Transcriptomic Analyses Revealed the Implications of Abscisic Acid in Mediating the Rate-Limiting Step for Photosynthetic Carbon Dioxide Utilisation in Response to Vapour Pressure Deficit in Solanum Lycopersicum (Tomato)

The atmospheric vapour pressure deficit (VPD) has been demonstrated to be a significant environmental factor inducing plant water stress and affecting plant photosynthetic productivity. Despite this, the rate-limiting step for photosynthesis under varying VPD is still unclear. In the present study, tomato plants were cultivated under two contrasting VPD levels: high VPD (3–5 kPa) and low VPD (0.5–1.5 kPa). The effect of long-term acclimation on the short-term rapid VPD response was examined across VPD ranging from 0.5 to 4.5 kPa. Quantitative photosynthetic limitation analysis across the VPD range was performed by combining gas exchange and chlorophyll fluorescence. The potential role of abscisic acid (ABA) in mediating photosynthetic carbon dioxide (CO2) uptake across a series of VPD was evaluated by physiological and transcriptomic analyses. The rate-limiting step for photosynthetic CO2 utilisation varied with VPD elevation in tomato plants. Under low VPD conditions, stomatal and mesophyll conductance was sufficiently high for CO2 transport. With VPD elevation, plant water stress was gradually pronounced and triggered rapid ABA biosynthesis. The contribution of stomatal and mesophyll limitation to photosynthesis gradually increased with an increase in the VPD. Consequently, the low CO2 availability inside chloroplasts substantially constrained photosynthesis under high VPD conditions. The foliar ABA content was negatively correlated with stomatal and mesophyll conductance for CO2 diffusion. Transcriptomic and physiological analyses revealed that ABA was potentially involved in mediating water transport and photosynthetic CO2 uptake in response to VPD variation. The present study provided new insights into the underlying mechanism of photosynthetic depression under high VPD stress.

Elevated CO2 Enhances Dynamic Photosynthesis in Rice and Wheat

Crops developed under elevated carbon dioxide (eCO2) exhibit enhanced leaf photosynthesis under steady states. However, little is known about the effect of eCO2 on dynamic photosynthesis and the relative contribution of the short-term (substrate) and long-term (acclimation) effects of eCO2. We grew an Oryza sativa japonica cultivar and a Triticum aestivum cultivar under 400 μmol CO2 mol−1 air (ambient, A) and 600 μmol CO2 mol−1 air (elevated, E). Regardless of growth [CO2], the photosynthetic responses to the sudden increase and decrease in light intensity were characterized under 400 (a) or 600 μmol CO2 mol−1 air (e). The Aa1, Ae2, Ea3, and Ee4 treatments were employed to quantify the acclimation effect (Ae vs. Ee and Aa vs. Ea) and substrate effect (Aa vs. Ae and Ea vs. Ee). In comparison with the Aa treatment, both the steady-state photosynthetic rate (PN) and induction state (IS) were higher under the Ae and Ee treatments but lower under the Ea treatment in both species. However, IS reached at the 60 sec after the increase in light intensity, the time required for photosynthetic induction, and induction efficiency under Ae and Ee treatment did not differ significantly from those under Aa treatment. The substrate effect increased the accumulative carbon gain (ACG) during photosynthetic induction by 45.5% in rice and by 39.3% in wheat, whereas the acclimation effect decreased the ACG by 18.3% in rice but increased it by 7.5% in wheat. Thus, eCO2, either during growth or at measurement, enhances the dynamic photosynthetic carbon gain in both crop species. This indicates that photosynthetic carbon loss due to an induction limitation may be reduced in the future, under a high-CO2 world.

The Bloom-Forming Dinoflagellate Karenia mikimotoi Adopts Different Growth Modes When Exposed to Short or Long Period of Seawater Acidification

Impacts of ocean acidification (OA) on noncalcifying organisms and the possibly responsible mechanism have aroused great research interests with the intensification of global warming. The present study focused on a noxious, noncalcifying, bloom-forming dinoflagellate, Karenia mikimotoi (K. mikimotoi), and its variation of growth patterns exposed to different periods of seawater acidification with stressing gradients was discussed. The dinoflagellates under short-time acidifying stress (2d) with different levels of CO2 presented significant growth inhibition (p < 0.05). The cell cycle was obviously inhibited at S phase, and the photosynthetic carbon fixation was also greatly suppressed (p < 0.05). Apoptosis was observed and the apoptotic rate increased with the increment of pCO2. Similar tendencies were observed in the key components of mitochondrial apoptotic pathway (the mitochondrial membrane potential (MMP), Caspase-3 and -9, and Bax/Bcl-2 ratio). However, under prolonged stressing time (8 d and 15 d), the growth of dinoflagellates was recovered or even stimulated, the photosynthetic carbon fixation was significantly increased (p < 0.05), the cell cycle of division presented little difference with those in the control, and no apoptosis was observed (p > 0.05). Besides, acidification adjusted by HCl addition and CO2 enrichment resulted in different growth performances, while the latter had a more negative impact. The results of present study indicated that (1) the short-time exposure to acidified seawater led to reduced growth performance via inducing apoptosis, blocking of cell cycle, and the alteration in photosynthetic carbon fixation. (2) K. mikimotoi had undergone adaptive changes under long-term exposure to CO2 induced seawater acidification. This further demonstrated that K. mikimotoi has strong adaptability in the face of seawater acidification, and this may be one of the reasons for the frequent outbreak of red tide. (3) Ions that dissociated by the dissolved CO2, instead of H+ itself, were more important for the impacts induced by the acidification. This work thus provides a new perspective and a possible explanation for the dominance of K. mikimotoi during the occurrence of HABs.

A warmer growing season triggers earlier following spring phenology

Under global warming, advances in spring phenology due to the rising temperature have been widely reported. However, the mechanisms underlying the warming-induced earlier spring phenology remain poorly understood. Here, using multiple long-term and large-scale phenological datasets between 1951 and 2018, we show that warmer temperatures during the previous growing season between May and September led to earlier spring phenology in the Northern Hemisphere. We also found that warming-induced increases in maximum photosynthetic rate in the previous year advanced spring phenology. Furthermore, we found a significant decline in the advancing effect of warming during previous growing season on spring phenology from cold to warm periods over the past decades. Our results suggest that observed warming-induced earlier spring phenology may be driven by increased photosynthetic carbon assimilation in the previous season, while the slowdown in the advanced spring phenology is likely due to decreased carbon assimilation when warming exceeding the optimal temperatures for photosynthesis. Our study suggested the vital role of photosynthetic carbon assimilation during growing season in spring phenology under global warming.