Improving the estimation of mesophyll conductance to CO2: on the role of electron transport rate correction and respiration

Mesophyll conductance (g m ) can markedly limit photosynthetic CO2 assimilation and is required to estimate the parameters of the Farquhar–von Caemmerer–Berry (FvCB) model properly. The variable J (electron transport rate) is the most frequently used method for estimating g m, and the correct determination of J is one of its requirements. Recent evidence has shown that calibrating J can lead to some errors in estimating g m , but to what extent the parameterization of the FvCB model is affected by calibrations is not well known. In addition to determining the FvCB parameters, variants of the J calibration method were tested to address whether varying CO 2 or light levels, possible alternative electron sinks, or contrasting leaf structural properties might play a role in determining differences in αβ, the product of the leaf absorptance (α) and the photosystem II optical cross-section (β). It was shown that differences in αβ were mainly attributed to the use of A/C i or A/PPFD curves to calibrate J. The different αβ values greatly influenced g m , leading to a high number of unrealistic values in addition to affecting the estimates of the FvCB model parameters. A new approach was devised to retrieve leaf respiration in the light from combined A/C i and A/C c curves and a framework to understand the high variation in observed g m values. Overall, a background is provided to decrease the noise in g m , facilitating data reporting and allowing better retrieval of the information presented in A/C i and A/C c curves

Photosynthesis and photoprotection in coffee leaves is affected by nitrogen and light availabilities in winter conditions

Coffee is native to shady environments but often grows better and produces higher yields without shade, though at the expense of high fertilization inputs, particularly nitrogen (N). Potted plants were grown under full sunlight and shade (50%) conditions and were fertilized with nutrient solutions containing either 0 or 23 mM N. Measurements were made in southeastern Brazil during winter conditions, when relatively low night temperatures and high diurnal insolation are common. Overall, the net carbon assimilation rate was quite low, which was associated with diffusive, rather than biochemical, constraints. N deficiency led to decreases in the concentrations of chlorophylls (Chl) and total carotenoids as well as in the Chl/N ratio. These conditions also led to qualitative changes in the carotenoid composition, e.g., increased antheraxanthin (A) and zeaxanthin (Z) pools on a Chl basis, particularly at high light, which was linked to increased thermal dissipation of absorbed light. The variable-to-maximum fluorescence ratio at predawn decreased with increasing A + Z pools and decreased linearly with decreasing N. We showed that this ratio was inadequate for assessing photoinhibition under N limitation. Expressed per unit mass, the activities of superoxide dismutase and glutathione reductase were not altered with the treatments. In contrast, ascorbate peroxidase activity was lower in low N plants, particularly under shade, whereas catalase activity was lower in shaded plants than in sun-grown plants, regardless of the N level. Glutamine synthetase activity was greater in sun-grown plants than in shaded individuals at a given N level and decreased with decreasing N application. Our results suggest that the photoprotective and antioxidant capacity per amount of photons absorbed was up-regulated by a low N supply; nevertheless, this capacity, regardless of the light conditions, was not enough to prevent oxidative damage, as judged from the increases in the H2O2 and malondialdehyde concentrations and electrolyte leakage. We demonstrated that N fertilization could adequately protect the coffee plants against photodamage independently of the anticipated positive effects of N on the photosynthetic capacity

Knockout mutants are due to altered organic acid accumulation and an increase in both stomatal and mesophyll conductance

Stomata control the exchange of CO 2 and water vapor in land plants. Thus, whereas a constant supply of CO 2 is required to maintain adequate rates of photosynthesis, the accompanying water losses must be tightly regulated to prevent dehydration and undesired metabolic changes. Accordingly, the uptake or release of ions and metabolites from guard cells is necessary to achieve normal stomatal function. The AtQUAC1, an R-type anion channel responsible for the release of malate from guard cells, is essential for efficient stomatal closure. Here, we demonstrate that mutant plants lacking AtQUAC1 accumulated higher levels of malate and fumarate. These mutant plants not only display slower stomatal closure in response to increased CO 2 concentration and dark but are also characterized by improved mesophyll conductance. These responses were accompanied by increases in both photosynthesis and respiration rates, without affecting the activity of photosynthetic and respiratory enzymes and the expression of other transporter genes in guard cells, which ultimately led to improved growth. Collectively, our results highlight that the transport of organic acids plays a key role in plant cell metabolism and demonstrate that AtQUAC1 reduce diffusive limitations to photosynthesis, which, at least partially, explain the observed increments in growth under well-watered conditions

The role of silicon in metabolic acclimation of rice plants challenged with arsenic

Silicon (Si) plays key roles in alleviating various abiotic stresses, including arsenic (As) toxicity, via physiological mechanisms that remain poorly understood. Here, we combined photosynthetic measurements with analyses of central metabolism and gene expression to explore the consequences of As-related alterations on primary metabolism and examined whether these consequences could be affected by the application of Si to rice (Oryza sativa L.) plants challenged with As but supplemented with Si. The negative effects of As on photosynthesis and carbohydrate status were largely reversed by Si. However, no major metabolic reprogramming was observed, as denoted by minor, if any, significant changes in (i) the activities of a range of enzymes associated with C metabolism; (ii) the levels of a wide range of organic acids and amino acids; and (iii) the pools of NAD(P)H/NAD(P)+ and the redox states of ascorbate and glutathione. Arsenic toxicity was apparently unrelated to oxidative stress. We suggested that the search for As-tolerant plants under real field conditions should not focus solely on oxidative stress, and hence the focus on photosynthesis might be of higher significance. In conclusion, we identified Si nutrition as a central player that restricts photosynthetic impairment in As-treated plants, in addition to limiting As uptake via modulation of the expression of genes with prime importance in As uptake and translocation

The interplay between irrigation and fruiting on branch growth and mortality, gas exchange and water relations of coffee trees

The overall coordination between gas exchanges and plant hydraulics may be affected by soil water availability and source-to-sink relationships. Here we evaluated how branch growth and mortality, leaf gas exchange and metabolism are affected in coffee (Coffea arabica L.) trees by drought and fruiting. Field-grown plants were irrigated or not, and maintained with full or no fruit load. Under mild water deficit, irrigation per se did not significantly impact growth but markedly reduced branch mortality in fruiting trees, despite similar leaf assimilate pools and water status. Fruiting increased net photosynthetic rate in parallel with an enhanced stomatal conductance, particularly in irrigated plants. Mesophyll conductance and maximum RuBisCO carboxylation rate remained unchanged across treatments. The increased stomatal conductance in fruiting trees over nonfruiting ones was unrelated to internal CO2 concentration, foliar abscisic acid (ABA) levels or differential ABA sensitivity. However, stomatal conductance was associated with higher stomatal density, lower stomatal sensitivity to vapor pressure deficit, and higher leaf hydraulic conductance and capacitance. Increased leaf transpiration rate in fruiting trees was supported by coordinated alterations in plant hydraulics, which explained the maintenance of plant water status. Finally, by preventing branch mortality, irrigation can mitigate biennial production fluctuations and improve the sustainability of coffee plantations.Universidad Federal de Viçosa, Brasl.Universidad Nacional, Costa Rica.Escuela de Ciencias Biológica