C 4 savanna grasses fail to maintain assimilation in drying soil under low CO 2 compared with C 3 trees despite lower leaf water demand

The slope of assimilation rate against stomatal conductance should not be used as a measure of water use efficiency or stomatal control over assimilation

Quantifying water use efficiency, and the impact of stomata on CO2 uptake are pivotal in physiology and efforts to improve crop yields. Although tempting, relying on regression slopes from assimilation-stomatal conductance plots to estimate water use efficiency or stomatal control over assimilation is erroneous. Through numerical simulations, I substantiate this assertion. I propose the term 'instantaneous transpiration efficiency' for the assimilation-to-transpiration ratio to avoid confusion with 'intrinsic water use efficiency' which refers to the assimilation-to-stomatal conductance ratio, and recommend to compute both metrics for each gas exchange data point.

C4 maize and sorghum are more sensitive to rapid dehydration than C3 wheat and sunflower

The high productive potential, heat resilience, and greater water use efficiency of C4 over C3 plants attract considerable interest in the face of global warming and increasing population, but C4 plants are often sensitive to dehydration, questioning the feasibility of their wider adoption. To resolve the primary effect of dehydration from slower from secondary leaf responses originating within leaves to combat stress, we conducted an innovative dehydration experiment. Four crops grown in hydroponics were forced to a rapid yet controlled decrease in leaf water potential by progressively raising roots of out of the solution while measuring leaf gas exchange. We show that, under rapid dehydration, assimilation decreased more steeply in C4 maize and sorghum than in C3 wheat and sunflower. This reduction was due to a rise of nonstomatal limitation at triple the rate in maize and sorghum than in wheat and sunflower. Rapid reductions in assimilation were previously measured in numerous C4 species across both laboratory and natural conditions. Hence, we deduce that high sensitivity to rapid dehydration might stem from the disturbance of an intrinsic aspect of C4 bicellular photosynthesis. We posit that an obstruction to metabolite transport between mesophyll and bundle sheath cells could be the cause.

Physiological responses to low CO2 over prolonged drought as primers for forest-grassland transitions

Savannahs dominated by grasses with scattered C₃ trees expanded between 24 and 9 million years ago in low latitudes at the expense of forests. Fire, herbivory, drought and the susceptibility of trees to declining atmospheric CO₂ concentrations ([CO₂]_a) are proposed as key drivers of this transition. The role of disturbance is well studied, but physiological arguments are mostly derived from models and palaeorecords, without direct experimental evidence. In replicated comparative experimental trials, we examined the physiological effects of [CO₂]_a and prolonged drought in a broadleaf forest tree, a savannah tree and a savannah C₄ grass. We show that the forest tree was more disadvantaged than either the savannah tree or the C₄ grass by the low [CO₂]_a and increasing aridity. Our experiments provide insights into the role of the intrinsic physiological susceptibility of trees in priming the disturbance-driven transition from forest to savannah in the conditions of the early Miocene.

Leaf physiological traits of plants from the Qinghai-Tibet Plateau and other arid sites in China: Identifying susceptible species and well-adapted extremophiles

Extreme environments, such as deserts and high-elevation ecosystems, are very important from biodiversity and ecological perspectives. However, plant physiology at those sites has been scarcely studied, likely due to logistic difficulties. In the present study, leaf physiological traits in native plants were analyzed from arid zones across an elevational transect in Western China, from Turpan Basin to the Qinghai-Tibet Plateau (QTP) at Delingha. The aim of this study was to use leaf physiological traits to help identifying potentially threatened species and true extremophiles. Physiological measurements in the field, and particularly in situ measurements of gas exchange and chlorophyll fluorescence, have been determined to be useful to determine the current state of plants at a given environment. Using this approach plus a combination of leaf traits, several species performing particularly well at the QTP were identified, e.g. Hedysarum multijugum, as well as at Manas drylands, e.g. Peganum harmala and Setaria viridis. On the other hand, several species showed marked signs of severe stress, in particular a very low photosynthetic rate over its potential maximum, as well as other negative traits, like low water and/or nitrogen-use-efficiency, which should be considered in conservation plans. Interestingly, all C₄ species studied except Setaria viridis were among the most stressed species. Despite their higher water use efficiency and drought-tolerance reputation, they presented a much larger photosynthesis depression than most C₃ species. This is an intriguing and interesting observation that deserves further studies.

A meta-analysis of responses of C3 plants to atmospheric CO2 : dose-response curves for 85 traits ranging from the molecular to the whole-plant level

Generalised dose-response curves are essential to understand how plants acclimate to atmospheric CO₂ . We carried out a meta-analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂ ] under relatively benign conditions, and derived dose-response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200-1200 µmol mol^-1 CO₂ , some traits more than doubled (e.g. area-based photosynthesis; intrinsic water-use efficiency), whereas others more than halved (area-based transpiration). At current atmospheric [CO₂ ], 64% of the total stimulation in biomass over the 200-1200 µmol mol^-1 range has already been realised. We also mapped the trait responses of plants to [CO₂ ] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂ ], and some traits (such as area-based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂ ] at different integration levels and offers the quantitative dose-response curves that can be used to improve global change simulation models.

Response of photosynthesis, growth and water relations of a savannah-adapted tree and grass grown across high to low CO2

BACKGROUND AND AIMS

By the year 2100, atmospheric CO2 concentration ([CO2]a) could reach 800 ppm, having risen from ~200 ppm since the Neogene, beginning ~24 Myr ago. Changing [CO2]a affects plant carbon-water balance, with implications for growth, drought tolerance and vegetation shifts. The evolution of C4 photosynthesis improved plant hydraulic function under low [CO2]a and preluded the establishment of savannahs, characterized by rapid transitions between open C4-dominated grassland with scattered trees and closed forest. Understanding directional vegetation trends in response to environmental change will require modelling. But models are often parameterized with characteristics observed in plants under current climatic conditions, necessitating experimental quantification of the mechanistic underpinnings of plant acclimation to [CO2]a.

METHODS

We measured growth, photosynthesis and plant-water relations, within wetting-drying cycles, of a C3 tree (Vachellia karroo, an acacia) and a C4 grass (Eragrostis curvula) grown at 200, 400 or 800 ppm [CO2]a. We investigated the mechanistic linkages between trait responses to [CO2]a under moderate soil drying, and photosynthetic characteristics.

KEY RESULTS

For V. karroo, higher [CO2]a increased assimilation, foliar carbon:nitrogen, biomass and leaf starch, but decreased stomatal conductance and root starch. For Eragrostis, higher [CO2]a decreased C:N, did not affect assimilation, biomass or starch, and markedly decreased stomatal conductance. Together, this meant that C4 advantages in efficient water-use over the tree were maintained with rising [CO2]a.

CONCLUSIONS

Acacia and Eragrostis acclimated differently to [CO2]a, with implications for their respective responses to water limitation and environmental change. Our findings question the carbon-centric focus on factors limiting assimilation with changing [CO2]a, how they are predicted and their role in determining productivity. We emphasize the continuing importance of water-conserving strategies in the assimilation response of savannah plants to rising [CO2]a.

A leaf-level biochemical model simulating the introduction of C2 and C4 photosynthesis in C3 rice: gains, losses and metabolite fluxes

This work aims at developing an adequate theoretical basis for comparing assimilation of the ancestral C₃ pathway with CO₂ concentrating mechanisms (CCM) that have evolved to reduce photorespiratory yield losses. We present a novel model for C₃ , C₂ , C₂  + C₄ and C₄ photosynthesis simulating assimilatory metabolism, energetics and metabolite traffic at the leaf level. It integrates a mechanistic description of light reactions to simulate ATP and NADPH production, and a variable engagement of cyclic electron flow. The analytical solutions are compact and thus suitable for larger scale simulations. Inputs were derived with a comprehensive gas-exchange experiment. We show trade-offs in the operation of C₄ that are in line with ecophysiological data. C₄ has the potential to increase assimilation over C₃ at high temperatures and light intensities, but this benefit is reversed under low temperatures and light. We apply the model to simulate the introduction of progressively complex levels of CCM into C₃ rice, which feeds > 3.5 billion people. Increasing assimilation will require considerable modifications such as expressing the NAD(P)H Dehydrogenase-like complex and upregulating cyclic electron flow, enlarging the bundle sheath, and expressing suitable transporters to allow adequate metabolite traffic. The simpler C₂ rice may be a desirable alternative.