The balance of carbon dioxide (CO2) and water vapour exchange between leaves and the atmosphere is strongly controlled by stomatal conductance. However, the influence of transport processes within ...leaves has recently been gaining prominence. Stable isotope techniques are at the forefront of understanding transport within leaves and the recent development of online, real-time optical isotope analysers has paved the way for new questions to be asked. In this insight, I outline these new techniques and the questions they can potentially address, including assessing possible coordination between mesophyll conductance to CO2 and leaf hydraulic conductance.
Summary
Past studies have established mesophyll diffusion conductance to CO2 (gm) as a variable and significant limitation to plant photosynthesis under steady‐state conditions. However, the role of ...gm in influencing photosynthesis (A) during the transient period of light induction is largely unknown.
We combined gas exchange measurements with laser‐enabled carbon isotope discrimination measurements to assess gm during photosynthetic induction, using Arabidopsis as the measurement species.
Our measurements revealed three key findings: (1) we found that the rate at which gm approached steady state during induction was not necessarily faster than the induction rate of the carboxylation process, contradictory to what has been suggested in previous studies; (2) gm displayed a strong and consistent coordination with A under both induction and steady‐state settings, hinting that the mechanism driving gm–A coupling does not require physiological stability as a prerequisite; and (3) photosynthetic limitation analysis of our data revealed that when integrated over the entire induction period, the relative limitation of A imposed by gm can be as high as > 35%.
The present study provides the first demonstration of the important role of gm in limiting CO2 assimilation during photosynthetic induction, thereby pointing to a need for more research attention to be devoted to gm in future induction studies.
► The intention of the present review is to update the consensus knowledge on gm. ► Gaps in knowledge and research priorities are indicated for the near future. ► In particular, how has gm evolved ...among phylogenetically distant groups? ► Can gm be uncoupled from the water path regulation? ► Mechanistic models of gm incorpored in photosynthesis models are needed.
Mesophyll diffusion conductance to CO2 is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of gm, and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance.
Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.
Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range ...of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ18O and δ2H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.
Brief Summary Statement
Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of evaporative enrichment of heavy isotopes in leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases.
The temperature response of mesophyll conductance to CO
2
diffusion (
g
m
) has been shown to vary considerably between species but remains poorly understood. Here, we tested the hypothesis that ...increases in chloroplast surface area with increasing temperature, due to the formation of chloroplast protrusions, caused observed positive responses of
g
m
to temperature. We found no evidence of chloroplast protrusions. Using simultaneous measurements of carbon and oxygen isotope discrimination during photosynthesis to separate total
g
m
(
g
m13
) into cell wall and plasma membrane conductance (
g
m18
) and chloroplast membrane conductance (
g
cm
) components, we explored the temperature response in genotypes of soybean and barley, and sunflower plants grown at differing CO
2
concentrations. Differences in the temperature sensitivity of
g
m18
were found between genotypes and between plants grown at differing CO
2
concentration but did not relate to measured anatomical features such as chloroplast surface area or cell wall thickness. The closest fit of modelled
g
m13
to estimated values was found when cell wall thickness was allowed to decline at higher temperatures and transpiration rates, but it remains to be tested if this decline is realistic. The temperature response of
g
cm
(calculated from the difference between 1/
g
m13
and 1/
g
m18
) varied between barley genotypes, and was best fitted by an optimal response in sunflower. Taken together, these results indicate that
g
m
is a highly complex trait with unpredictable sensitivity to temperature that varies between species, between genotypes within a single species, with growth environment, between replicate leaves, and even with age for an individual leaf.
• Leaf function is intimately related to the size, shape, abundance and position of cells and chloroplasts. Anatomy has long been assessed and quantified in two dimensions with 3D structure inferred ...from 2D micrographs.
• Serial block face scanning electron microscopy (SBF-SEM) was used to reconstruct 95 cells and 1173 chloroplasts from three wheat and nine chickpea leaves (three samples each from three chickpea genotypes).
• Wheat chloroplast volume was underestimated by 61% in mesophyll cells and 45% in bundle sheath cells from 2D micrographs, whereas chickpea mesophyll chloroplast volume was underestimated by 60% using simple geometrical models. Models of chickpea spongy and palisade cells both under- and overestimated surface area and volume by varying degrees. These models did not adequately capture irregular shapes such as flattening of chloroplasts or lobed spongy mesophyll cells.
• It is concluded that simple geometrical models to estimate chloroplast and cell 3D volume and surface area from 2D micrographs are inadequate, and that SBF-SEM has strong potential to contribute to improved understanding of leaf form and function.
Leaf day respiration Tcherkez, Guillaume; Gauthier, Paul; Buckley, Thomas N. ...
The New phytologist,
December 2017, Letnik:
216, Številka:
4
Journal Article
Recenzirano
Odprti dostop
It has been 75 yr since leaf respiratory metabolism in the light (day respiration) was identified as a low-flux metabolic pathway that accompanies photosynthesis. In principle, it provides carbon ...backbones for nitrogen assimilation and evolves CO2 and thus impacts on plant carbon and nitrogen balances. However, for a long time, uncertainties have remained as to whether techniques used to measure day respiratory efflux were valid and whether day respiration responded to environmental gaseous conditions. In the past few years, significant advances have beenmade using carbon isotopes, ‘omics’ analyses and surveys of respiration rates in mesocosms or ecosystems. There is substantial evidence that day respiration should be viewed as a highly dynamic metabolic pathway that interacts with photosynthesis and photorespiration and responds to atmospheric CO2 mole fraction. The view of leaf day respiration as a constant and/or negligible parameter of net carbon exchange isnow outdated and it should now be regarded as a central actor of plant carbon-use efficiency.
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