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The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO2 diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18–70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO2 diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.

Abstract The leaf economics spectrum (LES) is an ecophysiological concept describing the trade-offs of leaf structural and physiological traits, and has been widely investigated on multiple scales. However, the effects of the breeding process on the LES in crops, as well as the mechanisms of the trait trade-offs underlying the LES, have not been thoroughly elucidated to date. In this study, a dataset that included leaf anatomical, biochemical, and functional traits was constructed to evaluate the trait covariations and trade-offs in domesticated species, namely rice (Oryza species). The slopes and intercepts of the major bivariate correlations of the leaf traits in rice were significantly different from the global LES dataset (Glopnet), which is based on multiple non-crop species in natural ecosystems, although the general patterns were similar. The photosynthetic traits responded differently to leaf structural and biochemical changes, and mesophyll conductance was the most sensitive to leaf nitrogen (N) status. A further analysis revealed that the relative limitation of mesophyll conductance declined with leaf N content; however, the limitation of the biochemistry increased relative to leaf N content. These findings indicate that breeding selection and high-resource agricultural environments lead crops to deviate from the leaf trait covariation in wild species, and future breeding to increase the photosynthesis of rice should primarily focus on improvement of the efficiency of photosynthetic enzymes.

Ferns and fern allies have low photosynthetic rates compared with seed plants. Their photosynthesis is thought to be limited principally by physical CO2 diffusion from the atmosphere to chloroplasts. The aim of this study was to understand the reasons for low photosynthesis in species of ferns and fern allies (Lycopodiopsida and Polypodiopsida). We performed a comprehensive assessment of the foliar gas-exchange and mesophyll structural traits involved in photosynthetic function for 35 species of ferns and fern allies. Additionally, the leaf economics spectrum (the interrelationships between photosynthetic capacity and leaf/frond traits such as leaf dry mass per unit area or nitrogen content) was tested. Low mesophyll conductance to CO2 was the main cause for low photosynthesis in ferns and fern allies, which, in turn, was associated with thick cell walls and reduced chloroplast distribution towards intercellular mesophyll air spaces. Generally, the leaf economics spectrum in ferns follows a trend similar to that in seed plants. Nevertheless, ferns and allies had less nitrogen per unit DW than seed plants (i.e. the same slope but a different intercept) and lower photosynthesis rates per leaf mass area and per unit of nitrogen.

Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (g m), which is the most limiting factor for area-based photosynthesis (A N) in many Mediterranean sclerophyll species. However, despite their high LMA, these species have similar A N to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space (S m/S and S c/S), may offset the restrictions imposed by high LMA in g m and A N in these species. Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits. Mediterranean oaks showed specific differences in A N that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in g m, the most limiting factor for carbon assimilation in these species. Species with higher A N showed increased S c/S, which implies increased g m without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach A N values comparable to congeneric deciduous species despite their higher LMA.

Leaves are beautifully specialized organs designed to maximize the use of light and CO₂ for photosynthesis. Engineering leaf anatomy therefore holds great potential to enhance photosynthetic capacity. Here we review the effect of the dominant leaf anatomical traits on leaf photosynthesis and confirm that a high chloroplast surface area exposed to intercellular airspace per unit leaf area (S c) is critical for efficient photosynthesis. The possibility of improving S c through appropriately increasing mesophyll cell density is further analyzed. The potential influences of modifying mesophyll cell morphology on CO₂ diffusion, light distribution within the leaf, and other physiological processes are also discussed. Some potential target genes regulating leaf mesophyll cell proliferation and expansion are explored. Indeed, more comprehensive research is needed to understand how manipulating mesophyll cell morphology through editing the potential target genes impacts leaf photosynthetic capacity and related physiological processes. This will pinpoint the targets for engineering leaf anatomy to maximize photosynthetic capacity.

Leaf hydraulic conductance (K leaf) and mesophyll conductance (g m) both represent major constraints to photosynthetic rate (A), and previous studies have suggested that K leaf and g m is correlated in leaves. However, there is scarce empirical information about their correlation. In this study, K leaf, leaf hydraulic conductance inside xylem (K x), leaf hydraulic conductance outside xylem (K ox), A, stomatal conductance (g s), g m, and anatomical and structural leaf traits in 11 Oryza genotypes were investigated to elucidate the correlation of H2O and CO2 diffusion inside leaves. All of the leaf functional and anatomical traits varied significantly among genotypes. K leaf was not correlated with the maximum theoretical stomatal conductance calculated from stomatal dimensions (g smax), and neither g s nor g smax were correlated with K x. Moreover, K ox was linearly correlated with g m and both were closely related to mesophyll structural traits. These results suggest that K leaf and g m are related to leaf anatomical and structural features, which may explain the mechanism for correlation between g m and K leaf.

Foliage photosynthetic and structural traits were studied in 15 species with a wide range of foliage anatomies to gain insight into the importance of key anatomical traits in the limitation of diffusion of CO2 from substomatal cavities to chloroplasts. The relative importance of different anatomical traits in constraining CO2 diffusion was evaluated using a quantitative model. Mesophyll conductance (g m) was most strongly correlated with chloroplast exposed surface to leaf area ratio (S c/S) and cell wall thickness (T cw), but, depending on foliage structure, the overall importance of g m in constraining photosynthesis and the importance of different anatomical traits in the restriction of CO2 diffusion varied. In species with mesophytic leaves, membrane permeabilities and cytosol and stromal conductance dominated the variation in g m. However, in species with sclerophytic leaves, g m was mostly limited by T cw. These results demonstrate the major role of anatomy in constraining mesophyll diffusion conductance and, consequently, in determining the variability in photosynthetic capacity among species.

• Mesophyll conductance (g m) is the diffusion of CO₂ from intercellular air spaces (IAS) to the first site of carboxylation in the mesophyll cells. In C₃ species, g m is influenced by diverse leaf structural and anatomical traits; however, little is known about traits affecting g m in C₄ species. • To address this knowledge gap, we used online oxygen isotope discrimination measurements to estimate g m and microscopy techniques to measure leaf structural and anatomical traits potentially related to g m in 18 C₄ grasses. • In this study, g m scaled positively with photosynthesis and intrinsic water-use efficiency (TEi), but not with stomatal conductance. Also, g m was not determined by a single trait but was positively correlated with adaxial stomatal densities (SDada), stomatal ratio (SR), mesophyll surface area exposed to IAS (S mes) and leaf thickness. However, g m was not related to abaxial stomatal densities (SDaba) and mesophyll cell wall thickness (T CW). • Our study suggests that greater SDada and SR increased g m by increasing S mes and creating additional parallel pathways for CO₂ diffusion inside mesophyll cells. Thus, SDada, SR and S mes are important determinants of C₄-g m and could be the target traits selected or modified for achieving greater g m and TEi in C₄ species.

CONTENTS: 1064 I. 1064 II. 1066 III. 1066 IV. 1068 V. 1069 VI. 1070 VII. 1070 VIII. 1070 IX. 1071 X. 1072 XI. 1074 XII. 1074 1076 References 1076 SUMMARY: Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere. Guard cells determine stomatal aperture and must operate to ensure an appropriate balance between CO₂ uptake for photosynthesis (A) and water loss, and ultimately plant water use efficiency (WUE). A strong correlation between A and stomatal conductance (gₛ) is well documented and often observed, but the underlying mechanisms, possible signals and metabolites that promote this relationship are currently unknown. In this review we evaluate the current literature on mesophyll‐driven signals that may coordinate stomatal behaviour with mesophyll carbon assimilation. We explore a possible role of various metabolites including sucrose and malate (from several potential sources; including guard cell photosynthesis) and new evidence that improvements in WUE have been made by manipulating sucrose metabolism within the guard cells. Finally we discuss the new tools and techniques available for potentially manipulating cell‐specific metabolism, including guard and mesophyll cells, in order to elucidate mesophyll‐derived signals that coordinate mesophyll CO₂ demands with stomatal behaviour, in order to provide a mechanistic understanding of these processes as this may identify potential targets for manipulations in order to improve plant WUE and crop yield.

Increases in rates of individual leaf photosynthesis (Pn) are critical for future increases in yields of rice plants. Although many efforts have been made to improve rice Pn with transgenic technology, the desired increases in Pn have not yet been achieved. Two rice lines with extremely high values of Pn were identified among the backcrossed inbred lines derived from the indica variety Takanari, one of the most productive varieties in Japan, and the elite japonica variety Koshihikari (Koshihikari/Takanari//Takanari). The Pn values of the two lines at an ambient CO2 concentration of 370μmol mol–1 as well as at a saturating concentration of CO2 were 20–50% higher than those of the parental varieties. Compared with Takanari, these lines had neither a higher content nor a higher activity of ribulose 1,5-bisphosphate carboxylase/oxygenase when the leaf nitrogen contents were similar, but they did have high mesophyll conductance with respect to CO2 flux due to their higher density and more highly developed lobes of mesophyll cells. These lines also had higher electron transport rates. The plant growth rates of these lines were higher than that of Takanari. The findings show that it is possible to increase Pn significantly, both at the current atmospheric concentration of CO2 and at the increased concentration of CO2 expected in the future, using appropriate combinations of genetic resources that are available at present.