Growth of the C4 dicot Flaveria bidentis: photosynthetic acclimation to low light through shifts in leaf anatomy and biochemistry

In C4 plants, acclimation to growth at low irradiance by means of anatomical and biochemical changes to leaf tissue is considered to be limited by the need for a close interaction and coordination between bundle sheath and mesophyll cells. Here differences in relative growth rate (RGR), gas exchange, carbon isotope discrimination, photosynthetic enzyme activity, and leaf anatomy in the C4 dicot Flaveria bidentis grown at a low (LI; 150 μmol quanta m2 s−1) and medium (MI; 500 μmol quanta m2 s−1) irradiance and with a 12 h photoperiod over 36 d were examined. RGRs measured using a 3D non-destructive imaging technique were consistently higher in MI plants. Rates of CO2 assimilation per leaf area measured at 1500 μmmol quanta m2 s−1 were higher for MI than LI plants but did not differ on a mass basis. LI plants had lower Rubisco and phosphoenolpyruvate carboxylase activities and chlorophyll content on a leaf area basis. Bundle sheath leakiness of CO2 (ϕ) calculated from real-time carbon isotope discrimination was similar for MI and LI plants at high irradiance. ϕ increased at lower irradiances, but more so in MI plants, reflecting acclimation to low growth irradiance. Leaf thickness and vein density were greater in MI plants, and mesophyll surface area exposed to intercellular airspace (Sm) and bundle sheath surface area per unit leaf area (Sb) measured from leaf cross-sections were also both significantly greater in MI compared with LI leaves. Both mesophyll and bundle sheath conductance to CO2 diffusion were greater in MI compared with LI plants. Despite being a C4 species, F. bidentis is very plastic with respect to growth irradiance

Light and CO2 do not affect the mesophyll conductance to CO2 diffusion in wheat leaves

In C3 plants, diffusion of CO2 into leaves is restricted by stomata and subsequently by the intercellular airspaces and liquid phase into chloroplasts. While considerable information exists on the effect of environmental conditions on stomatal conductance (gs), little is known on whether the mesophyll conductance to CO2 diffusion (gm) changes with respect to photon flux density (PFD) and CO2 partial pressure (pCO2). In this study, the effects of PFD and/or pCO2 on gm were examined in wheat leaves by combining gas exchange with carbon isotope discrimination measurements using a membrane inlet mass spectrometer. Measurements were made in 2% O2 to reduce the fractionation associated with photorespiration. The magnitude of gm was estimated using the observed carbon isotope discrimination (Δ), ambient and intercellular pCO2, CO2 assimilation and respiration rates, either from an individual measurement made under one environmental condition or from a global fit to multiple measurements where PFD was varied. It was found that respiration made a significant and variable contribution to the observed discrimination, which associated with the difference in isotopic composition between CO2 in the greenhouse and that used for gas exchange measurements. In wheat, gm was independent of PFD between 200 and 1500 μmol m-2 s-1 and was independent of pi between 80 and 500 μbar

Exploiting transplastomically modified Rubisco to rapidly measure natural diversity in its carbon isotope discrimination using tuneable diode laser spectroscopy

Carbon isotope discrimination (Δ) during C3 photosynthesis is dominated by the fractionation occurring during CO2-fixation by the enzyme Rubisco. While knowing the fractionation by enzymes is pivotal to fully understanding plant carbon metabolism, littl

Using tunable diode laser spectroscopy to measure carbon isotope discrimination and mesophyll conductance to CO 2 diffusion dynamically at different CO 2 concentrations

In C3 leaves, the mesophyll conductance to CO2 diffusion, gm, determines the drawdown in CO2 concentration from intercellular airspace to the chloroplast stroma. Both gm and stomatal conductance limit photosynthetic rate and vary in response to the environment. We investigated the response of gm to changes in CO2 in two Arabidopsis genotypes (including a mutant with open stomata, ost1), tobacco and wheat. We combined measurements of gas exchange with carbon isotope discrimination using tunable diode laser absorption spectroscopy with a CO2 calibration system specially designed for a range of CO2 and O2 concentrations. CO2 was initially increased from 200 to 1000ppm and then decreased stepwise to 200ppm and increased stepwise back to 1000ppm, or the sequence was reversed. In 2% O2 a step increase from 200 to 1000ppm significantly decreased gm by 26-40% in all three species, whereas following a step decrease from 1000 to 200ppm, the 26-38% increase in gm was not statistically significant. The response of gm to CO2 was less in 21% O2. Comparing wild type against the ost1 revealed that mesophyll and stomatal conductance varied independently in response to CO2. We discuss the effects of isotope fractionation factors on estimating gm

Leaf Photosynthesis and Its Genetic Improvement from the Perspective of Energy Flow and CO2 Diffusion

Single-leaf photosynthesis is a fundamental process in plant biomass production, and is a major research topic in crop physiology. This paper reviews the recent achievements of research on the physiological determinants of the photosynthetic capacity from the perspective of energy flow and CO2 diffusion. Measurement of chlorophyll fluorescence is a popular method to diagnose the function of photosystem II, and is useful to assess the susceptibility to photoinhibition and allocation of energy, which are keys to improving both stress resistance and photosynthetic productivity. Mesophyll conductance (gm) is the conductance to CO2 diffusion from intercellular airspaces to the chloroplast, and was long thought to be determined by leaf anatomical properties. However, recent studies showed that environmental conditions affect g m. It is possible that g m is affected by the gating of the CO2-permeable aquaporins (cooporins). Stomatal morphology is revealed to be an important factor affecting gas exchange both in crop plants and inArabidopsis thaliana.The knowledge of the stomatal differentiation in Arabidopsis will be applicable to various crops. gm, stomatal conductance (gs) and leaf nitrogen content are the main factors to cause difference in leaf photosynthesis among rice lines, and recent activities are conducted to find genes to manipulate these factors. Although the association of leaf photosynthesis with crop productivity still has a large ‘missing link’, these achievements strongly suggest that the leaf photosynthetic capacity can be genetically improved in crop species