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Sunflecks are transient patches of direct radiation that provide a substantial proportion of the daily irradiance to leaves in the lower canopy. In this position, faster photosynthetic induction would allow for higher sunfleck-use efficiency, as is commonly reported in the literature. Yet, when sunflecks are too few and far between, it may be more beneficial for shade leaves to prioritize efficient photosynthesis under shade. We investigated the temporal dynamics of photosynthetic induction, recovery under shade, and stomatal movement during a sunfleck, in sun and shade leaves of Fagus sylvatica from three provenances of contrasting origin. We found that shade leaves complete full induction in a shorter time than sun leaves, but that sun leaves respond faster than shade leaves due to their much larger amplitude of induction. The core-range provenance achieved faster stomatal opening in shade leaves, which may allow for better sunfleck-use efficiency in denser canopies and lower canopy positions. Our findings represent a paradigm shift for future research into light fluctuations in canopies, drawing attention to the ubiquitous importance of sunflecks for photosynthesis, not only in lower-canopy leaves where shade is prevalent, but particularly in the upper canopy where longer sunflecks are more common due to canopy openness.

Climacteric ripening of tomato fruit is initiated by a characteristic surge of the production rate of ethylene, accompanied by an increase in respiration rate. As both activities consume O2 and produce CO2 , gas concentration gradients develop in the fruit that cause diffusive transport. This may, in turn, affect respiration and ethylene biosynthesis. Gas diffusion in fruit depends on the amount and connectivity of cells and intercellular spaces in 3D. We investigated micromorphological changes in different tomato tissues during development and ripening by visualizing cells and pores based on high-resolution micro-computed tomography, and computed effective O2 diffusivity coefficients based on microstructural features of the tissues. We demonstrated that mesocarp and septa tissues have larger cells but small and more disconnected pores than the placenta and columella, resulting in relatively lower effective O2 diffusivity coefficients. Cell disintegration occurred in the mesocarp and septa during ripening, indicating lysigenous air pore formation and resulting in a gradual increase of the effective O2 diffusivity. The results suggest that hypoxic conditions caused by the increasing size and, hence, diffusion resistance of the growing fruit may induce an increase of tissue porosity that results in a greatly enhanced O2 diffusivity and, thus, helps to alleviate them. ispartof: NEW PHYTOLOGIST vol:232 issue:5 pages:2043-2056 ispartof: location:England status: published

© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust The ratio of leaf intercellular to ambient CO 2 (χ) is modulated by stomatal conductance (g s ). These quantities link carbon (C) assimilation with transpiration, and along with photosynthetic capacities (V cmax and J max ) are required to model terrestrial C uptake. We use optimization criteria based on the growth environment to generate predicted values of photosynthetic and water-use efficiency traits and test these against a unique dataset. Leaf gas-exchange parameters and carbon isotope discrimination were analysed in relation to local climate across a continental network of study sites. Sun-exposed leaves of 50 species at seven sites were measured in contrasting seasons. Values of χ predicted from growth temperature and vapour pressure deficit were closely correlated to ratios derived from C isotope (δ 13 C) measurements. Correlations were stronger in the growing season. Predicted values of photosynthetic traits, including carboxylation capacity (V cmax ), derived from δ 13 C, growth temperature and solar radiation, showed meaningful agreement with inferred values derived from gas-exchange measurements. Between-site differences in water-use efficiency were, however, only weakly linked to the plant's growth environment and did not show seasonal variation. These results support the general hypothesis that many key parameters required by Earth system models are adaptive and predictable from plants’ growth environments.

Whole-plant morphology, leaf structure and composition were studied together with the effects of light orientation on the dorso-ventral regulation of photosynthesis and stomatal conductance in Paspalum dilatatum cv. Raki plants grown for 6 wk at either 350 or 700 µl l¿1 CO2. Plant biomass was doubled as a result of growth at high CO2 and the shoot:root ratio was decreased. Stomatal density was increased in the leaves of the high CO2-grown plants, which had greater numbers of smaller stomata and more epidermal cells on the abaxial surface. An asymmetric surface-specific regulation of photosynthesis and stomatal conductance was observed with respect to light orientation. This was not caused by dorso-ventral variations in leaf structure, the distribution of phosphoenolpyruvate carboxylase (PEPC) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) proteins or light absorptance, transmittance or reflectance. Adaxial/abaxial specification in the regulation of photosynthesis results from differential sensitivity of stomatal opening to light orientation and fixed gradients of enzyme activation across the leaf.