Vein density is independent of epidermal cell size in Arabidopsis mutants

Genomic regions associate with major axes of variation driven by gas exchange and leaf construction traits in cultivated sunflower (Helianthus annuus L.)

Stomata and leaf veins play an essential role in transpiration and the movement of water throughout leaves. These traits are thus thought to play a key role in the adaptation of plants to drought and a better understanding of the genetic basis of their variation and coordination could inform efforts to improve drought tolerance. Here, we explore patterns of variation and covariation in leaf anatomical traits and analyze their genetic architecture via genome-wide association (GWA) analyses in cultivated sunflower (Helianthus annuus L.). Traits related to stomatal density and morphology as well as lower-order veins were manually measured from digital images while the density of minor veins was estimated using a novel deep learning approach. Leaf, stomatal, and vein traits exhibited numerous significant correlations that generally followed expectations based on functional relationships. Correlated suites of traits could further be separated along three major principal component (PC) axes that were heavily influenced by variation in traits related to gas exchange, leaf hydraulics, and leaf construction. While there was limited evidence of colocalization when individual traits were subjected to GWA analyses, major multivariate PC axes that were most strongly influenced by several traits related to gas exchange or leaf construction did exhibit significant genomic associations. These results provide insight into the genetic basis of leaf trait covariation and showcase potential targets for future efforts aimed at modifying leaf anatomical traits in sunflower.

Impaired auxin signaling increases vein and stomatal density but reduces hydraulic efficiency and ultimately net photosynthesis

Auxins are known to regulate xylem development in plants, but their effects on water transport efficiency are poorly known. Here we used tomato plants with the diageotropica mutation (dgt), which has impaired function of a cyclophilin 1 cis-trans isomerase involved in auxin signaling, and the corresponding wild type (WT) to explore the mutation's effects on plant hydraulics and leaf gas exchange. The xylem of the dgt mutant showed a reduced hydraulically weighted vessel diameter (Dh) (24-43%) and conduit number (25-58%) in petioles and stems, resulting in lower theoretical hydraulic conductivities (Kt); on the other hand, no changes in root Dh and Kt were observed. The measured stem and leaf hydraulic conductances of the dgt mutant were lower (up to 81%), in agreement with the Kt values; however, despite dgt and WT plants showing similar root Dh and Kt, the measured root hydraulic conductance of the dgt mutant was 75% lower. The dgt mutation increased the vein and stomatal density, which could potentially increase photosynthesis. Nevertheless, even though it had the same photosynthetic capacity as WT plants, the dgt mutant showed a photosynthetic rate c. 25% lower, coupled with a stomatal conductance reduction of 52%. These results clearly demonstrate that increases in minor vein and stomatal density only result in higher leaf gas exchange when accompanied by higher hydraulic efficiency.

Limited plasticity in embolism resistance in response to light in leaves and stems in species with considerable vulnerability segmentation

Xylem resistance to embolism is a key metric determining plant survival during drought. Yet, we have a limited understanding of the degree of plasticity in vulnerability to embolism. Here, we tested whether light availability influences embolism resistance in leaves and stems. The optical vulnerability method was used to assess stem and leaf resistance to embolism in Phellodendron amurense and Ilex verticillata acclimated to sun and shade microenvironments within the same canopy. In both species, we found considerable segmentation in xylem resistance to embolism between leaves and stems, but only minor acclimation in response to light availability. With the addition of a third species, Betula pubescens, which shows no vulnerability segmentation, we sought to investigate xylem anatomical traits that might correlate with strong vulnerability segmentation. We found a correlation between the area fraction of vessels in the xylem and embolism resistance across species and tissue types. Our results suggest that minimal acclimation of embolism resistance occurs in response to light environment in the same individual and that the degree of vulnerability segmentation between leaves and stems might be determined by the vessel lumen fraction of the xylem.

Leaf anatomical adaptations have central roles in photosynthetic acclimation to humidity

Rates of photosynthesis can be lower in plants grown under conditions of high leaf-to-air vapour pressure difference (VPD) than under low VPD. Leaf phenotype plasticity is a primary factor determining photosynthetic responses to environmental stimuli. However, it remains unclear how changes in leaf anatomical traits drive photosynthetic acclimation to high VPD. Here, we examined the role of leaf anatomy in the differing photosynthetic responses of two tomato cultivars (Jinpeng and Zhongza) to long-term growth under high and low VPD. Photosynthesis was not affected by VPD in Jinpeng. This was attributed to homeostasis in stomatal conductance (gs) and, to a lesser extent, mesophyll conductance (gm). Disruption of synchronized changes to cell size in the epidermis and mesophyll meant that growth under high VPD reduced stomatal density in Jinpeng, but minor vein density remained unchanged. Thus, water supplied by the veins could support the increased transpirational demand, preventing stomatal closure. Variation in VPD did not affect mesophyll cell structures, and therefore gm, in Jinpeng. By contrast, photosynthesis in Zhongza was reduced under high VPD, which was primarily attributed to decreased gs and gm. The former was mainly induced by decreased stomatal aperture. Thus, transpirational demand exceeded water supply in Zhongza. This was likely due to coordinated decreases in stomatal and minor vein density driven by synchronized increases in epidermal and mesophyll cell size under high VPD. Liquid-phase limitation was primarily responsible for the reduced gm in Zhongza under high VPD. High VPD induced an increase in liquid-phase resistance by reducing the mesophyll surface area exposed to intercellular air spaces and increasing cytosolic resistance. These results suggest that plasticity in epidermal and mesophyll cell size provides an efficient means of regulating photosynthesis during acclimation to long-term high VPD.

Bundle sheath extensions affect leaf structural and physiological plasticity in response to irradiance

Coordination between structural and physiological traits is key to plants' responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light-saturated rates of photosynthesis, A_max ) and water transport capacity (leaf hydraulic conductance, K_leaf ). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near-isogenic lines grown at two different irradiance levels. K_leaf , minor vein density, and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased A_max , leaf intercellular air spaces, and mesophyll surface area exposed to intercellular airspace (S_mes ) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long-term water-use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function, and water supply and demand.