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.

Trait variation and performance across varying levels of drought stress in cultivated sunflower (Helianthus annuus L.).

Drought is a major agricultural challenge that is expected to worsen with climate change. A better understanding of drought responses has the potential to inform efforts to breed more tolerant plants. We assessed leaf trait variation and covariation in cultivated sunflower (Helianthus annuus L.) in response to water limitation. Plants were grown under four levels of water availability and assessed for environmentally induced plasticity in leaf stomatal and vein traits as well as biomass (performance indicator), mass fractions, leaf area, leaf mass per area, and chlorophyll content. Overall, biomass declined in response to stress; these changes were accompanied by responses in leaf-level traits including decreased leaf area and stomatal size, and increased stomatal and vein density. The magnitude of trait responses increased with stress severity and relative plasticity of smaller-scale leaf anatomical traits was less than that of larger-scale traits related to construction and growth. Across treatments, where phenotypic plasticity was observed, stomatal density was negatively correlated with stomatal size and positively correlated with minor vein density, but the correlations did not hold up within treatments. Four leaf traits previously shown to reflect major axes of variation in a large sunflower diversity panel under well-watered conditions (i.e. stomatal density, stomatal pore length, vein density, and leaf mass per area) predicted a surprisingly large amount of the variation in biomass across treatments, but trait associations with biomass differed within treatments. Additionally, the importance of these traits in predicting variation in biomass is mediated, at least in part, through leaf size. Our results demonstrate the importance of leaf anatomical traits in mediating drought responses in sunflower, and highlight the role that phenotypic plasticity and multi-trait phenotypes can play in predicting productivity under complex abiotic stresses like drought.