Cultivated sunflower (Helianthus annuus L.) has lower tolerance of moderate drought stress than its con‐specific wild relative, but the underlying traits remain elusive

Abstract

Cultivated crops are generally expected to have less abiotic stress tolerance than their wild relatives. However, this assumption is not well supported by empirical literature and may depend on the type of stress and how it is imposed, as well as the measure of tolerance being used. Here, we investigated whether wild and cultivated accessions of Helianthus annuus differed in stress tolerance assessed as proportional decline in biomass due to drought and whether wild and cultivated accessions differed in trait responses to drought and trait associations with tolerance. In a greenhouse study, H. annuus accessions in the two domestication classes (eight cultivated and eight wild accessions) received two treatments: a well‐watered control and a moderate drought implemented as a dry down followed by maintenance at a predetermined soil moisture level with automated irrigation. Treatments were imposed at the seedling stage, and plants were harvested after 2 weeks of treatment. The proportional biomass decline in response to drought was 24% for cultivated H. annuus accessions but was not significant for the wild accessions. Thus, using the metric of proportional biomass decline, the cultivated accessions had less drought tolerance. Among accessions, there was no tradeoff between drought tolerance and vigor assessed as biomass in the control treatment. In a multivariate analysis, wild and cultivated accessions did not differ from each other or in response to drought for a subset of morphological, physiological, and allocational traits. Analyzed individually, traits varied in response to drought in wild and/or cultivated accessions, including declines in specific leaf area, leaf theoretical maximum stomatal conductance (gsmax), and stomatal pore length, but there was no treatment response for stomatal density, succulence, or the ability to osmotically adjust. Focusing on traits associations with tolerance, plasticity in gsmax was the most interesting because its association with tolerance differed by domestication class (although the effects were relatively weak) and thus might contribute to lower tolerance of cultivated sunflower. Our H. annuus results support the expectation that stress tolerance is lower in crops than wild relatives under some conditions. However, determining the key traits that underpin differences in moderate drought tolerance between wild and cultivated H. annuus remains elusive.

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

“…It may be that the more important trait is the composite trait of theoretical g smax (calculated from stomatal density and pore length and depth; Bertolino et al 2019 ), but there were inconsistencies among studies in that response as well. While we found no consistent decline in g smax in response to drought, it has previously been shown to decline in response to drought in sunflower and other species ( Lei et al 2018 ; Janzen et al 2023 ; Tran et al 2024 ). The apparent discrepancy between sunflower studies may be due to differences in genotypes used or to differences in the logistics of drought implementation relative to leaf development (e.g.…”

“…For stomatal traits, average stomatal density increased and average stomatal pore length decreased consistent with other responses to drought ( Yu et al 2008 ; Sack and Scoffoni 2013 ; Sun et al 2014 ; Franks et al 2015 ; Carlson et al 2016 ; Ouyang et al 2017 ; Bresta et al 2018 ; Lei et al 2018 ; Mano et al 2023 ). However, other studies, including several for sunflower, documented no consistent effect on stomatal density ( Xu and Zhou 2008 ; Doheny-Adams et al 2012 ; Sack and Scoffoni 2013 ; Shahinnia et al 2016 ; Cardoso et al 2018 b; Tran et al 2024 ; Fig. S2 ) or a decline in both stomatal size and density ( Janzen et al 2023 ).…”

“…For intraspecific variation in crops and other herbaceous species, acclimation to drought generally results in development of new leaves with higher stomatal density and/or smaller stomatal size, as well as higher vein length density and hydraulic conductance (e.g. Yu et al 2008 ; Xu and Zhou 2008 ; Doheny-Adams et al 2012 ; Sack and Scoffoni 2013 ; Sun et al 2014 ; Franks et al 2015 ; Carlson et al 2016 ; Shahinnia et al 2016 ; Ouyang et al 2017 ; Wang et al 2018 ; Bresta et al 2018 ; Lei et al 2018 ; Dhakal et al 2021 ; Mano et al 2023 ; Figure S2 ), but not necessarily for sunflower ( Cardoso et al 2018 b; Janzen et al 2023 ; Tran et al , 2024 ). There is also some support for the expected negative correlation between stomatal density and stomatal size, as well as positive correlations between stomatal density and vein length density ( Doheny-Adams et al 2012 ; Carins Murphy et al 2014 ; Sun et al 2014 ; Shahinnia et al 2016 ; Xiong et al 2018 ; Bresta et al 2018 ; Lei et al 2018 ; but see Milla et al 2013 ).…”

“…Studies that compared the plasticity of traits as they respond (acclimate) to abiotic factors have found no generalities for which types of traits have the highest relative plasticity ( Stojnić et al 2015 ; Eziz et al 2017 ; Wang et al 2018 , 2020 ; Chen et al 2021 ; Zhao et al 2023 ), but greater plasticity in stomatal and vein traits may be associated with greater drought tolerance and/or adaptation to arid habitats (e.g. Adams et al 2016 ; Bresta et al 2018 ; Lei et al 2018 ; Tran et al 2024 ). Here, we investigate patterns of leaf stomatal and vein responses to varying levels of drought stress in cultivated sunflower ( Helianthus annuus L.).…”

“…In that experiment, genotypes differentiated in multivariate trait space along axes most strongly influenced by leaf traits related to gas exchange, hydraulics, and leaf construction, and stomatal density was negatively correlated with stomatal size and positively correlated with minor vein length density ( Earley et al 2022 ). Cultivated sunflower has been assessed as a moderate drought-tolerant crop, with documented responses including transcriptomic changes, ABA production, osmotic adjustment, high capacitance or buffering against water potential declines, stomatal closure, photosynthesis decline, low minimum transpiration when stomata are closed, xylem cavitation, adjustments of leaf morphology and anatomy, greater relative investment in roots, and reductions in biomass and yield ( Stiller and Sperry 2002 ; Cardoso et al 2018 b; Hussain et al 2018 ; Cardoso et al 2020 ; Barnhart et al 2022 ; Janzen et al 2023 ; Pereira et al 2024 ; Tran et al 2024 ). However, Cardoso et al (2018a) found no reduction in sunflower leaf vein length density in response to drought.…”