Coordination of trait responses to above and belowground resource limitation in cultivated sunflower (Helianthus annuus L.)

Abstract

O_LIIn the face of resource limitations, plants show plasticity in multiple trait categories, including biomass allocation, morphology and anatomy, yet inevitably also grow less. The extent to which passive mass-scaling plays a role in trait responses that contribute to increased potential for resource acquisition are poorly understood. Here we assessed the role of mass-scaling on the direction, magnitude and coordination of trait plasticity to light and/or nutrient limitation in cultivated sunflower (Helianthus annuus). C_LIO_LIWe grew seedlings of ten sunflower genotypes for three weeks in a factorial of light (50% shade) and nutrient (10% supply) limitation in the greenhouse and measured a suite of allocational, morphological and anatomical traits for leaves, stems, fine roots, and tap roots. C_LIO_LIUnder resource limitation, plants were smaller and more biomass was allocated to the organ capturing the most limiting resource, as expected. Traits varied in the magnitude of plasticity and the extent to which the observed response was passive (scaled with plant mass) and/or had an additional active component. None of the allocational responses were primarily passive. Plastic changes to specific leaf area and specific root length were primarily active, and adjusted towards more acquisitive trait values under light and nutrient limitation, respectively. For many traits, the observed response was a mixture of active and passive components, and for some traits the active adjustment was antagonistic to the direction of passive adjustment, e.g. stem height, and tap root and stem theoretical hydraulic conductance. Passive scaling with size played a major role in the coordinated response to light, but correcting for mass clarified that the active responses to both limitations were more similar in magnitude, although still resource and organ specific. C_LIO_LIOur results demonstrate that both passive plasticity and active plasticity can contribute to increased uptake capacity for limiting resources in a manner that is resource, organ and trait specific. Indeed, passive adjustments (scaling with mass) of traits due to resource stress extends well beyond just mass allocation traits. For a full understanding of plants response to environmental stress both passive and active plasticity needs to be taken into account. C_LI