Abstract
Under benign conditions, as plants grow, size dependent (allometric) scaling changes mass allocation between organs. In the face of resource stress plants grow less but also show plasticity in multiple trait categories, including biomass allocation, morphology and anatomy. The extent to which size dependent (allometric) vs active (beyond allometric) responses to resource limitations are consistent with expectations for increasing resource acquisition potential is poorly understood. Here we assess the impact of allometric scaling on the direction, magnitude and coordination of trait plasticity in response to light and/or nutrient limitations in cultivated sunflower (Helianthus annuus).
We grew seedings 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.
Under resource limitation, more biomass was allocated to the organ capturing the most limiting resource, as expected. Allometric scaling accounted for a substantial portion of many trait responses, especially for anatomical traits. Allometric and active responses were generally aligned in the same direction and for specific leaf area and specific root length under light and nutrient limitation, respectively, this alignment contributed to more acquisitive trait values. However, traits not generally associated with resource limitation showed the greatest active adjustment, e.g. tap root and stem theoretical hydraulic conductance. Although light limitation triggered a more pronounced coordinated trait adjustment than nutrient limitation, factoring out allometric scaling clarified that active coordinated trait responses to both light and nutrient limitation were similar in scale but resource specific.
The substantial contribution of allometric scaling to trait responses that are consistent with a functional increase in the uptake capacity of the most limiting resource suggests that both allometric and active trait adjustments need be considered as potentially adaptive.
Footnotes
Thanks to reviewer feedback updated the direction and focus of the paper towards allometric scaling and plasticity due to resource limitation.