Sink prioritization after carbon stress in Quillaja saponaria, a tree species endemic to Mediterranean Chile

Abstract

For plant species adapted to drought, such as those inhabiting Mediterranean regions, carbon (C) allocation to traits related to drought resistance could occur at the expense of growth. We hypothesized that in conditions of C limitation, drought-adapted species prioritize C allocation to osmoregulation and storage rather than growth. Juveniles of Quillaja saponaria were randomly assigned to full light (control) and 97.4% light reduction (shade) for 37 days; shaded plants were subsequently transferred to full light (i.e. reilluminated). Right before reillumination and for 21 days afterwards, structural biomass, relative growth rate in diameter (RGRd) and height (RGRh), concentrations of starch and soluble sugars across organs and osmotic potential at full turgor (πo), were measured in both control and reilluminated plants. Before reillumination, shaded plants had significantly lower RGRd, biomass, starch concentrations across organs, sugar concentrations in stems, and higher πo. During reillumination, biomass and RGRd remained lower in shaded plants, with no evidence of recovery over time. Concentrations of starch in leaves and of soluble sugars in stem, and leaf πo converged towards control values; stem and root starch concentrations in shaded plants did not recover. We conclude that under C limitation, juveniles of Quillaja saponaria prioritize the replenishment of storage in aboveground tissues and osmoregulation, over the allocation to growth. This prioritization is likely to be critical to sustain plant metabolic and osmotic requirements under the prolonged summer droughts characterizing the species’ native habitat.