Defoliation of the annual herb Abutilon theophrasti: mechanisms underlying reproductive compensation.

A number of studies have shown that under some conditions plants may fully or partially compensate for leaf tissue loss; however, the mechanisms underlying compensatory responses are not well understood. Previous work demonstrated that the annual herb Abutilon theophrasti fully compensated for 75% defoliation, but only when grown in the absence of stem competition. We examined potential mechanisms of compensatory response and how they are influenced by resource limitation (i.e., competition for light). Full compensation for these annual plants was defined as equal final reproductive output in defoliated and control plants. In the current study we observed substantial compensation in defoliated plants growing at low density, despite losing 75% of leaf area prior to the onset of flowering. Plant responses associated with compensation included (1) increased reproductive efficiency, which may in turn may have resulted from increased canopy light penetration and transient increases in leaf-level photosynthetic rates; (2) greater allocation to reproduction (RA); (3) changes in biomass allocation from roots to shoots; (4) lower leaf longevity, and (5) increased percent fruit set. Although some of these responses were also observed in defoliated plants grown at high density, the inability of high-density plants to compensate appeared to result from competition for light; these plants delayed reproduction and continued to produce new leaves late in the growing season after low-density, defoliated plants had shifted allocation of resources to reproduction.