DEFOLIATION OF THE ANNUAL HERB ABUTILON-THEOPHRASTI - MECHANISMS UNDERLYING REPRODUCTIVE COMPENSATION

A number of studies have shown that under some conditions plants may f ully or partially compensate for leaf tissue loss; however, the mechan isms underlying compensatory responses are not well understood. Previo us work demonstrated that the annual herb Abutilon theophrasti fully c ompensated for 75% defoliation, but only when grown in the absence of stem competition. We examined potential mechanisms of compensatory res ponse and how they are influenced by resource limitation (i.e., compet ition for light). Full compensation for these annual plants was define d as equal final reproductive output in defoliated and control plants. In the current study we observed substantial compensation in defoliat ed plants growing at low density, despite losing 75% of leaf area prio r to the onset of flowering. Plant responses associated with compensat ion included (1) increased reproductive efficiency, which may in turn may have resulted from increased canopy light penetration and transien t 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 pl ants grown at high density, the inability of high-density plants to co mpensate appeared to result from competition for light; these plants d elayed reproduction and continued to produce new leaves late in the gr owing season after low-density, defoliated plants had shifted allocati on of resources to reproduction.