Effect of plant age and severity of defoliation on regrowth of sheep's burnet during substrate moisture depletion

Regrowth of 3- and 4-month old (‘young’ and ‘old’ respectively) sheep's burnet Sanguisorba minor ssp. muricata (Spach) Briq.] was studied under limiting and non-limiting moisture conditions in a glasshouse. Moisture deficits were imposed by using a single cycle of withholding moisture until first wilting. Plants of each age were defoliated severely at three levels which represented approximately 80–100% canopy removal. These levels were based on the proportion retained of the eight most mature leaves on each plant and were referred to as complete 0% residual leaf area (rLA)] and partial 50% rLA (four leaves) and 100% rLA (eight leaves)] defoliation. Vegetative growth and total non-structural carbohydrate (TNC) levels were studied. Leaf number (0-8 leaves), area (0-115 cm²) and dry weight (0-1·0 g) differed (P < 0·05) between defoliation intensities at the start of regrowth, while stubble (1·2 g) and root (12.·6 g) dry weights were similar. Soluble sugars < 6% dry matter (DM)] and starch (< 1% DM) occurred in leaf, stubble and root. Old plants were morphologically and physiologically more developed than young plants. For example, stubble (2·0 g) and root (21·5 g) dry weights of old plants were greater (P < 0·05) than those of young plants (04 and 3·7 g respectively). Defoliation intensity had a major effect on regrowth, with completely defoliated plants at the final harvest having leaf numbers (forty-nine leaves) and areas (235 cm²) almost twice those of partially defoliated plants. Stubble soluble sugar levels (38% DM) were lower than those of partially defoliated plants (5·5% DM), and it was suggested that these contributed to regrowth. Moisture regime had a negligible influence on plant growth. However, plants in the dry regime had soluble sugar levels 1·4 (stubble) -1·7 (roots) times higher than those watered adequately, which suggested that plants adjusted to the water depletion. The effects of plant age on regrowth were similar for most characters, but the larger and physiologically more mature old plants would probably be more tolerant of successive defoliations.