EFFECT OF PLANT-AGE AND SEVERITY OF DEFOLIATION ON REGROWTH OF SHEEPSBURNET 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 u nder limiting and non-limiting moisture conditions in a glasshouse. Mo isture deficits were imposed by using a single cycle of withholding mo isture until first wilting. Plants of each age were defoliated severel y at three levels which represented approximately 80-100% canopy remov al. These levels were based on the proportion retained of the eight mo st mature leaves on each plant and were referred to as complete [10% r esidual leaf area (rLA)] and partial [50% rLA (four leaves) and 100% r LA (eight leaves)] defoliation. Vegetative growth and total non-struct ural 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% dr y matter (DM)] and starch (< 1% DM) occurred in leaf, stubble and root . Old plants were morphologically and physiologically more developed t han young plants. For example, stubble (2.0 g) and root (21.5 g) dry w eights of old plants were greater (P < 0.05) than those of young plant s (0.4 and 3.7 g respectively). Defoliation intensity had a major effe ct on regrowth, with completely defoliated plants at the final harvest having leaf numbers (forty-nine leaves) and areas (235 cm2) almost tw ice those of partially defoliated plants. Stubble soluble sugar levels (3.8% 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) ti mes 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 m ore mature old plants would probably be more tolerant of successive de foliations.