Perennial ryegrass regrowth after defoliation - physiological and molecular changes

Many aspects of plant growth and physiology following defoliation have been investigated in perennial ryegrass (Lolium perenne). The changes, however, have rarely been characterised at a molecular (gene) level. Perennial ryegrass plots were defoliated at the 3-leaf stage to 1 600 kg DM/ha (recommended grazing practice) and throughout the subsequent regrowth cycle, pasture mass, water-soluble carbohydrate (WSC) content and expression of carbon metabolism genes were determined. Plots accumulated 1 900 kg DM/ ha, 45% of this between the 2- and 3-leaf stages of regrowth. Following defoliation, stubble WSC content declined (P<0.001) until the first new leaf had emerged, with replenishment occurring during emergence of the second new leaf. In stubble tissue, expression of the fructan degradation gene was greatest immediately after defoliation (P<0.001), while expression of a fructan synthesis gene was greatest following emergence of the first leaf (P<0.05). Throughout regrowth, expression of photosynthesis genes increased (P<0.05) in leaf tissue, peaking at the 2-leaf stage of regrowth. These data indicate how perennial ryegrass plants alter their physiological processes following defoliation to survive and grow, and support recommendations for rotation lengths that are longer than the time taken to reach the 2-leaf stage of regrowth. Understanding how defoliation affects gene expression in perennial ryegrass could influence future plant breeding outcomes. Keywords: rotation length, leaf stage, water-soluble carbohydrate, fructan, photosynthesis, gene expression