Priority for allocation of water‐soluble carbohydrate reserves during regrowth of Lolium perenne

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Abstract
A glasshouse study was undertaken to determine the priority within the perennial ryegrass (Lolium perenne L.) plant for leaf and root growth and daughter tiller initiation after defoliation, in relation to various levels of water‐soluble carbohydrate (WSC) reserves at defoliation. Individual plants were arranged in mini‐swards, and underwent varying defoliation frequencies and ambient temperatures before defoliation, and harvest heights at defoliation, to obtain a gradient of WSC content at H1, the date when all plants were defoliated. Defoliation interval consisted of defoliating either three times at the one new leaf tiller–1 stage (1‐leaf stage) of regrowth, or once only at the 3‐leaf stage, up to H1, while night temperature in the week prior to H1 was altered from 15°C to either 8 or 20°C. At H1, plants were defoliated to a stubble height of either 20 or 50 mm. Plants were subsequently destructively harvested at days 4, 6, 8, 12, 18 and 27. Leaf and root extension and tiller dynamics were also measured.On a regrowth timescale, tiller initiation was most sensitive, root regrowth moderately sensitive, and leaf regrowth relatively insensitive to a decrease in WSC. The time of daughter tiller initiation also coincided with replenishment of stubble WSC levels.In contrast to this sequence of regrowth events following defoliation, the quantitative effects on growth were different, with elongation and survival of roots most affected by reduced WSC levels. A 30‐fold difference in stubble WSC at H1 between high and low WSC plants (1·52 vs. 0·05 mg tiller–1) produced only a 4‐fold increase in leaf dry matter (DM) after 27 d (2·2 vs. 0·6 g plant−1), while tiller number plant−1 increased 6‐fold (138 vs. 23% increase in tiller number from H1). Root elongation rate was 59 times higher in the high than in the low WSC plants (1·18 vs. 0·02 mm d−1).From a pasture management perspective, the study confirms that defoliation, coinciding with the 3‐leaf stage of regrowth and around a stubble height of 50 mm, optimizes persistence and productivity of perennial ryegrass. By allowing more rapid replenishment of WSC reserves, this optimal defoliation strategy enables a greater proportion of WSC to be allocated to maintain a more active root system, and promotes tillering, compared with more frequent and close defoliation.