| Περίληψη: | The increase in drought events due to climate change have enhanced the relevance of species with greater tolerance or avoidance traits to water restriction periods, such as <i>Bromus valdivianus</i> Phil. (<i>B. valdivianus</i>). In southern Chile, <i>B. valdivianus</i> and <i>Lolium perenne</i> L. (<i>L. perenne</i>) coexist; however, the pasture defoliation criterion is based on the physiological growth and development of <i>L. perenne</i>. It is hypothesised that <i>B. valdivianus</i> needs a lower defoliation frequency than <i>L. perenne</i> to enhance its regrowth and energy reserves. Defoliation frequencies tested were based on <i>B. valdivianus</i> leaf stage 2 (LS-2), leaf stage 3 (LS-3), leaf stage 4 (LS-4) and leaf stage 5 (LS-5). The leaf stage development of <i>Lolium perenne</i> was monitored and contrasted with that of <i>B. valdivianus</i>. The study was conducted in a glasshouse and used a randomised complete block design. For <i>Bromus valdivianus</i>, the lamina length, photosynthetic rate, stomatal conductance, tiller number per plant, leaf area, leaf weights, root growth rate, water-soluble carbohydrates (WSCs) and starch were evaluated. <i>Bromus valdivianus</i> maintained six live leaves with three leaves growing simultaneously. When an individual tiller started developing its seventh leaf, senescence began for the second leaf (the first relevant leaf for photosynthesis). Plant herbage mass, the root growth rate and tiller growth were maximised at LS-4 onwards. The highest leaf elongation rate, evaluated through the slope of the lamina elongation curve of a fully expanded leaf, was verified at LS-4. The water-soluble carbohydrates (WSCs) increased at LS-5; however, no statistical differences were found in LS-4. The LS-3 and LS-2 treatments showed a detrimental effect on WSCs and regrowth. The leaf photosynthetic rate and stomatal conductance diminished while the leaf age increased. In conclusion, <i>B. valdivianus</i> is a ‘six-leaf’ species with leaf senescence beginning at LS-4.25. Defoliation at LS-4 and LS-5 was optimum for plant regrowth, maximising the aboveground plant parameters and total WSC accumulation. The LS-4 for <i>B. valdivianus</i> was equivalent to LS-3.5 for <i>L. perenne</i>. No differences related to tiller population in <i>B. valdivianus</i> were found in the different defoliation frequencies.
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