Abstract
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 Bromus valdivianus Phil. (B. valdivianus). In southern Chile, B. valdivianus and Lolium perenne L. (L. perenne) coexist; however, the pasture defoliation criterion is based on the physiological growth and development of L. perenne. It is hypothesised that B. valdivianus needs a lower defoliation frequency than L. perenne to enhance its regrowth and energy reserves. Defoliation frequencies tested were based on B. valdivianus 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 Lolium perenne was monitored and contrasted with that of B. valdivianus. The study was conducted in a glasshouse and used a randomised complete block design. For Bromus valdivianus, 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. Bromus valdivianus 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, B. valdivianus 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 B. valdivianus was equivalent to LS-3.5 for L. perenne. No differences related to tiller population in B. valdivianus were found in the different defoliation frequencies.
Highlights
In temperate climates, a decline in rainfall and a concomitant increase in temperature during summer cause an increase in the soil matric potential [1], which can be exacerbated by climate change [2,3]
The lower defoliation frequencies leaf stage 4 (LS-4) and leaf stage 5 (LS-5) encouraged water-soluble carbohydrates (WSCs) and starch accumulation, shoot regrowth and root growth, but no effect was found related to the tiller population under the different defoliation frequencies
As the tiller population was the same for all treatments, the greater plant regrowth shown by leaf stage (LS)-4 and LS-5 was due to their greater tiller size, tiller leaf area and leaf elongation rates
Summary
A decline in rainfall and a concomitant increase in temperature during summer cause an increase in the soil matric potential [1], which can be exacerbated by climate change [2,3]. During a summer drought period, the soil matric potential in the first 20 cm closest to the surface can reach values greater than 1.543 MPa, which corresponds to the permanent wilting point [7]. Under these conditions, the survival and growth of shallow-rooted grasses, for example, L. perenne, can be compromised [7,8]. Plants with longer roots can access water in deep soil layers, increasing their capacity to avoid drought [5] and their resilience after drought [9]; they have higher growth rates than shallowrooted species [10]. The different growth strategies of the two species [13,14] probably favour the persistence of B. valdivianus during soil water restriction, as it can obtain water from deep soil layers to continue growing [1,7,11,13]
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