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| Growth - respiration relationships for individuals in a hierarchically structured plant stand |
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M. Lötscher and H. Schnyder
Chair of Grassland Science, Technische Universität München, D-85350 Freising
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The aim of the study was to investigate the relationship between C gain and C loss (respiration) of individuals that grew in different hierarchical positions in a plant stand. Alfalfa (Medicago sativa) stands were established by arranging singly potted plants at a density of 400 plants m-2. Root competition was excluded and all plants were well fertilized. Stands were grown in growth cabinets for four weeks and then cut. Investigations took place during regrowth. 13C steady-state labeling was used to assess C fixation (Cnew) by individuals in the stand during a photoperiod of 16 h. Respiration of shoots was measured during the following 8 h dark period. The amount of Cnew used in shoot respiration (Cnew, R) was calculated from the rate of respiration and the isotopic composition of the respiratory CO2. Fixation of C and respiration were compared among dominant individuals during the regrowth period (effect of time) and among individuals in different hierarchical positions in the fourth week of the regrowth period (effect of size hierarchy).
Effect of time: During regrowth daily C fixation per plant increased less than plant mass, reflecting decreasing C fixation rates per unit shoot mass (a measure of the relative growth rate, RGR). The rate of nocturnal respiration of C originating from recently fixed photosynthates (Cnew, R) was proportional to the C fixation during the preceding light period (Cnew): Cnew, R = 0.084 * Cnew, r = 0.94. Conversely, the rate of nocturnal respiration of non-labeled C (Cold, R, i.e. C respired using substrate fixed prior to labeling including reserve C) was proportional to shoot C mass. Cold, R per unit shoot mass decreased over the weeks, possibly due to an increasingly larger fraction of structural and metabolically less active tissues.
Effect of size hierarchy: Subordinate plants having a high leaf area density showed low RGR. In contrast, subordinate plants with low leaf area densities had a higher RGR which was similar to the RGR of dominant plants. Respiration of new C (Cnew, R) was proportional to the product of C fixation (Cnew) and shoot height (hi): Cnew, R = 0.165 * Cnew * hi, r = 0.98. In consequence, respiration of new C (Cnew, R) per unit C fixed (Cnew) and total respiration per unit shoot mass were higher in dominant than in subordinate plants. Again respiration of Cold, R was proportional to shoot C mass:
Cold, R = 0.0049 * C, r = 0.94.
In conclusion, respiration depended on biomass (Cold, R) and on plant growth (Cnew, R). For dominant plants, the percentage of Cnew, R in total respiration was constant with about 62 %. This percentage decreased with decreasing hierarchical position. The high correlation between the respiration of new C (Cnew, R) and RGR indicates a significant use of new C for growth respiration. In contrast, respiration of old C may have been preferentially used for maintenance.
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