Modeling Shoot and Root Biomass of Lucerne Crops—New Insights on the Seasonality of Dry Matter Partitioning and Root Maintenance Respiration

Abstract

Lucerne crops (Medicago sativa L.) are an important source of energy and protein for grazing and confined livestock in temperate and tropical countries. In contrast with annual crops, the modeling of above ground biomass of lucerne crops is often inaccurate when light interception (Ri) and radiation use efficiency (RUE) are used to estimate assimilation. The reason is that a large fraction of total assimilates are allocated to the root system and the pattern of partitioning differs in response to seasonal environmental stimuli. This dynamic has not been accurately quantified in lucerne models. In this study, a simple framework for a lucerne model to address the dynamics of shoots and roots is tested. Simulation results are compared with 2 years of field data from Lincoln University, Canterbury, New Zealand. The model incorporates recent findings on RUE, Ri and dry matter partitioning to roots in response to environmental factors. Shoot biomass and vegetative development were accurately simulated. In contrast, the model showed a systematic offset between simulated and measured root biomass caused by assuming constant rates of root maintenance respiration (Rm). A new hypothesis was tested by using the model as a tool to indicate the most appropriate rates of Rm to improve fits and resemble the seasonality of root and shoot biomass dynamically. Results indicated a systematic pattern of change in Rm from 3.5% in summer to <0.5% in winter that is consistent with the differences in metabolic processes that involve storage and translocation of carbon and nitrogen reserves.