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Modelling Sonchus arvensis root biomass allocation to below-ground shoot and fine root growth

ABSTRACT Below-ground (bg) shoot emergence rates of Sonchus arvensis are dependent on temperature and root weight. However, it is unknown to what extent this is due to a root depletion rate that depends on initial root weight, or due to differences in resource allocation to fine root and bg shoot growth. To resolve this, we retrieved data from an experiment in which plants were grown in the dark at constant temperature (4°C, 8°C, and 18°C) and harvested prior to or at shoot emergence. A dynamic mass-balance model, in which biomass of the initial root was allocated to bg shoot and fine root daily growth, and where respiration took place from all tissues, was used. The relative depletion rate of root biomass (RDR; d−1) and fraction of the depleted biomass allocated to bg shoots (SFRR) were estimated and calibrated to observed biomass. The RDR increased with initial root weight and temperature and SFFR was highest for light roots and lowest for heaviest roots, whereas the rest was allocated to fine root biomass. The length-to-biomass ratio of bg shoots decreased with initial root weight. Under between-year weather variations (2004–2010), the reduction in root biomass during the coldest April–May was simulated to be over 12 days delayed compared with the warmest spring. The influence of biomass allocation on bg shoot elongation of heavier roots was thus stimulated by a larger fraction of root biomass being depleted, but counteracted by a smaller fraction of it allocated into bg shoot elongation, compared with lighter roots. The complexity of shoot emergence based on root depletion estimates may be a reason why predictions based on only an accumulated root weight-specific temperature sum, as proposed by a previous study, are expected to be less uncertain than those based on root depletion estimates.