Modeling Carbon Allocation below Ground

Belowground carbon expenditure has been difficult to measure, and even more so to predict. Root growth is influenced by both direct effects on plant metabolism and indirect effects of soil drying and interactions with soil organisms. In Concord grape, crop load may compete with root growth when soil moisture is not limiting. Root lifespan also is difficult to predict but some success can be achieved by taking a cost-benefit perspective to unraveling some of the processes affecting root lifespan. Root respiration is affected by both shortand long-term temperature conditions. Short-term studies suggested that elevated soil temperature may have direct effects on root respiration that follow a Q10 response of about 1.7 2. More recent work indicates that plant respiratory responses to soil temperature may be a function of plant growing temperature and soil moisture. Citrus and grape roots exposed to increasing temperatures above 20 °C for more than three days exhibited homeostatic respiratory responses, presumably to avoid excess carbohydrate metabolism. Moreover, citrus roots in dry soil exhibited no temperature response but maintain respiration at a stable basal rate. INTRODUCTION Growth models in most crops, but particularly field crops, have largely ignored the factors controlling belowground carbon allocation. Environmental and cultural factors can strongly influence allocation to roots and mycorrhizal fungi. Seedling studies in citrus, for example, indicate that mycorrhizal colonization, soil temperature, plant age and soil moisture and phosphorus limitations all can influence the fraction of total photosynthate used for root growth and maintenance (Eissenstat et al., 1993; Kosola and Eissenstat, 1994; Bryla et al., 2001). Much less work has been done in bearing fruit trees in the field, which will be the focus of this review. Carbon allocation belowground is influenced by both the demands of the shoot for water and nutrients and the demands of the roots for photosynthate to provide the energy for maintenance of the tissue. Root:shoot communication is not well understood but the manifestations of resource limitations aboveor belowground on allocation are widely reported. In modeling belowground allocation, many of the responses are not linear and should be considered in this context. Moreover, most studies on root:shoot allocation have been conducted using young plants in pots. Seedling responses to resource limitations may be quite different from bearing trees. For example, in a perennial root system, the amount of roots available for water and nutrient absorption is a function of both root birth and root death. Root mortality has rarely been studied in the context of resource availability and carbohydrate competition between roots and shoots. An important first step in developing a predictive model is to have some notion of the patterns of the processes that are being modeled. In the case of root growth, root mortality and root respiration, few observations exist of these processes in relation to crop load, pruning and environmental stresses. Recent research in citrus and grape have provided some important new observations of these processes which should aid in modeling the amount of carbon allocated below ground. Proc. VII IS on Modelling in Fruit Research