A mathematical model of carbon acquisition and utilisation by kiwifruit vines

Abstract

A mathematical model of carbon acquisition and utilisation by a kiwifruit ( Actinidia deliciosa) vine during the growing season is described. The model includes specific features of deciduous fruit crops, such as maintenance of perennial biomass, growth of deciduous tissues, and hydrolysis, and restoration of carbon reserves. Canopy net photosynthesis is computed hourly according to incident radiation, solar angle, radiation attenuation through the canopy, radiation response of photosynthesis and leaf area, and summed to generate daily totals. Daily maintenance respiration is estimated using mean daily ambient temperature and biomass carbon and nitrogen contents. Daily carbon demands for biomass synthesis, including the costs of growth respiration, are estimated using potential growth rates for individual organs. The daily availability of carbon for partitioning is that from 7-day running means of daily photosynthesis and reserve hydrolysis. Carbon is partitioned first for maintenance of existing biomass, and then remaining carbon is partitioned fro growth of individual organs according to their respective sink strengths which, in turn, depend on their potential relative growth rates. A simulation of the seasonal carbon balance for a kiwifruit vine growing at Hamilton (New Zealand) indicated that maximum depletion of carbon reserves during spring was 103 g C m −2. Biomass synthesis for an entire growing season totalled 225 g C m −2 for the shoot (including the leaves), 319 g C m −2 for the fruit, and 58 g C m −2 for the fibrous roots. Biomass synthesis for the fibrous roots was very similar to simulated sensccene. Biomass synthesis of all vine components was limited by carbon supply, and the seasonal patterns of carbon allocation for growth of vine organs closely resembled field measurements reported elsewhere. Simulated carbon acquisition (photosynthesis) totalled 1773 g C m −2 for the growing season. For the whole vine, the carbon cost of maintenance exceeded that for growth. Synthesis and maintenance of shoots accounted for 40% of total carbon utilisation, while that for fruit accounted for 33%. Varying the fruit number led to proportionally similar changes in simulated total fruit biomass, so that average fruit size was relatively insensitive to fruit number. However, allocation of carbon for fibrous root growth and regeneration of reserves decreased with increasing fruit number. While simulated photosynthesis increased with shoot number and hence leaf area, the marginal gains at high shoot densities were less than the marginal costs of tissue synthesis and maintenance. Hence the optimum leaf area (M 2 m −2 ground areas), associated with maximum allocation of carbon to fruit growth and regeneration of reserves was about 3.5. The main advantages of this model include the value for identifying critical components of the whole plant C economy, the capacity to integrate plant—environment interactions at the whole plant level, and the potential for application to other deciduous fruit crops. A major limitation of the model at present is the lack of adequate published data for testing and validation.