DRY-MATTER PRODUCTION IN A TOMATO CROP - MEASUREMENTS AND SIMULATION

Simulation of dry matter production by the explanatory glasshouse crop growth model SUKAM (Gijzen, 1992, Simulation Monographs), based on SU CROS87 (Spitters, Van Keulen and Van Kraalingen, 1989, Simulation and systems management in crop protection), was validated for tomato. In t he model, assimilation rates are calculated separately for shaded and sunlit leaf area at different cumulative leaf area in the canopy, laki ng into account the different interception of direct and diffuse compo nents of light. Daily crop gross assimilation rate (P-gd) is computed by integration of these rates over total crop leaf area and over the d ay. Leaf photochemical efficiency and potential gross assimilation rat e al saturating light depend on temperature and CO2 concentration and are approximated as being identical in the whole canopy. Crop growth r esults from P-gd minus maintenance respiration rate (R(m); dependent o n temperature and crop dry weight), multiplied by the conversion effic iency (carbohydrates to structural dry matter; C-f). Growth experiment s (periodic destructive harvests) with different planting dates and pl ant densities and two datasets from commercially grown crops, were use d for model validation. Hourly averages for global radiation outside t he glasshouse, glasshouse temperature and CO2 concentration, together with measured leaf area index, dry matter distribution (for calculatio n of C-f) and organ dry weights (for calculation of R(m)) were the inp uts to the model. Dry matter production (both level and dynamic behavi our) was simulated reasonably well for most experiments, but final dry matter production was under-estimated by about 27% for the commercial ly grown crops. At low irradiance and with large crop dry weight, grow th rate was under-estimated, probably as a result of over-estimation o f R(m). This could almost completely explain the large under-estimatio n for the commercially grown crops, which had large dry weight. Final dry matter production was over-estimated by 7-11% if daily averages in stead of hourly input of climatic data were used. It is concluded that SUKAM is a reliable model for simulating dry matter production in a t omato crop, except for those situations where R(m) has a large influen ce on crop growth rate (low irradiance and large crop dry weight). An improved estimate of R(m) would take into account the influence of met abolic activity. A preliminary attempt to relate maintenance costs to relative growth rate (a measure for metabolic activity), showed promis ing results.