Modelling Dry Matter Production and Partitioning in Sweet Pepper

Models predicting growth and yield have been developed for a large number of crops. This paper describes a dynamic, mechanistic model for sweet pepper, addressing issues such as leaf area expansion, dry matter partitioning and validation. Leaf area formation and organ initiation are simulated as a function of temperature sum. Light absorption and photosynthesis are calculated for a multilayered uniform canopy. Leaf photosynthesis is calculated for the various leaf layers according to the biochemical model of Farquhar, and integrated to canopy photosynthesis. Net assimilate production is calculated as the difference between canopy gross photosynthesis and maintenance respiration. The net assimilate production is used for growth of the different plant organs and for growth respiration. Fruit set is simulated as a function of source and sink strength and temperature. Assimilate partitioning between vegetative parts and individual fruits is simulated on the basis of the concept of sink strengths. The sink strength of each individual fruit is calculated as a function of its temperature sum from anthesis. The sink strength of the vegetative parts is calculated as a function of temperature only. A wide range of experimental data show that leaf area is linearly related to the temperature sum from planting. The model was validated on the basis of six experiments in The Netherlands and France. Simulation of dry matter production and partitioning under a wide range of conditions showed that model results agreed well with measurements. Some directions for further improvements are discussed. INTRODUCTION Models are powerful tools to test hypotheses, to synthesize knowledge, to describe and understand complex systems and to compare different scenarios. Models may be used in decision support systems, greenhouse climate control and prediction and planning of production. Often descriptive and explanatory models are distinguished. Descriptive models, also called statistical, regression, empirical or black-box models, reflect little or none of the mechanisms that are the cause of the behaviour of a system, whereas explanatory models consist of a quantitative description of these mechanisms and processes (Penning de Vries et al., 1989). Explanatory models contain sub-models at least one hierarchical level deeper than the response to be described, e.g., crop photosynthesis and leaf area expansion are processes one hierarchical level below crop growth. Although the explanatory crop growth models in horticulture do, to some extent, reflect physiological processes, they do not incorporate all knowledge on biochemical mechanisms at the cellular level. On the other hand, if they did, the models would be impossible to manage and use for predictions and for analysis at the crop level. Models predicting growth and yield have been developed for a large number of crops, including a few models for sweet pepper (e.g., Marcelis et al., 1998; Buwalda et Proc. IIIrd IS on HORTIMODEL2006 Eds. L.F.M. Marcelis et al. Acta Hort. 718, ISHS 2006 122 al., 2006; Schepers et al., 2006). Developmental aspects such as fruit set and abortion, but also leaf area expansion are generally weak points of models. Furthermore, models are seldomly thoroughly validated. This paper describes a dynamic, mechanistic model for sweet pepper, addressing issues such as leaf area expansion, fruit set, dry matter partitioning and its validation in six experiments. MATERIALS AND METHODS In total eight experiments were conducted. Exp. 1 and 2 were used only for model calibration. Exp. 3 to 8 were used for model validation. However, Exp. 3 to 8 were also used for calibrating leaf area expansion. Although model validation should be done on different data as the data for model development and calibration, we choose to use data of all experiments for calibrating the simulation of leaf area expansion. Hence the simulation of leaf area expansion could not be validated; on the other hand if we had used only Exp. 1 and 2 for calibrating the simulation of leaf area expansion the model parameters would have been almost similar. All experiments were performed with sweet pepper (Capsicum annuum L.) grown on rockwool with drip irrigation in Venlo type glasshouses in Wageningen, The Netherlands (latitude 52°N; Exp. 1-5) or in Carquefou France (latitude 47°N; Exp. 6-8). Per location each experiment was in a different year, except for Exp. 2 and 4 which were in the same year. Cultivation was as much as possible comparable to standard practices of growers, but some treatments were out of the range of standard cultivation practice of growers. Plants were pruned to two main stems per plant. Exp. 1. ‘Mazurka’ (red fruits) was grown at three different planting densities: 1.56, 3.12 and 4.63 plants per m. The crop was planted on January 10 and ended on July 3, 1996. Exp. 2. ‘Red Spirit’ (red fruits) was grown at 3 different planting densities: 2.5, 3.8 and 5.0 plants per m. The crop was planted on December 13, 2001 and ended on September 4, 2002. Exp. 3. ‘Mazurka’ (red fruits) was grown at 3 different temperatures: average temperatures were 19, 22 and 25°C. The crop was planted on January 15 and ended on June 5, 1997. Exp. 4. ‘Meteor’ (red fruits) was grown at three different CO2 concentrations: 380, 580 and 780 ppm. As this experiment was performed in a greenhouse with mechanical cooling a very stable climate could be realized with constant CO2 concentration during day time. The crop was planted on February 7 and ended on June 19, 2002. Exp. 5. ‘Solution’ (red fruits) was planted on February 11 and the experiment ended on July 8, 2003. Exp. 6-8. ‘Triple 4’ (green fruits) was grown in 3 subsequent years (2003-2005). Planting was each year on December 17 and experiments ended in September/October. In these experiments there were also fertigation treatments. However, as these treatments had no major effects on crop growth only the reference treatments of these experiments are shown (for more details see Marcelis et al., 2005 and Brajeul et al., 2006). MODEL DESCRIPTION A mechanistic model for sweet pepper was developed that simulates leaf area expansion, dry and fresh weight growth of plant organs, flower formation, fruit set and fruit harvest. The model in fact also simulates plant-water relations and plant-nutrient relations, as described by Marcelis et al. (2005) and Brajeul et al. (2006). These parts of the model are not considered in this paper and the simulation runs shown assumed no limitation in availability of water or nutrients. The model is primarily based on the INTKAM model for simulation of plantwater relations and dry matter production (Gijzen, 1994). The simulation of dry matter partitioning, fruit set and fruit growth is primarily based on the cucumber model of Marcelis (1994). The model consists of routines for greenhouse radiation transmission, radiation interception by the crop, leaf and canopy photosynthesis, respiration, dry matter production, dry matter partitioning among plant organs (roots, stem, leaves and individual