Modelling biomass production and yield of horticultural crops: a review

Abstract

Descriptive and explanatory modelling of biomass production and yield of horticultural crops is reviewed with special reference to the simulation of leaf area, light interception, dry matter (DM) production, DM partitioning and DM content. Most models for prediction of harvest date (timing of production) are descriptive. For DM production many descriptive and explanatory models have been developed. Most explanatory models are photosynthesis-based models. Important components of photosynthesis-based models are leaf area development, light interception, photosynthesis and respiration. Leaf area is predominantly simulated as a function of plant developmental stage or of simulated leaf dry weight. Crop photosynthesis can be calculated as a function of intercepted radiation or more accurately by considering radiation absorption of different leaf layers in combination with a submodel for leaf photosynthesis. In most crop growth models respiration is subdivided into two components: growth and maintenance. There is reasonable consensus concerning the simulation of growth respiration, but the simulation of maintenance respiration is still an area of great uncertainty, which is especially important for large crops grown under winter conditions at relatively high greenhouse temperatures. DM partitioning can be simulated by descriptive allometry, functional equilibrium or sink regulation. The most suitable approach depends on the type of crop studied and the aim of the model. As opposed to most agricultural crops, the DM content of the harvestable product is of great importance to the yield of most horticultural crops. More attention should be paid to the simulation of DM content. It is concluded that the strong features of explanatory crop growth models are the simulation of light interception and gross photosynthesis, while the weak features are the simulation of leaf area development, maintenance respiration, organ abortion, DM content and product quality.

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, as discussed in more detail by Lentz (1998). Consequently, the interest in modelling of biomass production and yield of horticultural crops is still increasing as indicated by the increasing proportion of horticultural publications—listed in the CAB Abstract Database—that are dealing with models. The physical yield of a crop is determined by dry matter (DM) production, DM distribution and the DM content of the harvestable organs (Fig. 1). These attributes not only relate to product volume but also to product quality. DM production is primarily driven by photosynthesis, while photosynthesis to a great extent depends on the interception of light. Leaf area is an important determinant of light interception. Growth of the leaves is a function of the total DM production and the fraction of DM partitioned into the leaves. As horticultural products usually are sold on a fresh weight basis, yield is predominantly determined by water content.

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 submodels 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. At the lowest hierarchical level, submodels in an explanatory model are descriptive and the model's ability to explain is limited by its number of hierarchical levels (Larsen, 1990). Although the explanatory crop growth models in horticulture do, to some extent, reflect physiological structures, they do not incorporate the complete current knowledge of the biochemical relationships that we have for many of the mechanisms at the cellular level. On the other hand, if they did, the models would be impossible to manage and use for predictions and analysis at the crop level.

In this paper, descriptive and explanatory modelling of biomass production and yield of horticultural crops is reviewed with special reference to the simulation of leaf area, light interception, DM production, DM partitioning and DM content. The relation of growth and yield of plants to nutrition, water relations and plant architecture is not discussed in detail in this paper, as these aspects are reviewed by Le Bot et al. (1998), Jones and Tardieu (1998)and Prusinkiewicz (1998), respectively.