Benefits of Fundamental Modelling - the Case of Physiological Disorders

A model was developed assuming that the occurrence and the development of physiological disorders are effects of a balance between processes of disorder formation and scavenging of initiating compounds. Based on this (simplified) mechanism and applying the fundamental rules of chemical kinetics, the differential equations can be derived. The observed effects depend on initial levels of some nutrients (N), free radicals (Ra) and scavenging activity (SS). All three types of compounds do depend on the growing conditions prior to harvest, and hence on season, weather and climate. Postharvest development of disorders (PD) depends also on conditions during storage, e.g. temperature and CO2 level. Thus their influence was incorporated into a model based on fundamental knowledge in that sphere. The benefit of the developed model is that it allows simulating very different ranges in type of development of disorder. From very fast and reaching high level of occurrence, very fast but stabilised at low level of occurrence, to slow but reaching high level of occurrence, and slow and stabilised at low level of occurrence. And of course simulating of cases where the disorder occurrence is not observed at all. The model was checked against measured data on pears, indicating that the approach seems to be realistic and powerful enough to pursue. Existing data are however gathered in a manner not suitable for this approach. Therefore further studies on dynamics of physiological disorders require improvement of experimental methods. INTRODUCTION Traditionally, models in agriculture do not take the variance between batches or individuals into account. These models are useful for numerous practical applications, yet sometimes they lead to significant errors in predicted values. Fundamental modelling is based on two major pillars: the basic processes responsible for changes of observed attributes and theoretical information on these processes and on biological variance within simulated batch of fruits. Fundamental models of this type describe the changes of attributes within individuals depending on the actual state these individuals have at the moment the experiments start. With this approach various observed cases can be explained and described with the same model. Incorporating the effects of variance between individuals in fundamental process oriented models could possibly decrease the number of erroneous simulation. FORMULATION OF THE MODEL The observations of disorder occurrence showed that if disorders are found in a batch of fruits, only part of that batch is affected. The remainder of the fruits is not affected at all. This observed pattern is independent of fruit size, position in the canopy and many other investigated factors. Even in the batches of large fruits picked exclusively from upper part of canopy (which is a factor increasing the probability of disorder occurrence) one can find fruits without disorder. This leads to conclusion that there is probably some kind of "disorder seed" present in some fruits and absent in the others. Proc. 3 IS on Model IT Eds.: M.L.A.T.M. Hertog and B.M. Nicolaï Acta Hort. 674, ISHS 2005 158 This “seed” is a chemical element, compound or a concentration of specific atoms or molecules, come into being in favourable conditions and creating sensitivity to formation of a disorder. The general knowledge of chemistry and physiology (Tomala, 1999; Kruger et al., 1999; Lourens and Malherbe, 1999; Larrigaudiere et al., 2001) suggests that some physiological disorders are caused by free radicals. The analogy with development of chilling injury caused by free radicals (Tijskens et al., 1994) is evident. It is also widely accepted that occurrence of physiological disorders in fruits depends on their growing conditions (Tomala, 1999; Toivonen et al., 2003) Binding this information together one can say that the physiological disorder (PD) affects the tissue of substrate (S) when free radicals (Ra) are present. And that this process depends also on other factors exemplified by richness in nutrients (N), which (according to experts’ observations) enhance the occurrence of disorder. This process of disorder formation can be represented as: