Peter J.H. Sharpe’s research while affiliated with Texas A&M University and other places

Population dynamics of the southern pine beetle Dendroctonus frontalis has been the subject of intensive research in the USA for more than a decade. TAMBEETLE, an acronym for the mechanistic model of population dynamics of D. frontalis, was developed to abstract contemporary knowledge on the insect. This model was developed as a joint venture between the Biosystems Research Group of Industrial Engineering and the Department of Entomology at Texas A&M University. The approach used to develop the model and the structure of component submodels is described. Research leading to the development of the model involved a series of field and laboratory studies aimed at various aspects of the population system of the insect. This research is reviewed. Important areas needing further research are identified. Studies dealing with factors involved in initiation of infestions, biome-level epidemiology, host susceptibility, stand modeling, and dynamics of within-tree mortality were emphasized as important topics for further research.

S ummary The role of the epidermal cell has rarely been adequately investigated in studies of stomatal movements. In this study of Vicia faba , the changes in aperture in isolated epidermal strips and in intact leaf sections in response to a variety of CO 2 concentrations and temperatures in both light and darkness have been observed for guard cells that were in contact with live epidermal cells. The results indicate that the response of the stomatal system to CO 2 is largely dependent upon the degree to which the guard cell is functionally connected to the rest of the leaf. Stomata of guard cells in contact with live epidermal cells from leaf sections showed a significant decrease in aperture with increased CO 2 . Stomata of guard cells in contact with live epidermal cells on isolated epidermal strips, however, showed an increase in aperture with increased CO 2 which also appears to be related to temperature. Results can be interpreted as supporting the chemiosmotic hypothesis of stomatal movements. The data also indicate that there are two opposing responses in the stomatal system to CO 2 : an opening response located in the epidermis and an overriding closing response controlled by the mesophyll.

S ummary One of the least understood areas in guard cell movements remains the effect of CO 2 , particularly its effect on isolated guard cells. In this study the reactions of guard cells in both isolated epidermal strips and intact leaf sections from Vicia faba have been observed in a variety of temperatures and CO 2 concentrations in both light and darkness. The results are consistent with the hypothesis that stomatal aperture in the isolated guard cell is the result of two temperature‐dependent systems. A decrease in aperture consistently observed at 30 to 35°C in the light reflects the transition from one system to the other. The low temperature system appears consistent with the chemiosmotic hypothesis of phosphoenolpyruvate carboxylase regulation of malate. It demonstrates some sensitivity to CO 2 between 25 and 35 °C, resulting in a shift in optimum aperture to slightly higher temperatures. The high temperature system shows no apparent sensitivity to CO 2 . Stomata from intact leaf sections showed a considerable decrease in aperture as the CO 2 concentration increased from 0 to 60 μl 1 ⁻¹ . The stomatal response threshold to CO 2 appears to be at least as low as 60μ1 1 ⁻¹ and centred in the mesophyll tissue. All observable CO 2 responses in both strips and leaf sections disappeared at temperatures in excess of 35 °C in the light.

Previous mathematical analyses of stomatal mechanics have demonstrated, and experimental measurements have confirmed, that the turgor-generated force of the epidermal cells dominates that of the guard cells in determining aperture. DcMichele & Sharpe (1973) termed the phenomenon the mechanical advantage of the epidermal cells, while Cooke et al. (1976) expressed it as an antagonism ratio. Both of these formulations, however, have theoretical or practical limitations. This study presents a biophysical analysis demonstrating that the effective forces in the stomatal system may be studied in terms of simple stomatal geometry. From this analysis, the mechanical advantage can be redefined and interpreted based upon simple geometric relationships calculated from measurable anatomical dimensions.

Abstract In this revised formulation of guard cell mechanics, the material properties of the walls are re-examined. The observed elastic anisotropy of guard cell walls can be explained by non-random orientation of the cellulose micellae in the unstrained state. This micellar network is assumed to be loosely embedded in the wall matrix causing a two phase elongation process. In the first phase, the micellar network is ‘loose’ resulting in the walls behaving as an isotropic polymer when stretched. As the volume of the cell expands beyond some threshold, the network becomes ‘tightened’ and a second phase of elongation is initiated. During this anisotropic phase of cell expansion, wall elasticity reflectes changes in the orientation of the network to reduce its load. Using the above theoretical analysis, a turgor-pressure versus lumen volume relationship is simulated for Vicia faba. The relationship between aperture and water potential for this species is also established. The simulated results agree with the experimental evidence reported for Vicia faba. The estimated shear modulus of elasticity for guard cell walls is 2 MPa (20 bars) which is well within the limits of reported values for other biological tissues.

Abstract Vicia faba plants grown under water deficit were found to have guard cells considerably smaller than those of plants grown under well-watered conditions. Stomala of plants adapted to drought conditions have been observed in past studies to maintain opening at plant water potentials lower than those of plants not so adapted. By employing the geometric interpretation of the mechanical advantage (Wu, Sharpe & Spence, 1985), an anatomical/mechanical basis was found that helps explain how such opening in drought conditions can occur. The geometry and resulting mechanical properties of small stomata, in contrast to larger stomata, give them the capability of opening or maintaining open pores with lower guard cell turgor pressures, relative to the turgor of the surrounding epidermal cells.

A cotton crop model based on individual plant developmental behavior and variability was developed. Object-oriented simulation (OOS) provided the conceptual basis for the new model structure. The procedural model, COTSIM, provided the theoretical background for cotton plant development. Data collected during 1987 from field-grown cotton were used for model development and verification, and data from 1988 were used for model validation. The model predicted mass accretion and production of organs within the patterns and magnitudes observed in the field. The model also predicted crop development aspects that had not previously been described by procedural models. Age and size of leaves and fruit and associated developmental variability were included in the model through representation of objects and their variable behavior defined by their position on the plant and how this constrains their growth. Observed variability was the result of the aggregate behavior of components. Variability in our OOS model is an output as opposed to being an input in most procedural plant models. The model has recreated both realistic plants and populations in a mechanistic simulation. Object-oriented models are an important step towards common structures and languages for model design and the development of simulations. It was noted that increased mechanistic detail resulted in an increase of procedure calls (messages) and a five-fold increase in model run time.

This paper reviews and applies new hierarchical approaches to ecological modelling. These new approaches are made possible by the development of the object-oriented paradigm. This paradigm draws upon the notion of ‘universal’ or classes dating back to early Greek philosophy. It is an intriguing approach to simulation because it is based upon the concepts of hierarchy and taxonomy, two of the basic organizing principles in ecology. Adopting an object-oriented approach to simulation can result in a reduction of mathematical and statistical abstraction. The object-oriented approach lends itself directly to incorporation of mechanisms within appropriate hierarchies.