Dynamic Model of Leaf Photosynthesis with Acclimation to Light and Nitrogen

A simple model of photosynthesis in a mature C₃leaf is described,based on a non-rectangular hyperbola: the model allows the high-lightasymptote of that equation (P_max) to respond dynamically tolight and nitrogen. This causes the leaf light response equationto acclimate continuously to the current conditions of lightand N nutrition, which can vary greatly within a crop canopy,and through a growing season, with important consequences forgross production. Predictions are presented for the dynamicsof acclimation, acclimated and non-acclimated photosyntheticrates are compared, and the dependence of leaf properties onlight and N availability is explored. There is good correspondenceof predictions with experimental results at the leaf level.The model also provides a mechanism for a down regulation ofphotosynthesis in response to increased carbon dioxide concentrations,whose magnitude will depend on conditions, particularly of nitrogennutrition.Copyright 1998 Annals of Botany Company Leaf, photosynthesis, hyperbola, model, C₃, acclimation, light, nitrogen.     

Terrestrial carbon storage resulting from CO2 and nitrogen fertilization in temperate grasslands

Abstract. A temperate grassland model has been used to simulate carbon sequestration under various environmental conditions. The results suggest that the CO₂ and nitrogen fertilization that has occurred may contribute appreciably to the so-called missing carbon sink, which it has been suggested must exist to balance the global carbon budget.     

Photosynthesis in Stands of Green Peppers. An Application of Empirical and Mechanistic Models to Controlled-environment Data

The photosynthetic response of stands of green peppers to lightand CO² is examined by means of various mathematical models.Several single leaf response equations are considered, fromthe simple and ubiquitous rectangular hyperbola, to more sophisticatedforms incorporating photorespiration and the oxygen effect.By making the usual assumptions about stand structure and lightpenetration, mechanistic crop response equations arc derivedfor each of the different leaf response equations. First, asa purely empirical approach, the leaf response equations areapplied directly to crop data, and it is concluded that therectangular hyperbola with a term for dark respiration [eqn(2)] gives an adequate summary of crop response to light andCO₂. Second, four mechanistic equations of crop response areapplied to the data, and, although the results are equivocal,it is suggested that the simpler crop equations [eqns (15) and(17)] are satisfactory at the present time, and it is not yetpossible to detect the results of photorespiration and the oxygeneffect directly in the crop data.     

Response of Dry Matter Partitioning, Growth, and Carbon and Nitrogen Levels in the Tomato Plant to Changes in Root Temperature: Experiment and Theory

Growth, dry matter partitioning, and the levels of various substancesin tomato plants grown in liquid nutrient culture at four roottemperatures (15, 20, 25, and 30°C) in controlled environmentcabinets were measured. In order to understand the role of roottemperature in growth and development, the partitioning modelof Thornley (1972) was used to try and describe the observedbehaviour. The steady-state behaviour could be simulated reasonablyby the model, although certain aspects of the time-course datapresent problems. It was observed that relative growth rateis proportional to the product of the carbon and nitrogen levels(one of the two principal assumptions of the model). The modelalso made realistic predictions about the influence of roottemperature on the fraction of the plant dry matter in the root,and on the carbon and nitrogen levels. The effect of root temperaturewas incorporated into the model by assuming that certain valuesof specific root activity correspond to the different temperatures.In addition the model predicted an experimentally reasonablerelation between net assimilation rate and relative growth rate.     

Modelling Light Absorption and Canopy Net Photosynthesis of Glasshouse Row Crops and Application to Cucumber

A model of light absorption and photosynthesis applicable toglasshouse row crops is constructed and applied to cucumber.Light absorption is calculated using a method suggested fordiscontinuous canopies; photosynthesis is modelled with a non-rectangularhyperbola. The predictions of this model are compared with experimentaldata in the preceding paper. Here the model is used to simulateresponses to light and CO₂ concentration and especially to examinethe effects of varying the parameters of the crop that can becontrolled by the grower. These include the number of plantsin each row, the number and width of the rows, the gap betweenrows, and the height of the crop. For example, it is shown that,for high values of crop net photosynthesis, the number of rowsis more important at high light than at low light, whereas cropheight is more important at low light than at high light. Theimplications of these and other findings are discussed. Key words: Cucumis sativus L., glasshouse crops, cucumber, model, light absorption, photosynthesis, CO₂, row crops, simulation     

VERNALIZATION OF WINTER RYE BY ULTRASONICS

Petkus winter rye seed was subjected to ultrasonic vibrations of i Mcyc./sec. frequency in order to test the possibility of their having a vernalizing action. Seed which had not been soaked, fully imbibed seed, germinating seed (radicle showing) and seedlings with about 2 mm. long coleoptiles were treated with two intensities of ultrasonics for two periods of exposure.
There is a slight favourable effect on germination rate of the unsoaked seed, but injury or death is caused by higher doses applied to the imbibed or germinated seed. Time to flowering was not affected by this treatment and no vernalization resulted.     

A model of water flow through plants incorporating shoot/root 'message' control of stomatal conductance

Abstract. A model of water flow from the soil into the plant, and from the plant to the atmosphere is described. There are three state variables in the model: the soil, root and shoot water contents. The flow rate of water from the soil to the root is calculated by dividing the gradient in water potential by a resistance, comprising the resistance from the bulk soil to the root surface, and that from the root surface to the root interior. The resistance in the soil depends on the soil hydraulic conductivity, which in turn depends on the soil water potential. The flow rate from the root to the shoot is given by the gradient in water potential divided by a resistance, which depends on the structural dry mass of the plant. Transpiration is described by the Penman-Monteith equation. The plant water characteristics can be modified to take account of osmotic and cell wall rigidity parameters. The model incorporates the concept of shoot/root ‘messages’ of water stress, which influence stomatal conductance. The message works through the generation of a hormone as the pressure potential in the shoot (mesophyll) or root falls. This hormone induces a shift of osmoticum from the guard cells to the surrounding mesophyll cells, which causes an increase (i.e. closer to zero) in the osmotic potential in these cells. This, in turn, causes a decrease in their pressure potential, and so reduces stomatal conductance. The model is used as a framework to address some of the issues that have recently been raised concerning the role of water potential in describing water flow through plants. We conclude that, with the hormone present, there is unlikely to be a unique relationship between stomatal conductance and shoot total water potential, since stomatal conductance depends on the pressure potential in the guard cells, which may differ from that in other cells. Nevertheless, this does not imply that water potential is not an important, and indeed fundamental, component for describing water flow through plants. Other aspects of water flow through plants are also considered, such as diurnal patterns of shoot, root and soil water potential components. It is seen that these may differ from the commonly held view that, as the soil dries down, they all attain the same values during the dark period, and which, as we show, is largely unsubstantiated either theoretically or experimentally.     

A Model of Flowering in Chrysanthemum

A mathematical model of flowering in Chrysanthemum morifoliumRamat. is described which may be used to predict quantitiessuch as the number of primordia initiated by the apex, plastochronduration and apical dome mass before, during and after the transformationof the apical meristem from vegetative to reproductive development.The model assumes that primordial initiation is regulated byan inhibitor present in the apical dome. Within each plastochronthe apical dome grows exponentially, and the inhibitor concentrationdeclines through chemical decay and dilution. When the inhibitorconcentration falls to a critical level a new primordium isinitiated. There is instantaneous production of inhibitor, anda decrease in dome mass corresponding to the mass of the newprimordium. The process continues until the apical dome attainsa particular mass when the first bract primordium is produced.Subsequent primordia compete with the apical dome for substrates,and the specific growth rate of the dome declines with successiveplastochrons. Eventually, the net mass of the dome starts todecline until it is entirely consumed in the production of floralprimordia. Chrysanthemum morifoliumRamat, flowering, primordial initiation     

The magnitude of the temperature-driven contribution to within-plant transport

Abstract The contribution from temperature gradients within a plant to convective transport is calculated. Its magnitude depends primarily on the temperature differences in the plant and the radius of the conducting elements; the other quantities affecting it are well-established physical constants. Assuming a temperature difference in the plant of 1 K and a conductive element radius of 10^?4 m, the speed of convective flow is 0.5 cm h^?1 and this is independent of distance.     

A Model of Growth of the Fifth Leaf of Tomato

Some measurements on the fifth leaf of the tomato plant aredescribed: these include length, area, dry weight and freshweight. A model, which attempts to describe and interpret someof the data, is constructed. After fitting the model to thedata, it is used to rank the physiological and environmentalparameters, in terms of how much they affect the contributionthat the fifth leaf makes to the carbon economy of the wholeplant.