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Het gebruik van water ondervindt steeds meer beperkingen en de kosten lopen op. Vandaar is het van belang de watergift af te stemmen op de behoefte van de boom. Water geven op basis van meteorologische metingen is mogelijk met het WG-meteosysteem. In de tabellen informatie over: kwaliteitseisen voor water, maximale waterprijs bij Jon␁agold en Cox en een voorbeeldberekening watergift (mm)

Onderzoek naar de invloed van langdurige bodemvochtigheid op de fotosynthese en respiratie van jonge aardbeiplanten

Het onderzoek naar een milieuvriendelijke teelt van kasrozen is gericht op bemesting, invloed van licht op wateropname, het gebruik van tensiometers en bestrijding van bodemmoeheid

Stoken en luchten tegelijk is heel gebruikelijk. Het doel van deze maatregel is de luchtvochtigheid te verlagen en zo de verdamping te bevorderen. Het is de vraag of dit extra energieverbruik wel altijd nodig is. Voor komkommer hoeft een hoge luchtvochtigheid niet altijd nodig te zijn

Plant physiological research has revealed that stomatal aperture of many plant species is reduced by CO 2 . Therefore, the question has been raised as to how transpiration will be affected if the ambient C0 2 concentration increases. This study focuses on the prediction of changes in transpiration at the regional scale (10-100 km horizontal, 1-5 km vertical). A rather detailed, coupled vegetation- Planetary Boundary Layer (PBL) model has been constructed in order to identify important processes that control such changes.The coupled model uses the well-known "big-leaf' model for the vegetation part. Surface resistance (r s ) is described by means of an up-scaled ''A-g s '' model, where stomatal conductance is related to photosynthetic rate. The background of this model for (r s ) is outlined. A new parameterization to mimic stomatal humidity responses is proposed. The parameterization prescribes a linear relation between the specific humidity deficit at the leaf surface and the ratio of the internal C0 2 concentration to the external C0 2 concentration. The resulting ''A-g s '' model simulates stomatal responses to CO 2 , light, temperature, humidity as well as their synergistic interactions. The model is tested using data for grapevines ( VitisVinifera L., cv. Airen). The model is able to simulate the photosynthetic rate and the stomatal conductance of this species satisfactorily.The PBL part of the coupled model is a 1D, first-order closure model, which takes into account nonlocal turbulent transport by means of a countergradient correction. The PBL model also simulates C0 2 fluxes and concentrations. The surface flux of C0 2 is driven by photosynthetic rate from the up-scaled ''A-g s '' model. The complete coupled model realistically simulates the state of the PBL, (r s ) transpiration, and the most important aspects of the biosphere-atmosphere interaction for extensive, homogeneous, well-watered canopies with dry leaves.Systematic sensitivity studies using the co

This monograph presents the results of a detailed study in micrometeorology; one of the sciences that play an important role in production ecology. The purpose is to explain the microweather as a function of the properties of plant and soil, and of the weather conditions prevalent at some height above the canopy. So many, partly interacting, factors are involved in micrometeorology that a quantitative evaluation of their combined effect can only be successfully achieved with the aid of a simulation technique.In Chapters 2, 3 and 4 the submodels, that are the basic elements of the micrometeorological system, are described and discussed. In Chapter 5 the programming aspects are considered and in Chapter 6 some results of the composite model are given, together with an evaluation. The parameter values that are needed in the submodels are almost exclusively taken from literature sources. The structure of the submodels on the contrary is specially developed for this study to attain compatibility of these submodels and an optimal balance between accuracy and simplicity.In Chapter 2 some models are given for the radiation in plant canopies for different circumstances and assumptions.For horizontal leaves the extinction of radiant fluxes of one wavelength is exactly exponential, both for black and scattering leaves. The numerical model shows that for non-horizontal leaves the extinction is usually close to exponential. Only for low inclinations of direct radiation does the exponential profile underestimate the extinction in the top layers. The values of the extinction and reflection coefficients can be approximately calculated with a few simple equations, based on a generalization of the results for horizontal leaves. Ross (1975) had already shown that the leaf angle distribution can be approximately characterized by a single number. My evaluation shows that the values of the reflection and extinction coefficients are successfully estimated by the model and the simplified e

Onderzoek naar de verschillende faktoren die invloed hebben op het broei-effekt in verwarmde kassen (Venlo-type) en naar de invloeden hiervan op de gewastranspiratie (tomaat op steenwol)

In this book some physical aspects of greenhouse climate are analyzed to show the direct interrelation between microclimate and crop transpiration. The energy balance of a greenhouse crop is shown to provide a sound physical framework to quantify the impact of microclimate on transpiration and to identify the constraints set on climate management by the termodynamic behaviour of the canopy. Before the relationship among microclimate, canopy temperature and transpiration is rendered in mathematical terms, a good deal of experimental work is necessary to establish sub-models for the heat transfer of the foliage, for the radiative transfer within the canopy and for the canopy resistance to vapour transfer. The sub-models are merged in a combination-type equation to obtain the temperature of a greenhouse crop and its transpiration. The resulting estimates are shown to reproduce accurately the temperature and transpiration of a greenhouse tomato crop, as measured at time intervals of a few minutes.To illustrate the practical application of the model thus developed a number of examples are presented. In particular, it is shown that defining the transpiration rate as the criterion for the control of air humidity within a greenhouse would deliver a quantitative framework for that control. That would largely enhance the efficiency of the (expensive) procedures applied at present for the control of humidity in greenhouses., Een methode wordt ontwikkeld en op bruikbaarheid getoetst om de gewasverdamping in kassen kwantitatief te beschrijven, rekening houdend met het microklimaat dicht op het gewas. Regeling van het kasklimaat via deze methode kan de hoge kosten verminderen die nodig zijn voor de regeling van de vochthuishouding in kassen

De methoden om de verdamping van een vegetatie te bepalen zijn meestal ontwikkeld ten behoeve van de landbouw. Deze methoden zijn in principe ook geschikt voor toepassing voor gebieden met min of meer natuurlijke begroeiingen. De vaak complexe vegetatiestructuur maakt dat een aantal methoden minder geschikt zijn, of dat in het geval van formules de benodigde vegetatie-specifieke factoren niet of nauwelijks bekend zijn, of dat deze moeilijk te bepalen zijn. Voor drie van dergelijke formules zijn parameters en vegetatiefactoren verzameld. Dit betreft de formules van Penman, Priestley en Taylor en die van Rijtema/Monteith. Naast een bespreking van de voor- en nadelen van deze formules zijn in een rekenvoorbeeld van elk van de drie formules voor zowel een blauwgrasland als ook voor een loofbos verdampingsberekeningen uitgevoerd.

Theoretical and practical aspects of the transpiration of crops in the field are discussed and he concludes that the relationship between transpiration and total dry matter production is much less affected by growing conditions than has been supposed. In semi-arid and arid regions, this relationship depends on plant species and on free water evaporation, while in cloudy regions it depends mainly on plant species. This difference is due to light intensity; in cloudy regions assimilation is less than in regions with more bright sunshine. The relationship between transpiration and dry matter in plants grown in the field with limited available water is often similar to that for container-grown plants. Where water is not limited, however, transpiration tends to increase, although even when transpiration becomes considerably greater than free water evaporation, the relationship between transpiration and production in the field resembles that of plants growing in containers. The relationship between transpiration and total dry matter production is not affected by the ability of plants to withstand periods of drought, but the amount of marketable product and total amount of water which is transpired during the growing period may depend largely on their drought resistance.

The effect was estimated of light intensity, leaf temperature, and C0 2 concentration on photosynthetic rate in leaves of crop plants. The potential capacities of photochemical and biochemical processes and of C0 2 transport were compared.Resistance to C0 2 transport in various parts of the pathway from external air to reaction centre in the chloroplasts was estimated in absolute units. This allowed quantitative comparison of micrometeorological and physiological effects on photosynthesis.C0 2 transport towards the leaf is affected by the sum of resistances in external air (r a ), stomata (r s ), and mesophyll cells (r m ). Since r m is usually larger than r s of open stomata, r m is probably an important yield determining factor.With adequate water-supply, transpiration depends on r s + r a , but not on r m . Therefore higher values of r s and lower values of r m favour the water balance of crops.With strong light and leaf temperature around 22°C, C0 2 saturation of photosynthesis was reached with C0 2 concentrations between 01 and 0.15 %.Techniques were described for accurate measurement of C0 2 and water vapour, leaf temperature, light intensity and diffusion resistance. The importance of different types of light bulbs for photosynthesis was evaluated, and much attention was paid to accurate assessment of effective C0 2 concentration in the air around leaves in assimilation chambers.

An analytical study of the cyclic stomatal behaviour in leaves of bean, Phaseolus vulgaris L. 'Vroege Wagenaar', was made in order to explore the cycling, to study different aspects of plant water relations and of the action mechanism of the stomata. A general method of investigation was recording of the difference of temperature of leaf and air with thermocouples in a constant environment.Introductory observationsPlants grown at low light intensity showed a higher tendency to sustained cycling (higher instability of the stomatal apparatus) than those grown at higher light intensity. At the higher light intensity non-aeration of the nutrient solution increased the tendency to cycling with a more than additive effect. It is suggested that the lower capacity to take up water of non-aerated roots was involved in the higher tendency to cycling. Cycling over the whole plant was frequently disturbed by bringing cycling of a part of the active stomata out of phase. Cycling was induced by changing abruptly any factor that affected stomatal opening, by entrainment with cycling air vapour pressure, by entrainment with cycling plant water potential, and apparently spontaneously.Younger leaves showed shorter free-running periods than older ones. When both were attached on the same plant, cycling in the younger leaves entrained cycling in the older ones.Water relations of cyclingAmplitudes of cycling increased by decreasing air humidity and by increasing the water potential in the root medium. A time delay in the adjustment of the turgor of the guard cells to the plant water potential is assigned to be the cause of overshooting of the active stomatal movement and as to be the factor amplifying both passive opening and closing movement. Cycling only occurred when an important resistance to water uptake was present in the transpiration stream. In intact plants this resistance was located in the roots. The water potential cycled in the whole plant down to this resistance, synchroni