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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.

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

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 equations. The daily trends and the difference between the visible and the near-infrared regions are well represented by the model. The numerical results of the model show a reciprocity relation with interesting theoretical aspects and important implications for the remote sensing technique for example. The equations for the thermal radiation field can be kept simple because the temperatures of the leaves and the soil surface are considered as state variables.An extension of the model to a scattering coefficient of unity explains the radiance distribution of the standard overcast sky (SOC). Another model extension shows the effect of a deviation from the foregoing simplifying assumption that the reflection and transmission coefficients of the individual leaves should be equal. Subsequently it is shown that over the range from a regular to a clustered leaf arrangement, the reflection and extinction coefficients in a canopy decrease, as well as the difference between extinction of visible and near-infrared radiation. Both effects are also found in literature. The modelling of light interception by plants in rows shows that the loss of diffuse light between the rows is not too serious. Simplifying formulations are derived and evaluated for this situation.The models presented in this chapter are general enough on the one hand and their results are sufficiently simplified on the other to be successfully applicable in simulation models for photosynthesis and transpiration of plant stands.In Chapter 3 the energy and mass balances of single leaves, of the soil, of the canopy and of the air inside the canopy are evaluated by a partitioning of the incoming radiant energy over the leaves and the soil surface according to the theory of Chapter 2. The distribution of the available energy between transpiration or evaporation, sensible heat loss, heat storage and photosynthesis is derived from the specific properties of plants and soil, and from the aerial conditions. The plant water content, which feeds back on the stomatal resistance, is found by integration of calculated transpiration and water uptake from the soil. The uneven distribution of the radiation over the leaves and the presumed vertical gradients of air temperature and humidity require a classification of the leaves with respect to height and to incidence of radiation. The fluxes of heat and water vapour from the leaves and the soil surface are released into the air. The aerial profiles of temperature and humidity are found by integration of the net fluxes with respect to time for each horizontal layer inside the canopy.The soil temperatures are found by a similar procedure. Here the driving force is the soil heat flux at the surface. In the soil the diffusivity for heat is taken as constant. To obtain the values of the turbulent exchange in the air the theory of Chapter 4 is used.In Chapter 4 the turbulent echange in the air inside and above the canopy is calculated.In Section 4.2 the frequently observed logarithmic wind profile is used as a basis for the calculation of the exchange coefficient above the canopy. When there is a vertical temperature gradient in the air, the logarithmic wind profile is disturbed and the values of the exchange coefficient have to be modified. The relations that express this effect, are taken from literature and reformulated for the purpose of application in the simulation model. An important result is that the formation of an inversion layer (in which the turbulent exchange is reduced or even blocked) above the canopy during the night when the net radiation is negative, can be understood and described. In Section 4.3 the turbulent exchange inside the canopy is calculated. It is found that the profiles of both wind speed and exchange coefficient are reasonably well approximated by an exponential extinction with depth. The values of the extinction coefficient derived theoretically are in good agreement with experimental data. When temperature gradients exist in the canopy the exchange coefficient is corrected in a similar way to above the canopy. There is a lower limit for the exchange coefficient when the wind speed drops to zero, as might occur under an inversion, so that the exchange inside the canopy is then maintained. In Section 4.4 the aerodynamic macrocharacteristics, i.e. the zero plane displacement d and the roughness length Zo , are expressed in the aerodynamic microcharacteristics and geometry of the crop. The theoretical results for grassland, maize and a coniferous forest appear to agree well with the aerodynamic macrocharacteristics found experimentally. From a model analysis of the effect of spatial variability it is concluded in Section 4.5 that the one-dimensional scheme is adequate as long as one is only interested in horizontally averaged figures, Temporal variations such as those caused by gustiness of wind may have quite important effects on mean values of temperature and humidity.In Chapter 5 the programming aspects are discussed. In Section 5.2 first the initialization problem is discussed, which can be solved by the concept of the cyclic equilibrium. One spatial boundary of the system is chosen deep enough in the soil to eliminate the influence of the daily heat wave, and the other one is not taken higher than 3 in above the ground surface to keep horizontal advection as small as possible. The method of conversion of the weather data measured at this level is also given. In Section 5.3, the usefulness of the hierarchical approach as well as the conditions for its application are discussed. Some examples are given. Section 5.4 gives a survey of some solution techniques together with their merits and drawbacks for different problems (see also Fig. 27). The choice of the programming language is explained. In Section 5.5 a technique is given by which the number of computations in a simulation program for a stiff system is greatly reduced. This method is employed in the micrometeorological simulation program. The listings of the programs are printed in Section 5.6.Chapter 6 gives the results of the composite model. The daily courses of some typical output variables are simulated for a specific case and explained with the submodels. Here (wet soil) the simulated soil evaporation amounted to about one third of the total evapotranspiration. The dew on the leaves mainly consisted of over-distilled soil water. During the night an inversion situation developed. A water stress in the afternoon resulted in a serious depression of the rate of CO 2 -assimilation.The sensitivity analysis (Section 6.3) showed that slow sensors may be used to record input weather data, because hourly averages are sufficiently accurate. An exception must be made for wind speed since wind gustiness may seriously affect the simulated results. Incoming solar radiation should preferably be recorded in the diffuse and the direct components separately. Amount of dew and duration of leaf wetness cannot be calculated by an easy rule. The numerous complex relations must be integrated in a simulation program to evaluate their combined effect. The compartmentalization of the soil in ten layers was sufficiently accurate, for the canopy space only three layers seemed sufficient. Among the plant properties, the scattering coefficient of the leaves is of great importance. Other important factors are the cuticular resistance, especially when the leaf area index is high, and the stomatal resistance behaviour. Compared to these factors, the leaf angle distribution is of only minor importance, (already found by de Wit (1965)) so that Ross' characterization (Section 2.3.3) is certainly sufficiently accurate. For a closed canopy the leaf area index has little influence on CO 2 -assimilation. and transpiration and the aerodynamic crop characteristics are of moderate importance.The thermal properties of the soil have a large effect on the amplitude of the soil heat flux and hence on the amplitude of the soil temperature variation, but their phases are only little influenced. The effect of the thermal soil properties on the profile in the air is negligible in the daytime, but much larger during the night.The variation of the exchange coefficient with height is one of the predominant factors for the shape of the profiles of temperature and humidity in the air in the canopy space. The temporal variation of wind speed and exchange coefficient is also an important factor.In Section 6.4 the model results are compared with the measurements of micrometeorological factors in a maize crop for three different days. The profiles of net radiation, wind speed and leaf resistances are in sufficient agreement. However, the simulated profiles in the air showed a substantial deviation from the measured profiles.In Section 6.4.4 it is shown that this deviation is largely due to an overestimation of the relative turbulence intensity and to neglect of the effect of wind gustiness. A more correct implementation of both of them reduces the differences to much smaller values. Micrometeorological research should be concentrated on the physical knowledge of these phenomena where it is still lacking. The presented model is a sound and reliable basis for the calculation of the different components of microweather inside a crop. Its results can be fruitfully applied to simulation of crop growth and development, and of some of its pests and diseases.In future the model should be extended to other situations like grassland. Its results should be summarized in simple formulae as was done for the radiation models. A next step will be the bridging of the gap between where the crop is grown and meteorological observation site. Still it is good to realize that prediction of crop growth and yield by simulation is certainly a problem of weather forecasting, but even more a problem of understanding the physiology of plant growth.

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 equations. The daily trends and the difference between the visible and the near-infrared regions are well represented by the model. The numerical results of the model show a reciprocity relation with interesting theoretical aspects and important implications for the remote sensing technique for example. The equations for the thermal radiation field can be kept simple because the temperatures of the leaves and the soil surface are considered as state variables.An extension of the model to a scattering coefficient of unity explains the radiance distribution of the standard overcast sky (SOC). Another model extension shows the effect of a deviation from the foregoing simplifying assumption that the reflection and transmission coefficients of the individual leaves should be equal. Subsequently it is shown that over the range from a regular to a clustered leaf arrangement, the reflection and extinction coefficients in a canopy decrease, as well as the difference between extinction of visible and near-infrared radiation. Both effects are also found in literature. The modelling of light interception by plants in rows shows that the loss of diffuse light between the rows is not too serious. Simplifying formulations are derived and evaluated for this situation.The models presented in this chapter are general enough on the one hand and their results are sufficiently simplified on the other to be successfully applicable in simulation models for photosynthesis and transpiration of plant stands.In Chapter 3 the energy and mass balances of single leaves, of the soil, of the canopy and of the air inside the canopy are evaluated by a partitioning of the incoming radiant energy over the leaves and the soil surface according to the theory of Chapter 2. The distribution of the available energy between transpiration or evaporation, sensible heat loss, heat storage and photosynthesis is derived from the specific properties of plants and soil, and from the aerial conditions. The plant water content, which feeds back on the stomatal resistance, is found by integration of calculated transpiration and water uptake from the soil. The uneven distribution of the radiation over the leaves and the presumed vertical gradients of air temperature and humidity require a classification of the leaves with respect to height and to incidence of radiation. The fluxes of heat and water vapour from the leaves and the soil surface are released into the air. The aerial profiles of temperature and humidity are found by integration of the net fluxes with respect to time for each horizontal layer inside the canopy.The soil temperatures are found by a similar procedure. Here the driving force is the soil heat flux at the surface. In the soil the diffusivity for heat is taken as constant. To obtain the values of the turbulent exchange in the air the theory of Chapter 4 is used.In Chapter 4 the turbulent echange in the air inside and above the canopy is calculated.In Section 4.2 the frequently observed logarithmic wind profile is used as a basis for the calculation of the exchange coefficient above the canopy. When there is a vertical temperature gradient in the air, the logarithmic wind profile is disturbed and the values of the exchange coefficient have to be modified. The relations that express this effect, are taken from literature and reformulated for the purpose of application in the simulation model. An important result is that the formation of an inversion layer (in which the turbulent exchange is reduced or even blocked) above the canopy during the night when the net radiation is negative, can be understood and described. In Section 4.3 the turbulent exchange inside the canopy is calculated. It is found that the profiles of both wind speed and exchange coefficient are reasonably well approximated by an exponential extinction with depth. The values of the extinction coefficient derived theoretically are in good agreement with experimental data. When temperature gradients exist in the canopy the exchange coefficient is corrected in a similar way to above the canopy. There is a lower limit for the exchange coefficient when the wind speed drops to zero, as might occur under an inversion, so that the exchange inside the canopy is then maintained. In Section 4.4 the aerodynamic macrocharacteristics, i.e. the zero plane displacement d and the roughness length Zo , are expressed in the aerodynamic microcharacteristics and geometry of the crop. The theoretical results for grassland, maize and a coniferous forest appear to agree well with the aerodynamic macrocharacteristics found experimentally. From a model analysis of the effect of spatial variability it is concluded in Section 4.5 that the one-dimensional scheme is adequate as long as one is only interested in horizontally averaged figures, Temporal variations such as those caused by gustiness of wind may have quite important effects on mean values of temperature and humidity.In Chapter 5 the programming aspects are discussed. In Section 5.2 first the initialization problem is discussed, which can be solved by the concept of the cyclic equilibrium. One spatial boundary of the system is chosen deep enough in the soil to eliminate the influence of the daily heat wave, and the other one is not taken higher than 3 in above the ground surface to keep horizontal advection as small as possible. The method of conversion of the weather data measured at this level is also given. In Section 5.3, the usefulness of the hierarchical approach as well as the conditions for its application are discussed. Some examples are given. Section 5.4 gives a survey of some solution techniques together with their merits and drawbacks for different problems (see also Fig. 27). The choice of the programming language is explained. In Section 5.5 a technique is given by which the number of computations in a simulation program for a stiff system is greatly reduced. This method is employed in the micrometeorological simulation program. The listings of the programs are printed in Section 5.6.Chapter 6 gives the results of the composite model. The daily courses of some typical output variables are simulated for a specific case and explained with the submodels. Here (wet soil) the simulated soil evaporation amounted to about one third of the total evapotranspiration. The dew on the leaves mainly consisted of over-distilled soil water. During the night an inversion situation developed. A water stress in the afternoon resulted in a serious depression of the rate of CO 2 -assimilation.The sensitivity analysis (Section 6.3) showed that slow sensors may be used to record input weather data, because hourly averages are sufficiently accurate. An exception must be made for wind speed since wind gustiness may seriously affect the simulated results. Incoming solar radiation should preferably be recorded in the diffuse and the direct components separately. Amount of dew and duration of leaf wetness cannot be calculated by an easy rule. The numerous complex relations must be integrated in a simulation program to evaluate their combined effect. The compartmentalization of the soil in ten layers was sufficiently accurate, for the canopy space only three layers seemed sufficient. Among the plant properties, the scattering coefficient of the leaves is of great importance. Other important factors are the cuticular resistance, especially when the leaf area index is high, and the stomatal resistance behaviour. Compared to these factors, the leaf angle distribution is of only minor importance, (already found by de Wit (1965)) so that Ross' characterization (Section 2.3.3) is certainly sufficiently accurate. For a closed canopy the leaf area index has little influence on CO 2 -assimilation. and transpiration and the aerodynamic crop characteristics are of moderate importance.The thermal properties of the soil have a large effect on the amplitude of the soil heat flux and hence on the amplitude of the soil temperature variation, but their phases are only little influenced. The effect of the thermal soil properties on the profile in the air is negligible in the daytime, but much larger during the night.The variation of the exchange coefficient with height is one of the predominant factors for the shape of the profiles of temperature and humidity in the air in the canopy space. The temporal variation of wind speed and exchange coefficient is also an important factor.In Section 6.4 the model results are compared with the measurements of micrometeorological factors in a maize crop for three different days. The profiles of net radiation, wind speed and leaf resistances are in sufficient agreement. However, the simulated profiles in the air showed a substantial deviation from the measured profiles.In Section 6.4.4 it is shown that this deviation is largely due to an overestimation of the relative turbulence intensity and to neglect of the effect of wind gustiness. A more correct implementation of both of them reduces the differences to much smaller values. Micrometeorological research should be concentrated on the physical knowledge of these phenomena where it is still lacking. The presented model is a sound and reliable basis for the calculation of the different components of microweather inside a crop. Its results can be fruitfully applied to simulation of crop growth and development, and of some of its pests and diseases.In future the model should be extended to other situations like grassland. Its results should be summarized in simple formulae as was done for the radiation models. A next step will be the bridging of the gap between where the crop is grown and meteorological observation site. Still it is good to realize that prediction of crop growth and yield by simulation is certainly a problem of weather forecasting, but even more a problem of understanding the physiology of plant growth.

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, synchronizing cycling in the whole plant. During each cycle the root resistance increased during the sub-period with low rates of water uptake and decreased delayed at subsequently decreasing plant water potential. Due to the fact that the fall of root resistance was enhanced by increasing root temperature, both cycling of stomatal opening and cycling of root resistance were intensified.A diagrammatic conceptual model is presented, showing the pathway of the transpiration stream, of the transfer of water within the leaf, and of the stomatal control system of the water balance of the leaf. A negative feedback control circuit in it indicates how active stomatal movements function, while a positive feedback circuit shows how passive stomatal movements function during cycling. Circadian rhythm in steady and cycling stomatal openingsIn continuous light a circadian rhythm in non-cycling stomatal opening was found, which phase could be caused to shift. Cycling was most intense at the time that in similarly treated plants the maximum was attained in the circadian rhythm in the opening in non-cycling stomata and least intense half a circadian period later. In continuous darkness during the first day after the last photoperiod, stomata opened in a cyclic way with a daily pattern in the amplitudes as well as in the number of cycles. During photoperiods of 17 hours the noncycling stomatal opening in leaves, attached to plants with and without root systems, as well as in isolated leaves followed a daily pattern. During these photoperiods the modulation of amplitudes and periods showed the same daily pattern as during the circadian rhythm in continuous light. During the daily dark periods stomata remained closed. A daily pattern was observed in the onset of stomata] opening after short dark periods in the course of the daily photoperiod. The data suggest that one endogenous circadian oscillator, situated in the leaves, caused the circadian rhythm in non-cycling stomatal opening and modulated stomatal cycling in continuous light, continuous darkness and during the daily photoperiods.Environmental effects on cycling stomataThe time between the onset of light and the beginning of stomatal opening increased with increasing length of the. preceding dark period at equal phases of the circadian rhythm. After 1 hour in the light the effect of the preceding dark period had almost disappeared. After removal of the trifoliates the periods in the primary leaves gradually shortened in the course of some days. It is suggested that this increased activity was caused by a reduced competition for minerals, enzymes and proenzymes from the root.Cycling was frequently damped by increasing the irradiation intensity. Irradiation intensity beyond 34,000 erg cm -2sec -1affected the rate of CO 2 exchange, but not the period or amplitude of sustained cycling. When the irradiation intensity on the lower surface of the leaf during the diurnal photoperiod was decreased from 2100 to 800 erg cm -2sec -1, cycling continued. The amplitude decreased and the period increased, mainly due to the increase of the sub-period with minimal stomatal opening. During the photoperiod 150 erg cm -2sec -1on the upper surface of the leaf induced overshooting stomatal opening. The sub-period with minimal stomatal opening, being similar to STÅLFELT'S 'Spannungsphase' (build-up period), increased at decreasing irradiation intensity due to an increase in time required for the guard cells to exceed the counterpressure from the surrounding cells, hence, the product rule of STÅLFELT applied here.The build-up period of stomatal opening increased by decreasing the CO 2 -concentration of the air from 300 to 200 to 0 ppm.Enhancing the temperature in light decreased the period. This effect was clearly distinguished from the effect of increasing vapour pressure gradient. The Q 10 of decrease fell gradually from approximately 4 between 17° and 20°C to approximately 1.5 between 25° and 29°C. The sub-period with minimal stomatal opening was mainly affected. In light the same effect of temperature on cycling was found in CO 2 -free air as in normal air and the same on cycling in detached leaves as in attached leaves. A temperature increase in darkness induced stomatal cycling; this was assigned for the major part to the stimulation of the active opening component. Temperature is concluded to exert an influence independent of the effect of CO 2 -concentration and of the water balance.The effects of light and temperature on cycling support the existing hypothesis for an active ATP using potassium pump as a mechanism for stomatal opening.Conclusive remarksSeveral internal plant factors play a part in causing instability of the stomatal apparatus, but also the habit of the plant. Cycling must be expected to occur in the field and in the glasshouse, especially at low air humidity, when these factors favour instability. Severe stomatal cycling is suggested to decrease mean assimilation rate.

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.

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.

The transpiration of cut leaves (bean, tomato, Hyoscyamus ) in potometers was studied as affected by light intensity, leaf temperature and air humidity, with special attention to stomatal light response. Fick's diffusion law could be applied to evaporation of water and to transpiration of leaves. Evaporation was found to be proportional to Vα(V = wind velocity),αbeing 0.76 in turbulent air. Transpiration rate increased rectilinearly with light intensity below 5 x 10 4erg sec. -1cm -2, mainly through increase in leaf temperature by radiation with incandescent light, containing much infrared radiation; with fluorescent tubes, sodium lamps, or high- pressure mercury vapour lamps, transpiration increase was mainly due to increase in stomatal opening. Stomatal resistance to diffusion could be exactly determined by measurements on transpiration, leaf temperature and air humidity. Transpiration increases at higher light intensities (saturation) were entirely due to increase in leaf temperature.Regulation of transpiration by small changes in stomatal resistance to diffusion was most pronounced at small stomatal apertures. In normal air, the stomatal response to light depended on photosynthesis of the guard cell chloroplasts (use of CO 2 ) and the response of the guard cells to CO 2 (stomatal feed back, causing fluctuations in the degree of stomatal opening).

The transpiration of cut leaves (bean, tomato, Hyoscyamus ) in potometers was studied as affected by light intensity, leaf temperature and air humidity, with special attention to stomatal light response. Fick's diffusion law could be applied to evaporation of water and to transpiration of leaves. Evaporation was found to be proportional to Vα(V = wind velocity),αbeing 0.76 in turbulent air. Transpiration rate increased rectilinearly with light intensity below 5 x 10 4erg sec. -1cm -2, mainly through increase in leaf temperature by radiation with incandescent light, containing much infrared radiation; with fluorescent tubes, sodium lamps, or high- pressure mercury vapour lamps, transpiration increase was mainly due to increase in stomatal opening. Stomatal resistance to diffusion could be exactly determined by measurements on transpiration, leaf temperature and air humidity. Transpiration increases at higher light intensities (saturation) were entirely due to increase in leaf temperature.Regulation of transpiration by small changes in stomatal resistance to diffusion was most pronounced at small stomatal apertures. In normal air, the stomatal response to light depended on photosynthesis of the guard cell chloroplasts (use of CO 2 ) and the response of the guard cells to CO 2 (stomatal feed back, causing fluctuations in the degree of stomatal opening).

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, synchronizing cycling in the whole plant. During each cycle the root resistance increased during the sub-period with low rates of water uptake and decreased delayed at subsequently decreasing plant water potential. Due to the fact that the fall of root resistance was enhanced by increasing root temperature, both cycling of stomatal opening and cycling of root resistance were intensified.A diagrammatic conceptual model is presented, showing the pathway of the transpiration stream, of the transfer of water within the leaf, and of the stomatal control system of the water balance of the leaf. A negative feedback control circuit in it indicates how active stomatal movements function, while a positive feedback circuit shows how passive stomatal movements function during cycling. Circadian rhythm in steady and cycling stomatal openingsIn continuous light a circadian rhythm in non-cycling stomatal opening was found, which phase could be caused to shift. Cycling was most intense at the time that in similarly treated plants the maximum was attained in the circadian rhythm in the opening in non-cycling stomata and least intense half a circadian period later. In continuous darkness during the first day after the last photoperiod, stomata opened in a cyclic way with a daily pattern in the amplitudes as well as in the number of cycles. During photoperiods of 17 hours the noncycling stomatal opening in leaves, attached to plants with and without root systems, as well as in isolated leaves followed a daily pattern. During these photoperiods the modulation of amplitudes and periods showed the same daily pattern as during the circadian rhythm in continuous light. During the daily dark periods stomata remained closed. A daily pattern was observed in the onset of stomata] opening after short dark periods in the course of the daily photoperiod. The data suggest that one endogenous circadian oscillator, situated in the leaves, caused the circadian rhythm in non-cycling stomatal opening and modulated stomatal cycling in continuous light, continuous darkness and during the daily photoperiods.Environmental effects on cycling stomataThe time between the onset of light and the beginning of stomatal opening increased with increasing length of the. preceding dark period at equal phases of the circadian rhythm. After 1 hour in the light the effect of the preceding dark period had almost disappeared. After removal of the trifoliates the periods in the primary leaves gradually shortened in the course of some days. It is suggested that this increased activity was caused by a reduced competition for minerals, enzymes and proenzymes from the root.Cycling was frequently damped by increasing the irradiation intensity. Irradiation intensity beyond 34,000 erg cm -2sec -1affected the rate of CO 2 exchange, but not the period or amplitude of sustained cycling. When the irradiation intensity on the lower surface of the leaf during the diurnal photoperiod was decreased from 2100 to 800 erg cm -2sec -1, cycling continued. The amplitude decreased and the period increased, mainly due to the increase of the sub-period with minimal stomatal opening. During the photoperiod 150 erg cm -2sec -1on the upper surface of the leaf induced overshooting stomatal opening. The sub-period with minimal stomatal opening, being similar to STÅLFELT'S 'Spannungsphase' (build-up period), increased at decreasing irradiation intensity due to an increase in time required for the guard cells to exceed the counterpressure from the surrounding cells, hence, the product rule of STÅLFELT applied here.The build-up period of stomatal opening increased by decreasing the CO 2 -concentration of the air from 300 to 200 to 0 ppm.Enhancing the temperature in light decreased the period. This effect was clearly distinguished from the effect of increasing vapour pressure gradient. The Q 10 of decrease fell gradually from approximately 4 between 17° and 20°C to approximately 1.5 between 25° and 29°C. The sub-period with minimal stomatal opening was mainly affected. In light the same effect of temperature on cycling was found in CO 2 -free air as in normal air and the same on cycling in detached leaves as in attached leaves. A temperature increase in darkness induced stomatal cycling; this was assigned for the major part to the stimulation of the active opening component. Temperature is concluded to exert an influence independent of the effect of CO 2 -concentration and of the water balance.The effects of light and temperature on cycling support the existing hypothesis for an active ATP using potassium pump as a mechanism for stomatal opening.Conclusive remarksSeveral internal plant factors play a part in causing instability of the stomatal apparatus, but also the habit of the plant. Cycling must be expected to occur in the field and in the glasshouse, especially at low air humidity, when these factors favour instability. Severe stomatal cycling is suggested to decrease mean assimilation rate.

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.