N. Katsoulas

Effect of misting on transpiration and conductances of a greenhouse rose canopy

Abstract The influence of greenhouse humidity control on the transpiration rate ( λE c ), sensible heat flux ( H c ) and bulk stomatal conductance ( g c ) of a soilless rose canopy ( Rosa hybrida , cv. First Red) was studied in a greenhouse located in the coastal area of eastern Greece. Measurements were carried out during several days in the summer (i) without air humidity control and (ii) with a mist system operating when the relative humidity of the greenhouse air was lower than 75%. The diurnal course of g c was determined from the relation linking λE c to canopy-to-air vapour pressure deficit ( D c ) or from inversion of the Penman–Monteith equation. The two ways of estimating g c were in good agreement, showing a significant increase of g c under mist conditions. Covariation of radiation and humidity during the day caused diurnal hysteresis in λE c and g c . The hysteresis phenomena were less marked when the mist system was operating. Normalising g c by radiation removed most of the hysteresis and indicated a curvilinear stomatal response to vapour pressure deficit. The analysis of the energy partition at the canopy showed high negative values of the Bowen ratio ( β ≈−0.7) in both conditions, indicating that canopy transpiration played a major role in cooling the greenhouse atmosphere. The contribution of the mist system to total evaporative cooling was estimated to be about 20%, with only 40–50% of the mist water being effectively used in cooling. Calculation of the crop water stress index confirmed that the crop was less stressed under misting conditions. It was concluded that the prediction of short-term variations of λE c and g c in greenhouse environments must account for the magnitude and diurnal variation of air VPD.

Influence of whitening on greenhouse microclimate and crop energy partitioning

Abstract The influence of whitening a greenhouse roof on microclimate and canopy behaviour was studied during summer in a greenhouse located in the coastal area of eastern Greece. Measurements of microclimate variables, intercepted net radiation, canopy temperature and canopy transpiration rate ( λE c ) of a well-watered soilless rose canopy ( Rosa hybrida , cv. First Red) were carried out over several days before and after roof whitening. Whitening reduced the average glasshouse transmission coefficient for solar radiation from 0.62 to 0.31. As a consequence, air temperature, vapour pressure deficit and canopy-to-air temperature difference experienced drastic changes while transpiration rate was not strongly affected, being slightly higher (about 18%) after whitening. The Bowen ratio β was highly positive before whitening ( β ≈0.6) and negative ( β ≈−0.5) after whitening. Calculations of the canopy stomatal conductance ( g c ) and crop water stress index clearly showed that the crop was significantly less water stressed after whitening. It was found that the values of g c doubled the day after whitening and tended to increase slightly afterwards. The latter may be ascribed to the recovery of the crop and the emergence of new and healthy leaves. Globally, glass whitening was inexpensive, presented positive effects on both microclimate and crop behaviour and can be considered as an efficient means for alleviating the large heat load during summer in warm countries.

INFLUENCE OF AN INSECT SCREEN ON GREENHOUSE VENTILATION

The influence of an insect screen on ventilation rate was experimentally investigated in a multispan glass–covered greenhouse equipped with a continuous roof vent, located at the University of Thessaly near Volos in the coastal area of eastern Greece. Microclimate variables as well as the ventilation rate were measured during summer. Two measuring techniques were used for the determination of ventilation rate: (1) the decay rate tracer gas technique, using N2O as tracer gas, and (2) the water vapor balance technique. The influence of the insect screen on ventilation rate was studied using a wind–related coefficient identified by fitting a simple linear model to the experimental values. The two measuring techniques gave similar results, but the water vapor balance technique provided a better fit to the experimental data. The wind–related coefficient significantly decreased when an insect screen covered the vent. Finally, the influence of the insect screen on the discharge coefficient was investigated. The discharge coefficient was correlated to the aerodynamic properties of the screen using porous media flow analysis.

Effects on microclimate, crop production and quality of a tomato crop grown under shade nets

Summary The effects of four shade nets with different colour transmission properties and shading intensities (two black nets with shading intensities of 40% and 49%, a green net with a shading intensity of 34%, and a mixed green and black shade net with a shading intensity of 40%) on the screenhouse microclimate, and on the growth and development of a tomato crop, were investigated. Experiments were carried out during Spring and Summer 2003 in New Anchialos, a coastal area of Eastern Central Greece. Tomato seedlings were planted in soil and the following environmental variables were recorded at regular intervals: solar radiation, air temperature and relative humidity, leaf temperature, and several variables related to crop growth and development. Air temperature and vapour pressure deficit (VPD) values were similar under all four shade nets and without shading. However, canopy temperature and, accordingly, the canopy-to-air VPD were significantly lower under shading than without shading. Shading did not reduce the total plant dry matter content, but increased the leaf area index, the number of fruit per plant, and the total fresh tomato yield. Shading reduced losses caused by tomato cracking by 50%, and thus increased the marketable tomato fruit yield by approx. 50% compared to growth under non-shaded conditions.

INFLUENCE OF AN ALUMINIZED THERMAL SCREEN ON GREENHOUSE MICROCLIMATE AND CANOPY ENERGY BALANCE

Greenhouse microclimate, energy savings, and crop transpiration were investigated during winter in a glass-covered greenhouse cultivated with a rose crop and equipped with an aluminized thermal screen. Air temperature and humidity profiles were recorded at 0.3, 0.8, and 1.8 m heights inside the greenhouse. Net radiation over and under the rose crop, heating pipe temperature, canopy temperature at 0.3 and 0.8 m, and transpiration rate were simultaneously measured and recorded. When compared to unscreened conditions, it was found that the thermal screen provided a more homogeneous microclimate and increased the average air temperature and canopy temperature at 0.8 m by about 2.5.C and 3.0. C, respectively. The latter result was attributed to the observed increase (about 100%) in the net radiation absorbed by the canopy, along with reflection of long-wave radiation by the thermal screen. Higher canopy-to-air vapor pressure deficits (+0.3 kPa) were observed in the lower layer (bent shoots) under screened conditions. Canopy transpiration rate was of the same order of magnitude in both cases. Energy savings due to the screen was estimated to be about 15%. The body of results underlines that the basic effect of the studied screen on crop behavior was the doubling of the net radiation absorbed by the canopy, with positive consequences on both air and canopy temperature, and therefore on growth, development, and sanitary conditions of the rose plants.

THE IMPACT OF INSECT SCREENS AND VENTILATION OPENINGS ON THE GREENHOUSE MICROCLIMATE

The objective of this work was to experimentally investigate the influence of vent type (side, roof, or both) and of anti-aphid insect screens on airflow, air temperature, and air vapor pressure deficit distribution in a round arch, mono-span greenhouse with vertical side walls. The greenhouse was equipped with two side roll-up vents and a flap roof vent. A tomato crop planted in double rows was cultivated inside the greenhouse. The three components of air velocity were measured by a 3-D sonic anemometer, and the air temperature and relative humidity were simultaneously recorded at several positions inside the greenhouse. Concerning the effect of insect screens, it was found that the mean value of the normalized air velocity was 58% lower in the greenhouse with insect screens on the side vent openings than in the case of a greenhouse without screens. Furthermore, the spatial heterogeneity of the microclimate variables was reduced with screens in the vent openings. When the ventilation was provided by side openings only, the air velocity inside the greenhouse was characterized by a strong air current near the greenhouse ground and low air velocity near the roof; when the ventilation was provided by roof vents, a circulating current prevailing at the center of the greenhouse was observed. The combined use of roof and side openings increased air velocity and decreased air temperature inside the greenhouse but also increased the spatial heterogeneity of the greenhouse microclimate compared to the cases with side or roof vents only. The most homogeneous climate conditions were achieved with the use of roof openings only. The results of this study provide a better understanding of the plant environment behavior under different vent configurations and a high-resolution database for validating on-going efforts with computer simulations.

Greenhouse Crop Transpiration Modelling

Aim of this chapter is to present the parameters affecting greenhouse crop transpiration and the existing models for greenhouse crop transpiration simulation. In the first paragraphs of the chapter, the importance of crop transpiration on greenhouse microclimate and on crop is presented and discussed. Presentation, analysis and discussion of the parameters affecting greenhouse crop transpiration and the thermal and hydrological negative feedback effects follow. Finally, the existing models for greenhouse crop transpiration simulation are presented and discussed. Transpiration is an important component of canopy energy and water balance and thus, a major cooling mechanism of greenhouse crop canopies. Its estimation is essential for climate and irrigation control and that is why it has been given much attention in greenhouse climate research. Air temperature and vapour pressure deficit are parameters affecting the thermal and hydrological negative feedback effects existing in a greenhouse. In addition, the main factors affecting greenhouse crop transpiration are solar radiation, vapour pressure deficit and canopy and aerodynamic conductances. Several authors have proposed models that allow getting a more accurate estimation of the crop transpiration rate. More sophisticated transpiration models are based on leaf (canopy) transpiration and leaf energy balance models in which the transpiration is characterized by the canopy resistance, as proposed initially by Penman and modified by Monteith to account for the stomatal response of the crop (P-M formula). However, the use of the complete P-M formula requires the knowledge of several inputs or parameters that are not easily available. Particularly, the aerodynamic and stomatal leaf resistances have to be known for each crop species and possibly, for each cultivar. That is why researchers have tried to overcome the estimation of these resistances by using a simplified form of the P-M formula. Transpiration models with the greenhouse climate as a boundary condition were first developed in the northern regions of Europe and North America for horticultural crops. In these northern conditions, the glasshouse is generally poorly ventilated during a large part of the growing season. The boundary layer conductance for glasshouse crops tends to be much smaller than would be expected for similar crops growing outdoors. Thus, glasshouse crops are very strongly decoupled from the outside atmosphere by the presence of the glass, and the heat and the water released at crop surface will accumulate inside the

Electrical Conductivity and pH Prediction in a Recirculated Nutrient Solution of a Greenhouse Soilless Rose Crop

ABSTRACT The influence of nutrient solution (1) mixing rate, and (2) time of use on pH and electrical conductivity (EC) of a recirculated nutrient solution used for the irrigation of a greenhouse soilless rose crop was studied. Measurements of microclimate variables, pH, and EC of nutrient solutions and crop transpiration were conducted. The measurements of pH and EC values of nutrient solutions mixed with different mixing rates and applied for crop fertigation were used to develop and calibrate a model for pH and EC prediction in relation to nutrient-solution mixing rate and time of use. Application of the calibrated model gave satisfactory results. It was found that nutrient solutions with high mixing rates or volume equal to or double that of the total water consumed by the canopy during the conservation period had the most stable EC evolution and minimal pH changes.

Shading Effects on Greenhouse Microclimate and Crop Transpiration in a Cucumber Crop Grown Under Mediterranean Conditions

The aim of this study was to investigate the effects of greenhouse shading on greenhouse microclimate and energy balance, and on crop production. Experiments were carried out in the experimental farm of the University of Thessaly at Velestino, in three similar, plastic-covered greenhouses using hydroponically-grown cucumbers as a test crop. One of the greenhouses was used as a control (without shading); the other two were shaded using two different shade nets (shading intensity of approximately 35% and 50%, respectively). Climatic parameters were measured during two growing seasons from April to June and from September to November 2008 and seven selected days of the above periods are presented. The results showed that shading could not keep greenhouse air temperature and vapor pressure deficit below 30°C and 1.5 kPa, respectively, values that are considered acceptable for cucumber crop growth (Growers Books, 1980; Bakker et al., 1987; Olympios and Hanan, 1992). From the crop production data it was found that shading intensity should not exceed 35%. The analysis of greenhouse microclimate and energy balance showed that shading is necessary from the middle of spring, while even shading of approximately 50% was not sufficient to cool the greenhouse during noon time of summer days in Central Greece and that an additional cooling system was required.

Greenhouse Microclimate and Soilless Pepper Crop Production and Quality as Affected by a Fog Evaporative Cooling System

The influence of greenhouse humidity control on greenhouse microclimate, crop transpiration rate, and yield and fruit quality of a soilless grown pepper crop was studied in a greenhouse located at the coastal area of western Greece. Measurements were carried out during summer and autumn in two distinct greenhouse compartments involving: (1) no air humidity control and (2) a fog system operating when the relative humidity of the greenhouse air was lower than 80%. Under fog conditions, the greenhouse air and the crop leaf temperature were about 3°C lower than those measured under no fog conditions. The fog system allowed maintenance of the greenhouse air temperature under 30°C, while a maximum value of about 35°C was reached under no fog conditions. In addition, under fog conditions, the air vapor pressure deficit was lower than 2 kPa, even during the warmest part of the day, while under no fog conditions it reached values near 4 kPa. However, the transpiration rate was not affected to such an extent by fog as the air vapor pressure deficit, and under fog conditions it decreased by about 26%. This is attributed to the values reached by the bulk stomatal conductance, which were about 1.5 times higher under fog than under no fog conditions. The crop leaf area index values observed after the middle of the experimental period and later, under fog conditions, were higher than that observed under no fog conditions. Finally, the fog system enhanced the mean fruit weight and the percentage of marketable fruits but reduced appreciably the total number of fruits per plant. The free (titratable) acidity and the total soluble solids in the pepper fruit sap were slightly reduced by the fog cooling system, while the fruit size was increased. The use of a fog cooling system proved to be beneficial for summer crops of pepper grown under Mediterranean climatic conditions due to a favorable impact of the reduced vapor pressure deficit on both the mean fruit weight and the quality in terms of fruits graded Class I. Nevertheless, the high air humidity imposed by a cooling fog system may reduce the number of fruits per plant, thereby offsetting the benefits from the increased mean fruit weight in terms of total yield.