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- Author or Editor: Laurent Gauthier x
In soilless culture, the buffering capacity of the root environment for nutrients is low. This, combined with fluctuations of climatic factors and changes in nutrient uptake rates, can lead to nutrient imbalances. In order to achieve high yield and better quality, it is necessary to keep the nutrient concentrations in the root environment at the target levels. This requires frequent analysis and adjustments to the nutrient solution. Currently, leaching of the growing media or renewal of the nutrient solution is commonly used to avoid accumulation or depletion of nutrient in the root environment. However, this practice lowers the efficiency of fertilizers and can lead to the contamination of the ground water. One way to remedy to this problem is through the use of nutrients uptake models to track the composition of the nutrient solutions. The objective of this study was to develop such models. Such models can be used to maintain balanced nutrient solutions for longer periods. This can lead to reduced leaching and improved fertilizer use efficiency. Macronutrient (N, P, K, Ca, and Mg) uptake models were developed for tomato plants grown in an NFT system using data collected from experiments conducted in the Laval Univ. greenhouses. Analysis of the experimental results showed that the main factors affecting nutrients uptakes are light and transpiration.
To achieve high yield and better quality of soilless greenhouse tomato, it is necessary to keep the nutrient concentrations in the root environment at the target levels. Dynamic control of the nutrient solution composition can be used for this purpose. We developed a computer program that dynamically adjusts nutrient solution compositions based on various climatic and agronomic characteristics. The program integrates nutrient uptake and crop transpiration models and is part of a general-purpose greenhouse management and control software system developed at Laval University (GX). The architecture of the system and some simulation results comparing the effect of various control scenarios on the evolution of the composition of nutrient solutions are presented.
A software system (SAGE) was built for on-farm decision support. The objective was to provide a framework for constructing and deploying knowledge-based decision support in the areas of integrated pest management, fertilization, and field operations. The framework is open by design and includes a generic model of an agro-ecosystem as well as various mechanisms allowing for the continued growth in scope and function of the software. The SAGE system is designed to provide a number of building blocks and predefined decision-support strategies that can be adapted to specific needs and situations. It operates on a personal computer and is based on the use of an objectoriented technology for software construction and operation. A prototype of the system has been built and is being used to build commodity-specific decision-support modules.
A dynamic management strategy for supplemental lighting in greenhouses was developed. It makes use of a plant growth model and of a rule-based decisionmaking protocol within the framework of a generic greenhouse climate management software system. The model, an adapted version of SUCROS87, tracks plant growth and predicts dry weight production based on measured or estimated values of light intensity, temperature, and CO2 concentration. A set of logical predicates (rules) implements the strategy's behavior. Optimization of lamp use was conducted as a function of economic criteria that enable a comparison between the additional income associated with yield increases due to supplemental lighting and incurred cost increases. Although the model is not perfectly reliable in its predictions, the system can be used to simulate the effect of changes to economic parameters on the decisions of the management strategy. The dynamic strategy described here differs from conventional supplemental lighting scenarios in the sense that it increases the length of the period of supplemental lighting when the daily solar light integral is low, and reduces or eliminates the use of supplemental lighting when the weather forecast predicts that the daily solar light integral will exceed plant requirements.
Transpiration is essential to the performance of tomato plants. In greenhouses, transpiration can be impeded by low vapor-pressure deficits (VPD). An experiment was conducted to measure the effect of VPD on transpiration rates for greenhouse tomatoes grown on a nutrient film. Four treatments were applied: high (0.8 kpa) day and night VPDs; high day and low night (0.4 kPa) VPDs; low day and low night VPDs; and variable VPDs. The VPD was controlled using fogging and ventilation. Hourly transpiration values were recorded. Results show a significant difference between treatments. The measured transpiration rates were compared to the values calculated with a transpiration model. A good fit between measured and calculated values was observed. The model is being used within a dynamic VPD control strategy.
Tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in peat bags, rockwool slabs, and NFT in a greenhouse to examine the effects of nutrient solution electrical conductivity (EC) and potential evapotranspiration (PET)-dependent EC variation on plant water relations. Peat bags were irrigated by a PET-dependent irrigation system. EC was varied from 1 to 4 mS·cm-1 according to PET under –5 and –9 kPa of substrate water potential setpoints (SWPS). The plants in rockwool and NFT were treated with ECs of 2.5, 4, and 5.5 mS·cm-1. Peat bags and rockwool slabs were overwatered once a week to wash out the accumulated salts. Leaf water potential (ψ1) and relative water content (θ) were measured before and after plants were overwatered. Turgor (P) and osmotic π potentials were estimated from the pressure-volume method. Before plants were overwatered, ψ1 was significantly lower in the plants with high EC and low SWPS treatments and also lower in variable EC-treated plants, but P maintained close to the control value. After plants were overwatered, ψ1 recovered close to the control level and P became higher because of the lower π in the treatments of high EC, variable EC, and/or low SWPS. At a given ψ1 the plants with high EC, variable EC, and/or low SWPS maintained higher θ. The analysis of the pressure-volume curve showed that the leaves treated with high EC, variable EC, and/or low SWPS had higher turgid water content, higher symplasmic (osmotically active) water content, lower apoplasmic (osmotically inactive) water content, and lower θ point of zero turgor (incipient plasmolysis). Maintenance of P after overwatering was directly proportional to photosynthetic capacity. We suggest that osmotic adjustment occurs in response to high EC, low SWPS, or both and that overwatering substrates and varying EC can not only avoid salinity stress, but also improve turgor maintenance.
`Capello' tomato plants (Lycopersicon esculentum Mill.) were grown in a greenhouse in peat-based substrate (70% sphagnum peat and 309'. perlite, by volume) and supplied with nutrient solutions of high (4.5 mS·cm-1) or low (2.3 mS·cm-1) electrical conductivity (EC) under high (95% ± 5%) or low (55% ± 8% of capillary capacity) soil water conditions. Three weeks after treatments started, stomatal transpiration (TRst) and cuticular transpiration (TRcu) rates were measured by three methods: 1) analyzing TRst and TRcu from a water retention curve obtained by drying excised leaves in air under a photosynthetic photon flux (PPF) of 400 μmol·m-1·s-1, 2) analyzing TRst and TRcu from a transpiration decline curve obtained by measuring transpiration rates after cutting the leaf from the stem of the dehydrated plant in the gas-exchange system, and 3) measuring transpiration rates under light and in dark respectively using the gas-exchange method. TRst and TRcu were decreased by high EC and/or low soil water content. For method 1, the transpiration decline curve shows two distinct phases: the initial steep slope that indicates TRst and the gently sloped section that indicates TRcu. Both slopes were lower for high EC and/or water-stressed plants compared to the control (low EC and high soil water content). The tangent lines of these two phases of the curve intersect at one point (t, w). The value oft that indicates the time for stomatal closure was longer and the value of w that indicates the critical tissue water level for stomatal closure was lower for high EC and/or water-stressed plants. In method 2, the initial rate of total transpiration was higher in high EC and/or water-stressed plants. Leaf wax content increased, especially under high EC stress. This suggests that increased deposition of wax prevents water loss from the cuticle. A delay in complete stomatal closure, complete closure at lower RWC, and reduced TRcu or an increase in wax deposit were adaptations to water and salinity stresses in tomato plants under our controlled environmental conditions.
Greenhouse tomato plants (cv. Capello) were grown in peat moss based substrate and treated with high and low nutrient solution electric conductivity (EC) under high (95±5%) and low (55±8%) soil water conditions. Photosynthesis (Pn) was decreased about 25% 1 day after soil water content reached 55%. However, as the soil water deficit was prolonged, the effect of water deficit diminished, with the decreasing extent of ca 15% on the 11th and 16th days from starting treatments. This suggested that tomato plants acclimated to soil water deficit. One day after soil water content reached 55%, leaf turgor potential (TP) decreased largely as leaf water potential (WP) dropped. However, as soil water level was kept constant for a period, TP recovered to a quite large extent even at the same WP level. This TP recovery accounted for the photosynthetic recovery. Drying the constantly well watered plants to the same soil water level decreased Pn to a much larger extent than did the prolonged soil water deficit. This supports the above-mentioned photosynthetic acclimation. The effects of high EC were not observed 1 day after the treatments started, but became larger and larger as the treatments were prolonged. Build-up of EC in the soil accounted for this result