OPTIMALIZATION OF OXYGEN LEVELS IN ROOT SYSTEMS AS EFFECTIVE CULTIVATION TOOL

Autor: W. Holtman, C. Blok, A. Blaakmeer, B. van Duijn
Rok vydání: 2005
Předmět:
Zdroj: Acta Horticulturae. :57-64
ISSN: 2406-6168
0567-7572
Popis: To investigate the influence of oxygen concentrations in the root system on plant development, young cucumber plants were grown during three weeks on stone wool blocks. A continuous flow of nutrient solution (0.75 L h), containing 0.5, 3.5 or 10 mg L dissolved oxygen, was led through the substrates. Already after 3 days, the impact of the various oxygen levels in the root systems became evident. Plant development was reduced when plants were subjected to the lowest oxygen concentration. In time, increasing differences in leaf area were observed when plants were grown under different oxygen levels, showing largest leaf area at 10 mg L oxygen. Also, root mass was significantly reduced when plants were grown under low oxygen conditions. Bacteria and other micro-organisms that consume oxygen can reduce oxygen levels. Oxygen consumption was detected in samples that were taken at different places in greenhouses, e.g. irrigation water storage tanks, irrigation supply system and substrates. During the winter period, the contribution of micro-organisms in oxygen consumption was low. However, in April oxygen consumption in the samples was significantly higher. Sometimes, oxygen in nutrient solution, present in the substrates, was fully consumed within 30 minutes, indicating that roots would suffer anoxia. Frequent refreshment of nutrient solution in the irrigation supply system had a major impact on oxygen levels. Continuous measurements showed that during the day oxygen levels in the irrigation system were high just after refreshment of the nutrient solution, but between two refreshments oxygen levels in the system dropped towards very low oxygen values. If technically feasible, in future the control of oxygen levels in root systems may become a new tool for growers to manage cultivation in horticulture, in addition to light, temperature, carbon dioxide, nutrition and water. INTRODUCTION In horticulture, many parameters, such as PAR light, air humidity, air temperature, carbon dioxide, water content in the slab, and the drain of nutrient water, are measured. Most parameters, which are used to feed back process computers, are derived from environmental conditions in the green house, while besides water content, EC and temperature, conditions in the root environment inside substrates are poorly taken into account. This may be surprising since the root system is used for uptake of nutrients and water, and is essential for growth. Many processes inside the roots require energy, which is generated by oxygen dependent dissimilation. Hence, depletion of oxygen in the root system could result in reduced root activity and development and thereby growth retardation, and eventually reduced yield. Additionally, hypoxia may result in enhanced susceptibility to diseases (Zheng et al.). Measurements of oxygen in the root system has been focused mainly on monocots (Armstrong and Gaynard, 1976; Saglio et al., 1984). Research on oxygen supply in substrates in horticulture is scarce, and was hampered for long time due to the lack of a reliable sensor, which could measure oxygen in the root environment. Electrochemical oxygen sensors may be useful in those locations of the green house where water is constantly moving, but these sensors are not useful in a non-stirred environment such as substrates. First of all, electrochemical sensors do consume oxygen, and therefore affect the oxygen levels in its direct environment. Secondly, these sensors need frequent calibration, and are fragile due to the membrane present. The introduction of an optical oxygen sensor, a few years ago, could Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 58 overcome most of the limitations of the electrochemical sensors. The optical oxygen sensor is robust, it does not consume oxygen, and needs little maintenance, which makes it useful for measurements in substrates. Figure 1 shows an optical sensor, which we developed for the use of measurements non-stirred liquid environment, such as substrates. The availability of the optical oxygen sensor gave us the opportunity to investigate the relevance of oxygen in the root environment for plant quality. So, primarily the aim of the research presented here was to carry out an experiment, in which could be determined whether oxygen supply in the root environment is essential for plant development (proof of principle). In addition, we have investigated oxygen profiles at different locations in the green house, e.g. irrigation water storage tanks, irrigation supply system and substrates. Finally, the contribution of micro-organisms in oxygen consumption was determined on different locations, and during different periods during the growing season. It will be discussed, which management actions can be taken by growers to control oxygen levels in substrates. MATERIALS AND METHODS Growing Conditions for Proof of Principle (Experiment 1) To investigate the role of oxygen levels in the root environment for plant development and plant quality (proof of principle), young cucumber plants (Cucumber sativis L. cv. Aviance) were grown in a green house on stone wool blocks. Therefore, cucumber seeds were germinated on vermiculite, and after 10 days seedlings were transferred to stone wool blocks, attached in special Perspex boxes, for further growing. An experimental set-up was used that allowed us to keep all root environment parameters equal, except for the oxygen concentration. The stone wool block was placed in a first box that had an inlet on top of the stone wool block, and an outlet at the bottom (solution flow through the system). The first box was placed inside a second box with an outlet at the level of the nutrient solution of the first box. This made it possible to subject the growing plant to a continuous flow (0.75 L h) of standard nutrient solution for cucumber (Fig. 2). Plants were grown on nutrient solution, which contained either 0.5 mg L dissolved oxygen, 3.5 mg L per, or 10 mg L. An oxygen concentration of 0.5 mg Loxygen represents a situation of anoxia, while 10 mg L represents a situation of saturation with oxygen. Previous work (data not shown) has shown that 3.5 mg L dissolved oxygen represent a critical level, meaning that below these concentrations reduction of metabolic processes inside root cells may occur, resulting in reduced activity. Nutrient solution was pumped to the plants from storage tanks of about 300 liter. For each condition 5 plants were cultivated, which were randomly located in the green house to avoid differences in plant growth due to local climate differences. Using the optical oxygen sensor (Fig. 1), oxygen levels inside the substrates were controlled continuously. In Figure 2 the set up is depicted. Plant Measurements for the Proof of Principle (Experiment 1) After 3 weeks, the 15 cucumber plants (5 for each oxygen condition) were harvested, and the total leaf area was determined for each plant. Detection of Oxygen Levels for the Proof of Principle (Experiment 1) Oxygen levels inside the substrates were detected using the optical oxygen sensor (Fig. 1). The principle of the optical sensor is based on an oxygen sensitive dye, which fluorescence changes depend on oxygen pressure. Consumption of Oxygen by Micro Organisms (Experiment 2) To determine the contribution of micro organisms to oxygen consumption in the water system of a green house, samples were taken at different locations, e.g. irrigation water storage tanks, irrigation supply system. In the green house where the samples were taken, productive cucumber plants (Cucumber sativis L. cv. Aviance) were cultivated. Samples were taken both in February, and in April. Samples (1 mL) were taken with a syringe, and concentrated 10 fold on a 0.2 μm filter. The filter, containing concentrated nutrient solution, was used for oxygen measurements. For oxygen measurements, a measurement device was
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