Gas Concentration in the Root Zone of Rosa hybrida L. Grown in Different Growing Media

The subject of this study was to get more information about the root zone, mainly the gas composition in growing media, which is used in hydroponic systems. Besides the plant growth parameters of roses, yield and nutrient uptake, the results of the gas concentrations in the root zone are presented. Gas sampling at 2 or 3 heights from root zone did function well, results ranged from 530 ppm to 1570 ppm CO2. Results are significant between the different substrates and measurement spots within one substrate. The organic substrates “Coir” (coconut fibre) contains most CO2. From top to the bottom the CO2 level was increasing. The CO2 level is influenced by microbial and root respiration. But compared with recommended figures the determined CO2 concentration is no limiting growth factor. The O2-concentration in substrates was determined between 39 % to 99 % saturation of disolved oxygen (DO). The daily time course was for each substrate different and specific. The results shown a influence by the plant activity (radiation) and the irrigation schedule. The Coir substrates contained with about 40 % significant less oxygen than inert substrates (Perlite, Grodan and Sawagrow). O2-level of substrates can be an indicator for oxygen deficiency direct in root zone. During the experiment the O2-concentration decreased from the top to the bottom of substrates; for CO2 the opposite trend was found. The new method of monitoring the oxygen level in substrates via fiber optic sensors did function well. The artifact of oxygen deficiency correlating with low yield or quality wasn’t found during the experiment. Experiments should be continued with different oxygen levels in substrates due to excess irrigation or substrates with high water content levels to get deficiency conditions for plants roots and significant results. INTRODUCTION The use of substrates in horticulture has greatly developed recently, mainly because: i) growing media are less prone to soil-borne diseases than soils; ii) their uniform nature allows better control of their nutritional status than soil; iii) the inherent high porosity of most substrates and their high available water content, enable the use of high water potentials without risks of root hypoxia (Raviv et al., 2002). But the plant growth, nutrient uptake and yield in hydroponic systems, with restricted root environment, and consequently, low buffering capacity is more related to water, nutrient solution, and oxygen supply. Water and nutrients are supplied with the nutrient solution, which is regulated by the composition of the solution and the irrigation control. Available oxygen is mainly determined by the layout of the hydroponic system and the physical substrate properties. The oxygen diffusion rates into the water depends directly on volumetric air content, the partial oxygen pressure, and temperature. Within the hydroponic systems there is an oxygen gradient for design flow techniques and flow rates (Vestergaard, 1984; Bunt, 1991; Baas et al. 2000; Wever et al., 2000). In hydroponic systems, the plants are grown in low volume of substrates. For this reason high frequencies with and even excess watering (about 30%) is needed and used for irrigation control. In consequence the substrate is always almost completely saturated with nutrient solution. During summer, when plants have high water consumption, the solution passes through the substrates a few times per day. In periods with low radiation, the water consumption is low and solution exchange limited as well as the gas exchange and oxygen supply. In addition, the substrate parameters change at the end of each crop, mainly as result of decomposition of organic substrates or root and increasing root mass (Wever and van Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 2005 50 Leeuwen, 1995). Oxygen deficiency may be a limiting factor for plant growth. Otherwise oxygen deficiency can be tolerated by roots for a short time, even for several days (Buwalda, 1991). Plants roots require dissolved oxygen (DO) for respiration, nutrient and water uptake. The growth of most horticultural species was reduced as DO in solution decreased below 60% of saturation, while below 2.5% root tips die and growth is arrested (Jackson, 1980; Soffer et al., 1991). In addition, oxygen is consumed by microorganisms inhabiting the root zone solution (Vestergaard, 1984). Accumulation of root respiration products (mainly CO2) can inhibit plant growth (Strojny et al., 1998). CO2 concentration of 10,000 ppm can partially inhibit root growth while 1000 ppm may stimulate it (Geisler, 1963; Radin and Loomis, 1969). Investigations with soilless systems have shown high CO2 in the root zone (Schroeder, 1994; Schroeder and Lieth, 2004). The CO2 in the water film surrounding the roots can be higher than in the bulk of the substrate. Adequate aeration of the root zone is required for oxygen flow from the atmosphere, into the gas phase of the medium and dissolution into its liquid phase. Media traits and irrigation regime can affect O2 and CO2 in the root zone (Schwarz, 1995; Strojny et al., 1998). Available oxygen is mainly determined by the layout of the growing system and the physical substrate properties. Oxygen diffusion rates (ODR) depends on volumetric air content, partial oxygen pressure, and temperature. Different substrates have different ODRs (Vestergaard, 1984; Bunt, 1991; Baas et al. 1997; Wever et al., 2000; Wever and van Leeuwen, 1997). The process of gas exchange in substrates is firstly driven by diffusion through the pores of the growing medium along the partial pressure gradient in the gas phase. Large pores permit air entry into the medium shortly after irrigation. The second stage of the process is the dissolution of oxygen into the films surrounding the roots. The fact that oxygen may be supplied to the rhizosphere in a dissolved state by frequent mass flow of oxygen-saturated water is one of the main factors justifying frequent irrigation regime. Stagnant water cannot supply sufficient oxygen for most horticultural crops. Moreover, the inherent low volume of containerized systems, coupled with the intensive growth rate of most horticultural crops and the high greenhouse temperatures, may result with oxygen deficiency. Therefore a concomitant study of water and oxygen dynamics is important even in porous media. MATERIAL AND METHODS The experiments with Rosa hybrida L. ‘Sacha’ were carried out from April to July 2003 at the University of Applied Science Dresden (Germany). To investigate the plant growth and root zone, a hydroponic system with standard drip irrigation and different substrates (Table 1) was used. All substrates were used like in commercial practice, Grodan and Sawagrow are available as a slab, Perlite and Coir were filled up in containers of 10 liter. The plant density of 7.6 plants/m2 was the same. The climate and the nutrient solution supply in the climate cabins was monitored by computer. A standard nutrient solution (Sonneveld and Straver, 1988) was used adjusting EC and pH. The irrigation control was based on time and radiation, independent of the substrate. The bending technique of roses was used. The total yield and nutrient content of roses (dry matter) was determined. For analyses of carbon dioxide (CO2) and dissolved oxygen (DO) different methods were used. For analyses of carbon dioxide a gas sampling system was used to get gas samples from the root zone. Gas sampling cells are located at 2 heights in slabs (2 and 4 cm from the bottom) and at 3 heights in containers (5cm, 10 cm and 15 cm from the bottom). The gas sampling cells of 40 ml were air tight at one end and the other and closed with a tape so a plastic syringe could used to take air samples. CO2 gas samples of 20 ml were analyzed with an infrared (IR) sensor Multiwarn II (Dräger, 2004). The DO was measured in substrates with a new fiber optic mini sensor and the “Fibox 3” device (PreSens, 2004), which allows an online measurements in situ monitored by computer. The principle of the measurement is based on the effect of dynamic luminescence quenching by molecular Oxygen. The collision between the luminophore in its excited state and the oxygen results in radiation less deactivation. A relation exists between the oxygen concentration in the sample and the luminescence intensity. The oxygen results representing the dissolved oxygen in solution, as percentage of dissolved oxygen saturation, at the surface