Effects of Plant Root Oxygen Deprivation in Relation to Water and Nitrate Uptake for Rose

Plants need oxygen to perform cellular respiration. Plants absorb oxygen through their roots. Past research has shown that reducing the concentration of oxygen in the rootzone of hydroponically grown rose plants will compromise the plants’ ability to absorb nitrate and water, although this effect has not been quantified. The objective of this research was to quantify the effects of different oxygen concentrations in the rootzone on water and nitrate absorption rates. It was hypothesized that absorption rates would be reduced at the point at which the oxygen concentration in the rootzone became a limiting factor on the plants’ ability to perform cellular respiration. Hydroponically grown rose plants, Rosa hybrida ‘Kardinal’, were exposed to different oxygen concentrations and the water and nitrate absorption rates of each plant were measured. No noticeable correlation between water and nitrate absorption rates and rootzone oxygen concentration were observed. These results were contrary to past research and have led to the conclusion that data at lower concentrations of oxygen must be gathered to demonstrate a critical oxygen concentration for water and nitrate absorption. Data from lower oxygen concentrations may demonstrate the point at which the rootzone oxygen concentration becomes a limiting factor on cellular respiration. INTRODUCTION The commercial use of hydroponics for production of food and ornamental crops has increased in recent years. In 2000, the worldwide area of hydroponically grown agriculture was about 25,000 ha and was considered to be increasing (Jeong et al., 2001). Hydroponics is an effective management intensive system for crop production that allows environmentally sensitive water management. Optimization of many hydroponic techniques is still under development because the methods are relatively new to commercial production. Significant research has been completed regarding optimal fertilization and irrigation (fertigation) for plants (Lieth and Burger, 1989; Chimonidou Pavlidou and Papadopoulos, 1999; Malorgio et al., 2001; Raviv et al., 2001; Jovicich et al., 2003; Venezia et al., 2003). Plants require oxygen for aerobic cellular respiration to produce energy consumed during growth Research has shown that reduced oxygen (O2) concentrations in the rootzone reduce the plant absorbtion of nitrate and water (Bradford and Hsiao, 1982; Smit and Stachowiak, 1988; Morard and Silvestre, 1996; Kozlowski, 1997; Visser et al., 2003), but the concentration at which absorption rates are reduced have not been determined. The objective of this research was to quantify the effects of different oxygen concentrations in the rootzone on the water and nitrate absorption rates of rose plants. Outcomes will help commercial flower growers efficiently manage fertigation schedules. Efficient management of irrigation and fertilization will reduce water and fertilizer waste and make commercial hydroponics more economical and environmentally sensitive. MATERIALS AND METHODS Four six-month-old rose plants, Rosa hybrida ‘Kardinal’, were grown from cuttings hydroponically in a growth chamber. Each plant was grown in an 8-liter Proc. XXVII IHC-S5 Ornamentals, Now! Ed.-in-Chief: R.A. Criley Acta Hort. 766, ISHS 2008 54 hydroponic unit filled with modified half-strength Hoagland’s solution (Hoagland and Arnon, 1950). The nutrient solution was changed three times per week to replenish absorbed nutrients. Plants received 1200 μmoles m s of light from 3 HID lamps from 0100-1600 HR and the temperature was maintained at 25°C while the lights were on and 18°C while the lights were off. Each plant was subjected to a different concentration of oxygen in its nutrient solution. These concentrations varied between 2.5 and 8 mg/L. No one plant was held at extreme low or high oxygen concentrations for an extended period of time. Instead, the assignment of the oxygen concentration each plant received was changed three times each week on a rotating schedule; therefore each rose plant was given time to recover from possible periods of oxygen stress in the rootzone. The desired oxygen concentrations were programmed into a Campbell Scientific CR23X datalogger. The datalogger continuously recorded the oxygen concentration of each pot and maintained the desired oxygen concentration by regulating the bubbling of air and nitrogen gas into the pot. Nitrogen gas generated by a Balston HFX-1 nitrogen generator was bubbled through the nutrient solution to reduce the oxygen concentration of the pot. Oxygen diffused down a concentration gradient into the bubbles of nitrogen and was carried to the surface and out of the solution by the bubbles. Air was bubbled through the nutrient solution to raise the concentration of oxygen as oxygen tended to move out of the air bubbles and into the surrounding solution. Oxygen probes (Kernco Instruments) were used to constantly monitor the oxygen concentration in each hydroponic unit. Readings were recorded by the datalogger and compared the actual oxygen concentration to the programmed target concentration level. A water pump was placed in each container to move water over the oxygen probe and maintain accurate concentration readings. The sensors were recalibrated using an Oxan 600 Oxygen Sensor. Samples (approximately 60 ml) were collected daily from the nutrient solution of each container. These samples were analyzed with a Nitrogen Analyzer (rapid diffusion conductivity method; Carlson, 1986) to determine the amount of nitrate present in the solution. From this, the amount of nitrate absorbed by each plant could be calculated with the following equation. Absorption = ([initial NO3] x initial volume) – ([final NO3] x final volume) (1) The pH of each sample was measured with an Oakton pH meter during sample analysis. Water absorbed by each plant was measured daily gravimetrically. The volume of water lost via transpiration was replaced with deionized water before samples were taken so that a known volume of 8 L could be used to calculate nitrate absorption. The difference in these weights was due to water lost by transpiration. The volume of each plant’s rootzone was measured by water displacement and the fresh weight of the plant was measured daily. These measurements were used to determine the effect of oxygen concentrations on the plant health and growth. RESULTS All four plants grew based on fresh weight increase, during the experiment (Fig. 1). The largest plant was plant 1, the smallest plant was plant 2, and plants 3 and 4 were of intermediate size. Plant 1 had an increase of about 40 g of fresh weight. Plants 1, 3, and 4 showed no relationship of water absorption rate to dissolved oxygen concentration (Fig. 2). Plant 2 showed a weak positive trend of increased water absorption rate with increased dissolved oxygen. There was no correlation between dissolved oxygen levels and nitrate absorption rate for plants 1, 3, and 4 (Fig. 3). Plant 2 may show a weak positive trend of increased nitrate absorption rate with increased dissolved oxygen if the outlying negative value is excluded. We were unable to achieve dissolved oxygen levels lower than 2 mg/L.