EFFECT OF WATER STRESS ON ROOT DYNAMICS OF OPUNTIA FICUS-INDICA AND O. ROBUSTA - H. A. Snyman

Department of Animal, Wildlife & Grassland Sciences, University of the Free State, Bloemfontein 9300, South Africa

There is a lack of information concerning the adaptability of different cactus pear cultivars and species to a range of environmental conditions.  This study was aimed at quantifying the effects of water stress on the growth of tap roots, side roots and rain roots of the species Opuntia ficus-indica (L.) Miller (cultivar Morado – green cladode) and O. robusta Wendl. (cultivar Monterey – blue cladode).  One-year-old cladodes were planted in root boxes (650 mm long, 100 mm wide and 900 mm deep) that were kept in the greenhouse at day/night temperatures of 25-30/15-18oC.  The boxes faced north/south, and were placed at a 15% angle so that the roots growing down could easily be seen through the glass side of the box.  The measurements took place over a soil-water gradient.  The highest daily tap root growth was 43 mm for O. ficus-indica and 37 mm for O. robusta.  Tap root growth increased in the morning with water stress for both species, but decreased in the afternoon.  At night both species showed an increase in tap root growth with water stress.  Side root growth increased with water stress, with that of O. robusta more (p<0.01) per tap root than O. ficus-indica.  Opuntia robusta has a finer root system than O. ficus-indica.  The side roots grew as much as 7 mm per day for O. ficus-indica and 4 mm for O. robusta.  Whitish rain roots developed on the established roots within the first hour after rewetting the soil and grew for only 3 days.  The rain roots grew up to 6 mm within a day for O. ficus-indica and 5 mm for O. robusta.  Light rain showers of only a few millimeters, which are not always of value to other fodder plants, can thus be utilized effectively by the unique root system of the cactus pear.