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Poster 4

INFLUENCE OF WATER STRESS ON ROOT/CLADODE RATIO AND ROOT LENGTH 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

Due to the regular occurrence of drought in southern Africa, there is a need for more research on drought tolerant fodder plants, such as Opuntia species.  The influence of various water application strategies was evaluated in terms of root and cladode mass and root length for one-year-old cactus pear plants of the most important domesticated varieties of Opuntia, namely Opuntia ficus-indica (L.) Miller (cultivar Morado) and one of the most important wild species, O. robusta Wendl. (cultivar Monterey).  One-year-old cladodes were planted in pots (210 mm diameter and 550 mm deep soil) and grown in the greenhouse at day/night temperatures of 25-30/15-18oC.  The water treatments applied were 0-25%, 25-50%, 50-75% and 75-100% depletion of total plant available water.  Root mass decreased and root length increased significantly for both species with water stress.  Due to the finer root system of O. robusta, the root mass was lower (p<0.01) than that of O. ficus-indica.  In contrast, the root length/root mass ratio was higher (p<0.01) for O. robusta than for O. ficus-indica.  The influence of water stress on root die-back was clearly observed in O. ficus-indica, but less noticeable in O. robusta.  The side roots per tap root increased significantly with water stress, with as many as 35 for O. ficus-indica and 70 for O. robusta.  On average for all water treatments the roots of O. ficus-indica and O. robusta composed only 12% and 10% respectively of the total plant biomass.  A water application of only 11 mm was sufficient for filling up the cladodes of both species at the lowest water treatment.  It is clear that O. ficus-indica is more sensitive to water stress than O. robusta.  Marginal drier areas with shallow soils can therefore be utilized to its full potential by the cactus pear plant.

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