A Case Study on in situ Rooting Profiles and Water-Use Efficiency of Cactus Pears, Opuntia ficus-indica and O. robusta
Keywords:cactus pear, root/cladode mass, root distribution, root length, root mass, root thickness, wateruse efficiency.
Root distribution of cactus pear with distance from the mother plant and depth was determined in the field for one-year-old Opuntia ficus-indica (L.) Miller (cultivar Morado – green cladode) and O. robusta Wendl. (cultivar Monterey – blue cladode) in a semiarid climate. The roots were expressed in terms of root mass, root length and root thickness. In addition, the evapotranspiration water-use efficiency (cladode dry-mass production per unit of evapotranspiration) was quantified for these two spineless cactus pear species during the 2002/2003 growing season. In both species most roots were concentrated over the first 100 mm soil layer. After only one growing season of establishment the roots spread as far as 1.7 m and 1.8 m from the stem for O. robusta and O. ficus-indica, respectively. The root mass, root length, and root thickness decreased (P < 0.05), as expected, with depth and distance. The total root mass per plant, calculated for all depths were 107.5 and 131.6 g m-2 for O. ficus-indica and O. robusta, respectively. The root length and root mass relationship showed that a root mass of 1 g m-2 was equal to a root length of 3.17 and 3.79 m m-2 for O. ficus-indica and O. robusta, respectively. O. robusta showed a finer root system than that of O. ficus-indica. The evapotranspiration water-use efficiency for the two species ranged between 1.19 to 1.57 kg DM ha-1 mm-1 for O. robusta and O. ficus-indica respectively. The roots composed only 21% and 13% of the total plant biomass for O. robusta and O. ficus-indica, respectively. In the one-year old plants no deep root system occurred in both species and was only characterized by a shallow horizontally spreading root system. These above-mentioned root characteristics of the cactus pear make it more appropriate for arid and semiarid crop production. The marginal drier areas with shallow soils can, therefore, be utilised to their full potential by the cactus-pear plant.