Root dynamics and water studies on Opuntia ficus-indica and O.robusta

English: Over the last few years a great interest in cactus pear was shown in terms of both fresh fruit and fodder production. However, there is a lack of knowledge on roots and therefore this study aimed at determining the root dynamics in relation to soil-water content and to quantify or characterise the different root types for two Opuntia species namely Opuntia ficus-indica (cultivar Morado) and 0. robusta (cultivar Monterey). This was carried out in the glasshouse (pots and root boxes) as well as in the field. Cactus pear planted in asbestos pots were used to determine the impact of soil-water content on root development and also their short time of recovery after rewetting the soil. The water treatments included 0 to 25% depletion of plant available (PAW) (not stressed), 25 to 50% depletion of PAW (mildly stressed), 50 to 75% depletion of PAW (moderately stressed) and 75 to 100% depletion of PAW (severely stressed). The root mass and root/shoot ratio decreased (P<0.05) with water stress for both species. Water-use efficiency decreased for 0. ficus-indica and increased for 0. robusta with water stress. The root length, root length/root mass ratio and amount of water needed to fill the cladodes increased (P<0.05) with water stress for both species. The root boxes were used for observation and characterizing the different root types and their response to soil drying and rewetting. Main roots grow up to 42 and 36 mm per day for Opuntia ficus-indica and 0. robusta respectively. The rain roots, which develop after only one hour of rewetting the soil, grow only for three days until dying due to soil drying. The side roots also emerge in response to soil-water but diners from rain roots in that they continue to grow until they die back as soil dries. Only 10 to 14 mm water is needed to fill the cladodes after a water stress. The rain roots therefore play an important role in rapid water up-take, which can fill the cladodes overnight. The root distribution with depth and distance was studied in the field, where root mass, root length, root length/root mass relationship and root thickness were measured at different depths and distances for each species. It has been found that most roots were concentrated over the first 100 mm soil layer for both Opuntias. After only one season of establishment the root distribution can spread as far as 1.6 and 1.7 m from the stem for O. robusta and O. ficus-indica respectively. The root mass, root length and root thickness decreased (P<0.05) with depth and distance. In conclusion, the cactus pear is characterised by a shallow and extensive root system, which contributes a lot to its water-use efficiency. These characteristics make it more appropriate for arid and semi-arid crop production. The marginal drier areas with shallow soils can therefore be utilised to their full potential by the cactus pear plant.