Patterns of root growth and water uptake of a maize-cowpea mixture grown under greenhouse conditions

This study investigated the patterns of root growth and water uptake of mai ze (Zea mays L.) and cowpea (Vigna unguiculata (L.) Walp) grown in a mixtur e under greenhouse conditions. The plants were grown in root boxes for 5 we eks under 2 watering regimes; fully irrigated and water stress conditions, followed by a 5-day drying cycle imposed during the 6th week of growth. Wat er uptake patterns were analysed during the drying cycle. The two-dimension al distribution of the roots of both plants in the boxes was determined imm ediately at the end of the drying cycle. Under well-irrigated conditions, t he roots of the component plants grew profusely into all sections of the ro ot box and intermingled considerably. Water stress resulted in the decline of root growth of maize and cowpea but the root:shoot ratios of maize and c owpea were not affected, suggesting that there was no significant effect of water stress on root:shoot partitioning. However, water stress affected th e biomass distribution between fine and coarse roots in cowpea. About 64% b y weight of cowpea roots under water stress were coarse whereas as against 48% under well-irrigated conditions. Furthermore, water stress generally re stricted the lateral extent of the roots of both maize and cowpea with a te ndency of clumping together of the root systems and a reduced degree of int ermingling. Thus, the extent of mixing of the root systems was apparently c ontrolled by the availability of soil water. Water uptake from the well-irr igated soil in the root boxes was initially restricted to the sections dire ctly below the base of each plant. Although roots of both plants were prese nt in almost all sections of the root box, all the sections did not contrib ute simultaneously to water uptake by each plant. Water uptake was delayed from the middle intermingled zones. In effect, uptake patterns did not rela te generally to the root distribution. The tendency was for the component p lants to initially 'avoid' water uptake from zones of intense intermingling or competition.