Allometric root/shoot relationships and predicted water uptake for desert succulents

Abstract

Root morphology, shoot morphology, and water uptake for Agave deserti and Ferocactus acanthodes of various sizes were studied using allometric relationships (y = axb) and a previously developed water uptake model. Shoot surface area increased with shoot volume with an exponent b of 0.75 for both species. Root length and the ground area explored by the roots increased with shoot surface area with b's of 0.72 for A. deserti and 0.92 for F. acanthodes. Various sized individuals had about the same ratio of root length to explored ground area, with higher values occurring for A. deserti. Predicted water uptake averaged over the explored ground area was approximately constant over a 104-fold range in shoot surface area, suggesting that shoot size confers no intraspecific competitive advantage for water uptake. For the root lengths per explored ground area observed in the field, water uptake was predicted to be 85 per cent of maximal; water uptake could be increased by the production of more rain roots. When differences in shoot volume were accounted for by allometry, small plants had relatively less shoot surface area and relatively more root length per shoot volume than did large plants, which may be important for the water relations of seedling establishment. © 1987 Annals of Botany Company.