Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Abstract

Cultivated Agave mapisaga and A. salmiana can have an extremely high above-ground dry-weight productivity of 40 Mg ha−1 yr−1. To help understand the below-ground capabilities that support the high above-ground productivity of these Crassulacean acid metabolism plants, roots were studied in the laboratory and in plantations near Mexico City. For approximately 15-year-old plants, the lateral spread of roots from the plant base averaged 1.3 m and the maximal root depth was 0.8 m, both considerably greater than for desert succulents of the same age. Root and shoot growth occurred all year, although the increase in shoot growth at the beginning of the wet season preceded the increase in growth of main roots. New lateral roots branching from the main roots were more common at the beginning of the wet season, which favoured water uptake with a minimal biomass investment, whereas growth of new main roots occurred later in the growing season. The root: shoot dry weight ratio was extremely low, less than 0.07 for 6-year-old plants of both species, and decreased with plant age. The elongation rates of main roots and lateral roots were 10 to 17 mm d−1, higher than for various desert succulents but similar to elongation rates for roots of highly productive C3 and C4 agronomic species. The respiration rate of attached main roots was 32 μmol CO2 evolved kg−1 dry weight s−1 at 4 weeks of age, that of lateral roots was about 70% higher, and both rates decreased with root age. Such respiration rates are 4- to 5-fold higher than for Agave deserti, but similar to rates for C3 and C4 agronomic species. The root hydraulic conductivity had a maximal value of 3 × 10−7 ms−1 MPa−1 at 4 weeks of age, similar to A. deserti. The radial hydraulic conductivity from the root surface to the xylem decreased and the axial conductivity along the xylem increased with root age, again similar to A. deserti. Thus, although roots of A. mapisaga and A. salmiana had hydraulic properties per unit length similar to those of a desert agave, their higher growth rates, their higher respiration rates, and the greater soil volume explored by their roots than for various desert succulents apparently helped support their high above-ground biomass productivity