Table 2Aboveground parameters in J. curcas seedlings grown in three different soils.Root diameters and volumes for the five primary roots in J. curcas differed significantly among soil types (P < 0.001). Secondary root growth (thickening) was lower for plants in sandy soil than those in sandy-loam or clay-loam soils. Roots in sandy- and clay-loams had similar basal and mostly apical diameters. They also had a greater number of branches thicker than 2.0mm and larger volumes than those in sandy soil. All taproots in sandy-loam and clay-loam soils developed secondary roots thicker than 2.0mm, whereas only 13% of the taproots in sandy soil developed such roots. However, root lengths did not differ significantly among treatments (P > 0.05).

Stem mass, leaf mass, and root system mass were lower at the conclusion of the three-month growing period in sandy soil than in clay-loam or sandy-loam soils (Table 2, Figure 5). Despite these differences, allocation of biomass was greater to stems than to roots in all three soil types (Table 3). Within root systems, the greatest proportion of biomass and volume was allocated to taproots (Table 4). The uppermost 10cm of soil contained the majority of root volume (Figure 6).Figure 5Digitized Jatropha curcas root systems grown in three different soils.Figure 6Root volume distribution by depth in sandy, sandy-loam and clay-loam soils.Table 3Average �� SD dry matter allocation in J. curcas curcas grown in three different soil types.Table 4Average �� SD below-ground parameters in J. curcas seedlings grown in three different soils.3.3.

Root Structure, Similarity Indices, and Plasticity to SoilRoot system symmetry index scores were typically 0.146 �� 0.05 (mean �� SD); mean SI did not statistically differ among soil types (Table 4). Of the main parameters defining J. curcas root structure, taproot inclination, and primary lateral root distribution (��) had the lowest plasticity index scores (0.05 and 0.06, respectively). Biomass allocation to roots and the inclination angles (��), length, and apical diameters of the five primary coarse roots also showed low plasticity (PI < 0.31). However, the number of secondary lateral roots thicker than 2mm, as well as root mass, was highly plastic (P > 0.63) (Table 4).4. Discussion4.1. Growth and Mass DistributionJ. curcas had a significant growth response to soil conditions. In sandy soil, it displayed characteristics typical of plants grown in arid conditions, including reduction of leaf area and defoliation. These responses reduce transpirational surface area and are common adaptations of species with photosynthetic stems, such as J. curcas [24]. Additionally, the low nutrient availability of the sandy soil strongly reduced stem GSK-3 and leaf growth [25, 26].