Data collection
Plants were harvested at 30, 50 and 70 d of growth, representing developmental stages of early vegetative growth, late vegetative or early reproductive growth, and middle to late reproductive growth respectively. At each stage, six individual plants were randomly chosen from each plot, making a total of 6 replicates × 3 plots × 3 densities × 2 soils × 3 stages = 324 samplings. For each individual plant, the following traits were measured if applicable: diameter at the basal of the main root, length and number of lateral roots (above or equal to 1 mm in diameter along the main root). Morphological root traits were not measured at 30 d of growth due to small sizes of plants. Each individual was then separated into roots, stems, petioles, leaves, reproductive organs and branches (if any), enveloped respectively, and oven-dried at 75 oC for 2 d and weighed.