Anatomical convergence and diversity in the taproots of 16 herbaceous perennial species from subhumid New South Wales, Australia

Herbaceous perennial plants are an important part of the biodiversity of the subhumid western slopes of New South Wales, Australia. However, these species are usually 'out of sight, out of mind' as while they are perennials, they are usually relatively small and they die back to an underground perennating structure during dry periods. These structures are critical for survival as they store non-structural carbohydrates and moisture that allow resprouting to occur. Their anatomical features have rarely been investigated. The taproots or similar of 16 frequently recorded dicotyledonous herbaceous perennials of the South West Slopes, NSW were anatomically examined. One species from each of 14 families and two species from the Asteraceae were sampled. All species had roots with a high proportion of parenchyma and relatively few secondary xylem vessels that generally had a narrow lumen diameter (average 26 μm). The ratio of secondary cortex (tissues external to the vascular cambium excluding the phellem) to secondary xylem was highly variable between species (ranged from 0.24 to 75.13), with most species having a greater area of secondary cortex than xylem (median ratio 5.22). The species exhibited a wide array of anatomical variation, probably related to the wide taxonomic diversity. The high proportion of parenchyma facilitates storage of non-structural carbohydrates and water that are used for resprouting after adverse environmental periods. The relatively low density of vessels and small lumen diameters could be expected given that these plants generally have small leaves and short stems that are short lived. The high percentage of parenchyma and the small area of small diameter vessels show a form of functional convergence across the 15 families, while the wide variation in secondary cortex to xylem ratio and the wide variation in distribution and pattern of vessels and fibres reflects the wide taxonomic diversity of the studied species.