Root system hydraulic conductivity in species with contrasting root anatomy

Previous research suggested that the hydraulic properties of root systems of intact plants could be described by two parameters: the hydraulic conductivity (Lp(r)) or the slope of the flow-density/water potential gradient relationship, and the offset or minimum water potential gradient required to induce flow. In this study, Lp(r) and offset were correlated with anatomical features of the root radial path in plants with contrasting root anatomy. Two woody and three herbaceous species were examined which exhibit a range of root anatomical features: Asparagus densiflorus (Kunth) Jessop (asparagus), Dendrobium superbum Rchb. f. (dendrobium), Glycine max (L.) Merr. (soybean), Prunus persica (L.) Batsch. (peach), Citrus aurantium L. (sour orange). Lp(r) varied about 8-fold, and the offset varied about 6-fold among the five species. Lp(r) was inversely related to root diameter (r(2) = 0.39) and cortex width (r(2) = 0.55), suggesting that species with thinner roots or roots with a thin cortex had the highest Lp(r). Further observations suggested that the cortex width was a stronger determinant of Lp(r) than root diameter. However, the offset was not correlated with root diameter, stele diameter or cortex width, but was >2-fold higher in species having an exodermis in the root radial path (sour orange, asparagus, and dendrobium) compared to those lacking an exodermis (peach and soybean). The data of root Lp(r) obtained were similar to those given in the literature for both intact plants and excised roots which have been measured with different techniques. It is concluded that Lp(r) and offset, which describe the flow-water potential relationship for intact root systems, are related to differences in the root cortex; specifically, its thickness and the presence/absence of a suberized exodermis. Hence, these anatomical differences may, in part, cause the variability in root hydraulic properties that exists among plant species.