Root system hydraulic conductivity in species with contrasting root anatomy

Previous research suggested that the hydraulic properties of root systems o f intact plants could be described by two parameters: the hydraulic conduct ivity (Lp(r)) or the slope of the flow-density/water potential gradient rel ationship, and the offset or minimum water potential gradient required to i nduce 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 (asparagu s), Dendrobium superbum Rchb. f. (dendrobium), Glycine max (L.) Merr. (soyb ean), 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) an d 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 sugge sted that the cortex width was a stronger determinant of Lp(r) than root di ameter. However, the offset was not correlated with root diameter, stele di ameter or cortex width, but was >2-fold higher in species having an exoderm is in the root radial path (sour orange, asparagus, and dendrobium) compare d 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 pla nts and excised roots which have been measured with different techniques. I t is concluded that Lp(r) and offset, which describe the flow-water potenti al relationship for intact root systems, are related to differences in the root cortex; specifically, its thickness and the presence/absence of a sube rized exodermis. Hence, these anatomical differences may, in part, cause th e variability in root hydraulic properties that exists among plant species.