Drought‐induced changes in hydraulic conductivity and structure in roots of Ferocactus acanthodes and Opuntia ficus‐indica

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Abstract
Summary: Drought‐induced changes in hydraulic conductivity (Lp) and anatomy were investigated for 1‐, 3‐ and 12‐month‐old roots of Ferocactus acanthodes (Lem.) Britton & Rose and Opuntia ficus‐indica (L.) Miller. Responses were similar for the two species but varied for the three root ages. Drying in soil for 30 d caused little change in Lp or internal structure for 1‐month‐old roots, primarily due to the formation of soil sheaths in the root hair zone. For 3‐month‐old roots, Lp declined three‐ to five‐told after 30 d of drying, partly because of lacunae caused by rupture of cortical cells. For 12‐month‐old roots after 30 d of drying, the dehydration of suberized peridermal layers led to a two‐ to three‐fold decline in Lp. Measurements of axial hydraulic conductance per unit pressure gradient (Kh) before and after pressurization indicated that air embolism, caused by 30 d of drying, decreased Khby 53, 97, and 98% for 1‐ 3‐ and 12‐month‐old roots, respectively. Rewetting after 30 d of drying restored Kh to initial values within 7 d for all three root ages. Lpafter rewetting attained 100 % of its initial value for 1‐month‐old roots arid about 60% for 3‐ and 12‐month‐old roots. The formation of soil sheaths around young roots, the reduction in radial conductivity for older roots by cortical lacunae and peridermal dehydration, and the reduction in Kh by embolism thus caused a moderate, partly reversible decline in Lp that can help limit water loss from roots to the soil during drought.