Thermal and Water Relations of Roots of Desert Succulents

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Abstract
Two succulent perennials from the Sonoran Desert, Agave deserti Engelm. and Ferocactus acanthodes (Lem.) Britton and Rose, lose little water through their roots during drought, yet respond rapidly to light rainfall. Their roots tend to be shallow, although absent from the upper 20 mm or so of the soil. During 12–15 d after a rainfall, new root production increased total root length by 47 per cent to 740 m for A. deserti and by 27 per cent to 230 m for F. acanthodes; root dry weight then averaged only 15 per cent of shoot dry weight. The annual carbon allocated to dry weight of new roots required 11 per cent of shoot carbon dioxide uptake for A. deserti and 19 per cent for F. acanthodes. Elongation of new roots was greatest near a soil temperature of 30°C, and lethal temperature extremes (causing a 50 per cent decrease in root parenchyma cells taking up stain) were 56°C and -7°C. Soil temperatures annually exceeded the measured tolerance to high temperature at depths less than 20 mm, probably explaining the lack of roots in this zone. Attached roots immersed in solutions with osmotic potentials above -2·6 MPa could produce new lateral roots, with 50 per cent of maximum elongation occurring near -1·4 MPa for both species. Non-droughted roots lost water when immersed in solutions with osmotic potentials below -0·8 MPa, and root hydraulic conductance decreased markedly below about -1·2 MPa. Pressure-volume curves indicated that, for a given change in water potential, non-droughted roots lost three to five times more water than droughted roots, non-droughted leaves, or non-droughted stems. Hence, such roots, which could be produced in response to a rainfall, will lose the most tissue water with the onset of drought, the resulting shrinkage being accompanied by reduced root hydraulic conductance, less contact with drying soil, and less water loss from the plant to the soil.