Soil O2and CO2Effects on Apparent Cell Viability for Roots of Desert Succulents

Abstract

Roots of desert succulents occupy the upper layers of porous, well-aerated soils. However, roots of Agave deserti, Ferocactus acanthodes, and Opuntia ficus-indica all tolerated many days of soil anoxia; 0% 02 in the soil gas phase for 30 d reduced the fraction of cells taking up the vital stain neutral red, an average of only 18% for the cortex and 6% for parenchyma cells within the stele of perennial established roots. Ephemeral rain roots, induced by watering as branches on the established roots, were more susceptible to 0% 02 in the soil gas phase; 19 d abolished stain uptake for cortical cells and 32 d for stelar parenchyma cells. Soil C02 levels above the 0-1% observed in the root zone in the field rapidly reduced uptake of neutral red; the fraction of cortical cells taking up the stain decreased 30% in 10 h at 0-5% C02 and was abolished in 9 h at 2% and 7 h at 10% C02 averaged for the three species. Rain roots were somewhat more susceptible than established roots to elevated soil C02 levels, and stelar parenchyma cells were much less susceptible than were cortical cells. When uptake of the vital stain was abolished by elevated soil C02, no anatomical evidence of cellular damage was observed. For A. deserti exposed to 2% C02, the pH of macerated root tissue decreased about 0-35 pH unit over 10 h; C02 apparently entered the cells, lowered the intracellular and/or cell wall pH, and prevented the accumulation of neutral red. Elevated soil C02 also inhibits root respiration for the three desert succulents considered. Hence, the restriction of such species to porous soils may reflect the relatively rapid inhibiting effects of elevated soil C02 levels rather than a requirement for high soil 02 levels, consistent with the observation that desert soils tend to have low gas-phase C02 levels near 01% compared with 1% or more in the root zone of non-desert species.