Mucilage in Cacti: Its Apoplastic Capacitance, Associated Solutes, and Influence on Tissue Water Relations

Mucilage content in the stems of four sympatric cactus species varied from none for Ferocactus acanthodes, 19 percent by dry weight for Opxmtia basilaris, 26 percent for Opuntia acanthocarpa, and 35 percent for Echinocereus engelmannii. Although the mucilage differed chemically among the species (the arabinose content ranged from 17 percent to 51percent of the sugar monomers), its relative capacitance (change in relative water content per unit change in water potential)remained about 15 MPa '. The relative capacitance of the water-storage parenchyma averaged 1-04 MPa"1 and was consistent with the mucilage content, being lowest for F. acanthodes and highest for E. engelmannii. Mucilage isolated from hydrated tissue was accompanied by solutes with an osmotic pressure of about 0-2 MPa. Such associated solutes influence the water-release characteristics of mucilage and hence its role as an apoplastic capacitor. In particular, extracellular solutes can facilitate the release of appreciable mucilage-bound water to the cells at tissue water potentials occurring during the initial phases of drought.