Diel water movement between parenchyma and chlorenchyma of two desert CAM plants under dry and wet conditions

Abstract. Electric‐circuit analogue models of the water relations of crassulacean acid metabolism (CAM) succulents such as Agave deserti and Ferocactus acanthodes have predicted diel movement of water between the water‐storage parenchyma and the photo‐synthetic chlorenchyma. Injection of tritiated water into either tissue in the laboratory confirmed substantial and bidirectional water movements, especially under conditions of wet soil. For A. deserti, water movement from the water‐storage parenchyma to the chlorenchyma increased at night as the chlorenchyma osmotic pressure increased. Although nocturnal osmotic pressure increases and transpiration for both species were minimal in the field under dry conditions, diel changes in the deuterium: hydrogen ratio (expressed as ΔD) were similar for the water‐storage parenchyma and the chlorenchyma. Such indication of [substantial mixing of water between the tissues over a 24‐h cycle was more evident under wet conditions in the field. For A. deserti, ΔD then increased by 32%o from the afternoon to midnight and was essentially identical in the water‐storage parenchyma and the chlorenchyma. For F. acanthodes, the diel changes in ΔD were one‐third those of A. deserti, and ΔD was always slightly higher for the chlorenchyma than for the water‐storage parenchyma, apparently reflecting the lower surface‐to‐volume ratio of A. deserti. In summary, data obtained using radioactive and stable isotopes strongly supported model predictions concerning diel cycles of internal water distribution for these CAM species.