Water loss from leaf mesophyll stripped of the epidermis

Water vapour flux (rate of water loss) from the mesophyll of isolated Agapanthus praecox Willd. leaf pieces without an epidermis was investigated by loss of mass into unstirred air at relative humidities (RHs) of 0.993-0.850, compared with the rate from a water atmometer (rate of evaporation). The point at which relative evaporation (RE, the rate of water loss divided by the rate of evaporation) reaches <1 inadequately identifies the onset of mesophyll regulation because values >1 were found. For RHs of 0.993-0.967, RE varied in daily cycles from 0.6 to ∼3, with a period of ∼24h, maxima at mid-afternoon, minima at or near dawn. For RH<0.950, the cycles were suppressed. An initial rate of RE ≈1.2, RE declined towards zero. In leaf pieces supplied with water via vascular strands (rate of transpiration), the daily cycle persisted down to RH 0.850, where maximal RE ≈ 2. Transpiration from one surface of field leaves gave the rate of transpiration in the same range. These data require the maximum RE for each vapour pressure deficit as the value identifying the onset of mesophyll regulation (possibly by aquaporins), which produces cyclic changes in the rates of water loss and transpiration. At RH<0.95, the decline of RE below 1 is probably regulated by cell wall water status. Possible functions of the two types of regulation are discussed.