Nocturnal Accumulation of Acid in Leaves of Wall Pennywort (Umbilicus rupestris) Following Exposure to Water Stress

Abstract

Physiological responses to water stress (drought) have been investigated in Umbilicus rupestris (wall pennywort) by comparing control (well-watered) and draughted plants with respect to (i) diurnal fluctuations in the acid content of the leaves, (ii) CO2 exchange patterns and (iii) stomatal conductance. Control plants show no diurnal fluctuations in acid content, whereas after 6 d of drought a clear CAM-type pattern (nocturnal acidification followed by deacidification in the light) is observed. In control plants, the CO2 exchange pattern over a 24 h period is of a typical C-3 ‘square-wave’ type, with extensive CO2 uptake in the light and CO2 output in the dark. In droughted plants the day-time CO2 uptake is confined to a morning ‘burst’, whilst night-time CO2 output is markedly reduced. There is however no net noctural uptake of CO2. In control plants, stomatal conductance is high during the day (especially in the first half of the day) falling to a low level at the onset of darkness, and then rising slowly through the remainder of the night. In droughted plants, stomatal conductance is very low, except that there is morning ‘burst’ of high conductance and a period during the night when conductance is higher than in control plants. These results are discussed in relation to the response of U. rupestris to drought both in laboratory and in field conditions.