Circadian regulation of leaf hydraulic conductance in sunflower (Helianthus annuus L. cv Margot)

The circadian regulation of leaf hydraulic conductance (K(leaf)) was investigated in Helianthus annuus L. (sunflower). K(leaf) was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. K(leaf) was 4.0 e-4 kg s-1 m-2 MPa-1 during the light period (LL) and 30-40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for K(leaf) at their subjective light or dark periods, K(leaf) adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed K(leaf) oscillating from light to dark values in phase with their subjective photoperiod indicating that K(leaf) changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (R(leaf)) of both LL and DL to the same value (1.0 e + 3 MPa m2 s kg-1) that equalled the vascular resistance (R(v)). The contribution of the non-vascular leaf resistance (R(nv)) to R(leaf) was of 71.9% in DL and of 58.4% in LL. The dominant R(nv) was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of K(leaf).