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 m(2) 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.