BIOPHYSICS OF LEAF GROWTH OF HYBRID POPLAR - IMPACT OF OZONE

Biophysical parameters controlling leaf expansion in hybrid poplar (Populus nigra L. x P. deltoides Bartr. ex Marsh = P. x euramericana) were measured for first-year cuttings growing under controlled conditions, with or without ozone fumigation. As individual leaves expanded, leaf water potential remained constant whilst solute potential declined and turgor (P) increased from approximately 0.5 MPa to over 1.0 MPa as leaf size increased. Yield turgor (Y) measured by stress-relaxation of leaf discs within psychrometers, increased from approximately 0.2 MPa to over 1.0 MPa during the same period. As the value of Y approached that of P, the effective turgor for growth (P-Y) declined and became zero as leaf growth ceased. Cell wall extensibility (measured as percentage plasticity by the 'Instron' technique) was related to leaf growth rate such that highest values of plasticity corresponded to maximum rates of growth. As growth ceased, wall extensibility declined to very low values. Fumigation with 50 nl 1-1 ozone for 16 h d-1 had no effect on any of the measured parameters with the result that growth and mature leaf size were unaffected by comparison with control plants receiving charcoal-filtered air. Additional episodes of 120 nl 1-1 ozone on three consecutive days during the period of maximum leaf growth similarly had no effect on expansion or final leaf size.