Fig. 1.
Response of stomatal conductance, gsw, normalized by the maximum stomatal conductance value, gswmax, to (A) irradiance (PPF), (B) leaf temperature (Tl) and (C) leaf-to-air water vapour pressure deficit (VPD) in ‘Fuji’ and ‘Braeburn’ apple cultivars. In (A), [gsw/gswmax = (aP PPF + bP)/(cP PPF + dP)] fitting parameters: ‘Fuji’: aP = 66·10−4, bP = 0·716, cP = 66·10−4, dP = 1·716 (R2 = 0·93); ‘Braeburn’: aP = 90·10−4, bP = 0·856, cP = 90·10−4, dP = 1·856 (R2 = 0·94). In (B), (gsw/gswmax = aT Tl2 + bT Tl + cT) fitting parameters: ‘Fuji’: aT = –49·10−4, bT = 0·28, cT = –2·94 (R2 = 0·57); ‘Braeburn’: aT = –39·10−4, bT = 0·22, cT = –2·12 (R2 = 0·61). In (C), the threshold VPD value ensuring gsw = gswmax is noted. Equation beyond this value (gsw/gswmax = aD VPD + bD) uses fitting parameters: ‘Fuji’: aD = –0·17, bD = 1·23 (R2 = 0·89); ‘Braeburn’: aD = –0·21, bD = 1·39 (R2 = 0·75). Results of the Mann–Whitney test: in (A) P = 0·034 at PPF = 100 µmol m−2 s−1 but P > 0·05 at the other PPF levels; in (B) P > 0·05 at three temperatures; in (C) P = 0·029 between the threshold values. The ANCOVA procedure using VPD as the covariate detected no significant differences (P > 0·05) between the cultivars. Responses to PPF, Tl and VPD were recorded under standard conditions (see Materials and Methods).