Photosynthetic capacity shows diurnal variability
Photosynthetic capacity (both Vcmax andJmax ) at 25 °C shows significant diurnal change with a consistent decline of ~50% towards the end of day, with the largest declines beginning at ZT8 in Populus deltoides (2nd order polynomial responses;Vcmax : t52 = -6.67,P < 0.0001; Jmax :t52 = -7.30, P < 0.0001; Fig. 2a, d), ZT8 in Capsicum annuum (Vcmax :t33 = -5.24, P < 0.0001;Jmax : t33 = -4.10,P = 0.0003 ; Fig. 2b, e), and ZT2 in Malus domestica(Vcmax : t17 = -9.25,P < 0.0001); Jmax :17 = -9.13, P < 0.0001; Fig. 2b, e). Populus trichocarpa showed slight but significant declines in Vcmax at 25 °C (t19 = -3.39, P = 0.003) andJmax (t19 = -2.81,P = 0.01) (Fig. 2b, e). Pinus ponderosa did not show any significant declines in Vcmax(t8 = -1.50, P = 0.17) orJmax (t8 = -1.71, P= 0.13; Fig. 2c, f). Photosynthetic capacity measured at 30 °C shows significant diurnal changes in Vcmax forQuercus muehlenbergii (t24 = -6.39,P < 0.0001; Fig. 2a), Rosa grandiflora(t24 = -5.57, P < 0.0001, Fig. 2b), and Ginkgo biloba (t14 = -5.90,P < 0.0001; Fig. 2c) but not Asclepias speciosa(t24 = -1.77, P = 0.09; Fig. 2a) andCedrus deodar (t20 = 0.648, P = 0.52; Fig. 2c). For Jmax at 30 °C, there are linear declines in Quercus muehlenbergii(t24 = -5.75, P < 0.0001; Fig. 2d), Rosa grandiflora (t24 = -5.95,P < 0.0001; Fig. 2e), Ginkgo biloba(t14 = -9.29, P < 0.0001; Fig. 2f) and Asclepias speciosa (t24 = -2.09,P = 0.047; Fig. 2d), but not for Cedrus deodar(t20 = -0.10, P = 0.92; Fig. 2f). Across all species, apparent Jmax andVcmax were strongly correlated as expected (slope = 1.99, intercept = -13.54, F1,293 = 1826,R 2 = 0.86, P < 0.0001; Fig. 3; Wullschleger, 1993). Diurnal variation in relative photosynthetic capacity is quite variable (Fig. 4), with convergent responses forCapsicum annuum , Ginkgo biloba , Malus domestica , and Rosa grandiflora , but varying patterns in the remaining species.