Light Environment and Branch Photosynthesis at Inner Canopy

Light environment for the shade shoots of Quercus on a sunny day is characterized by the temporal fluctuation of diffuse light with low PPFD and sun fleck with high PPFD (Fig. 7a). Light interception in the leaves along the shoot was reduced (6.4 +1.9 mol photons m-2 day-1; mean + SD for three shoots x 12 light environments) as compared to that at horizontal plane

10 12 14 Time of day

16 18

10 12 14 Time of day

16 18

10 12 14 Time of day

16 18

Figure 7: Example diurnal course of light interception (a), leaf temperature (b), photosynthetic rate (c), and stomatal conductance to water vapor (d) in a shaded shoot of Quercus.

(9.3 + 2.8molm~2day~1), due to various leaf angles and self-shading (but smaller than the sun shoots). According to the diurnal changes in light interception, leaf temperature changed in fluctuated manner with maximum of about 23 and 31 °C for the two air temperature regimes; the maximum leaf temperature was observed when the leaves were receiving sun flecks (Fig. 7b). The gsw and A tracked the fluctuated changes of light interception, but higher temperature condition reduced both gsw and A especially during midday hours, when the light incidence and leaf temperature were maximum throughout the daytime (Figs. 7c, d).

Aday of the horizontal and not self-shaded leaves was 119.1 + 20.0 and 115.3 +19.1 mmol m~2 day-1 under the two temperature regimes of 15-20 °C and 15-28 °C, respectively. At shoot level, Aday was 90.9 + 16.5 and 88.7+ 15.3 mmol m-2day-1, respectively. Increasing temperature led only to 2.4% reduction in Aday, while transpirational water loss increased by about 20% (data not shown). Calculation of photosynthetic rate solely dependent on light interception (equation (2)) underestimated A at diffuse light incidence (Fig. 7c), and this difference accounted for 7 to 50% underestimation of Aday in a case of 15-20 °C regime (data not shown). This might be partly due to the temperature effects on respiration and photosynthetic processes calculations during the diffuse light with lower temperature than under sun flecks.

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