Branch Architecture and Photosynthetic Carbon Gain at the Canopy

3.2.1 Diurnal Patterns of Plant Ecophysiological Performance

Three-dimensional shoot architectures of Betula and Quercus (for sun and shade branches) were measured and reconstructed with a structural-functional model, Y-plant (Fig. 3). Leaf angle of the shoots were different among the tree species or growth light environment, i.e., 65.0712.2° for Betula (mean7SD for 40 leaves from three shoots), 31.5712.1° for sunlit (56 leaves), and 24.5712.8° for shaded (62 leaves) shoots of Quercus. Reflecting the differences in leaf angle and length of petiole, self-shading in Betula shoots was smaller than those of Quercus shoots, and projected leaf area to the light source (the sun) in midday hours is reduced in Betula, which would be effective in avoiding high light stresses and realizing moderate light interception . The leaf display was different between the sunlit and shaded shoots of Quercus, suggesting that the species changes its shoot architecture by responding to the local light environment within its canopy, as well as leaf photosynthetic characteristics. Leaf arrangement was flatter and self-shading was smaller in shaded shoots (Fig. 3). These characteristics in shaded shoots are effective in receiving light incidence in the light-limited environment of inner canopy and understory positions (Niinemets, 1998).

Figs. 4 and 5 show the diurnal patterns of the relationships between temperature regimes, stomatal conductance (gsw), photoinhibition,

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Figure 3: Architectures of a shoot of Betula, sunlit and shaded shoots of Quercus reconstructed by a simulation program Y-plant. The structures were reconstructed on the basis of the measured geometrical parameters (see text). Images indicate the views from sun angle in early morning (0700 hr) and noon (1200 hr). The darker areas of the shoots indicate self-shading.

photosynthetic rate (A), and light utilization efficiency (LUE — A/absorbed PPFD) for Betula and Quercus in a high light environment based on simulation results by Y-plant. Owing to the steep leaf angles and resulting small projection of leaf area to the sun in Betula, light absorption on the leaf area basis was largely reduced as compared to that for horizontal plane

Figure 4: Example diurnal course of simulated light interception (a) sto-matal conductance to water vapor (b) photosynthetic rate as a function of light incidence and temperature (c) photosynthetic rate with and without photoinhibition (d) and light utilization efficiency (LUE) and (e) of a shoot of Betula. Light interception, stomatal conductance, and photosynthetic rate are expressed on the basis of leaf area averaged over the sample shoot. Photosynthetic rate shown in (c) were calculated by solely light dependence (gray line) or by considering the temperature dependence of photosynthetic reaction and stomatal conductance to vapor pressure under two air temperature conditions (Tair; minimum-maximum of 15-20 °C, dotted line; and 15-28 °C, bold line). LUE shown in (e) were calculated from the data on (a) and (c).

Figure 4: Example diurnal course of simulated light interception (a) sto-matal conductance to water vapor (b) photosynthetic rate as a function of light incidence and temperature (c) photosynthetic rate with and without photoinhibition (d) and light utilization efficiency (LUE) and (e) of a shoot of Betula. Light interception, stomatal conductance, and photosynthetic rate are expressed on the basis of leaf area averaged over the sample shoot. Photosynthetic rate shown in (c) were calculated by solely light dependence (gray line) or by considering the temperature dependence of photosynthetic reaction and stomatal conductance to vapor pressure under two air temperature conditions (Tair; minimum-maximum of 15-20 °C, dotted line; and 15-28 °C, bold line). LUE shown in (e) were calculated from the data on (a) and (c).

Figure 5: Example diurnal course of simulated light interception (a), stomatal conductance to water vapor (b), photosynthetic rate solely light dependent (gray line) and or by considering the temperature dependence of photosynthetic reaction and stomatal conductance to vapor pressure under two air temperature conditions (Tair; minimum-maximum of 15-20 °C, dotted line; and 15-28 °C, bold line) (c), photosynthetic rate with and without photoinhibition (d) and light utilization efficiency (e) of a sunlit shoot of Quercus.

Figure 5: Example diurnal course of simulated light interception (a), stomatal conductance to water vapor (b), photosynthetic rate solely light dependent (gray line) and or by considering the temperature dependence of photosynthetic reaction and stomatal conductance to vapor pressure under two air temperature conditions (Tair; minimum-maximum of 15-20 °C, dotted line; and 15-28 °C, bold line) (c), photosynthetic rate with and without photoinhibition (d) and light utilization efficiency (e) of a sunlit shoot of Quercus.

(Fig. 4a). Different daily maximum air temperature (20 and 28 °C) led different maximum leaf temperature of about 20.5 and 28.5 °C, respectively. Consequently, gsw and A, and hence LUE was reduced by this rising leaf temperature: A was the highest in a case that was estimated solely from light absorption, and was reduced by incorporating the temperature and stomatal effects on photosynthetic rate (Figs. 4c, e; daily values will be shown below). The reduced gsw and A at midday hours are called "midday depression'', and would have considerable limitation to the daily photosyn-thetic carbon gain in high light environments. Daily accumulation of light absorption (PPFD dose) led to decline in Fv/Fm at midday hours with minimum of 0.64 and hence reduction of photosynthetic carbon gain in afternoon hours (Fig. 4d).

In the sun shoot of Quercus (Fig. 5), the maximum leaf temperatures were higher than Betula, about 23.5 °C at air temperature of 20 °C, and 30.5 °C at 28 °C. These higher leaf temperatures of Quercus than Betula were induced by the smaller leaf angles and resulting large projection leaf area to the sun in midday hours that receive larger amount of light in midday hours (Fig. 5). The high leaf temperature led to considerable reduction of gsw and hence A and LUE in Quercus (Figs. 5b, c, e). Also in Quercus, exposure of the leaves to direct sun light led to reduction of Fv/Fm to the minimum of 0.60 in afternoon and that photoinhibition reduced A (Fig. 5d).

3.2.2 Effects of Shoot Architecture and Stomatal Closure on Daily Photosynthesis

Daily integrated values of light absorption, photosynthesis, and photo-synthetic limitation by stomatal closure and photoinhibition are shown in Fig. 6. To clarify the effects of shoot architecture (leaf angle and self-shading) on the plant performance, simulation results for virtual plants with horizontal and non-self-shaded leaves are also shown. In horizontal leaves with no self-shading, they can receive 58.0 mol photons m~ 2 day"1 of PAR (expressed as daily PPFD: PPFDday) under a clear open sky of a sunny day. But this PPFDday was reduced to 28.973.5molm~2day~1 in Betula or 35.473.1 in Quercus shoots. These reductions were due to the variation of leaf angle within the shoots and self-shading among the leaves.

The high PPFDday in horizontal leaves led to high daily photosynthesis (Aday) than those for shoot-level performance in all cases of calculation of photosynthesis, i.e., solely light dependent, including stomatal effects and photoinhibition in both Betula and Quercus (Figs. 6a, b, c). Owing to the interception of full sunlight throughout the day, horizontal leaves had higher leaf temperature by 2 °C and lower Fv/Fm (ca. 0.4) than the leaves along the actual shoot architecture (Fv/Fm of ca. 0.6-0.65): these high light stresses led to larger stomatal and temperature limitation ( — reduction of

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Aday as compared to light-dependent Aday; Fig. 6d) and/or photoinhibitory limitation ( — reduction of Aday as compared to stomatal and temperature-limited photosynthesis; Fig. 6e) of photosynthesis. In horizontal leaves of Betula, stomatal and temperature limitation of Aday was 9.7 and 20.0% for air temperature regimes of 15-20 and 15-28°C, respectively, while those for actual shoots were 9.9 and 14.7%. In Quercus, stomatal limitation of horizontal leaves were 13.9 and 32.8% for the two air temperature regimes, while those for actual shoots were 11.7 and 24.4%, respectively. These results indicate important aspects in canopy photosynthesis that (1) stomatal limitation has quantitatively remarkable effect on daily photosynthetic carbon gain for plants in high light and high air temperature (and VPD) environments, and (2) leaf angles and foliage geometry affect estimation of canopy photosynthesis considerably.

LUE was dependent on the shoot architecture (leaf orientation) and tree species (Fig. 6f, g). LUE was higher in leaves having their natural orientation along the shoot than in horizontal leaves, and higher in Betula than in Quercus. These results were caused by the balance between light interception and photosynthesis, which were affected by leaf angle and photosyn-thetic activity under high light. Thus use of LUE term in ecosystem-scale models (e.g., Running et al., 2004) requires the ecophysiological understanding of these consequences for accurate and reliable estimates.

The larger midday depression of gsw and A in Quercus than Betula can be attributed to the smaller leaf angles and lower g0. As shown in Figs. 4a and 5a, the difference in leaf angles and hence shoot architecture, i.e., projected leaf area to the sun (Fig. 3), led to different light absorption with higher maximum PPFD in Quercus. This led to higher leaf temperature and transpirational load. Stomatas are suggested to close by responding to the transpirational load (Mott and Parkhurst 1991), and therefore the higher leaf temperature could induce stomatal closure more remarkably in Quercus

Figure 6: Daily integrated performance of Betula and Quercus shoots under high light conditions with two air temperature regimes (minimum-maximum of 15-20 °C, light gray bar; and 15-28 °C, solid bar). Daily photosynthesis (Aday) solely light dependent (a), temperature and stomatal dependent (b), and including the effect of photoinhibition on photosynthesis (c), stomatal limitation (d), and photoinhibitory limitation (e) to photosynthesis, and daily light utilization efficiency in which Aday was temperature and stomatal dependent (f) and photoinhibitory limited (g). For each tree species, "horizontal" indicates the calculated data for a model shoot with horizontal leaves with no self-shading and "shoot" indicates the data for the measured architectures (mean 7 SD of three shoots x four azimuth angles). Note that Aday solely light dependent (a) are not different between the temperature regimes.

(Fig. 4b vs. 5b). Moreover, midday gsw is highly dependent on g0 (equation (1)). Quercus leaves have lower g0 of 0.096 mol m-2 s-1 as compared to that of Betula (0.243 mol m-2s-1). The lower g0 would contribute to the conservative use of water but highly limits CO2 transfer into leaves via stomatal pore. On the other hand, the higher g0 and gsw in Betula would be effective in CO2 transfer as well as transpirational leaf cooling, which should be important in this pioneer species.

3.2.3 Effects of Shoot Architecture and Photoinhibition on Daily Photosynthesis

The smaller leaf angle also led to larger photoinhibitory limitation on photosynthesis (Fig. 6e). Photoinhibition due to accumulated light interception reduced Aday by 9-10% in horizontal leaves in both tree species, while it was 6% in Betula shoots and 10% in Quercus shoots. Reduction of photosynthesis by rising leaf temperature, stomatal closure, and photoinhibition decreased daily LUE from 0.19 to 0.15 in Betula and from 0.14 to 0.10 in Quercus shoots, as compared to the case of solely light-dependent photosynthesis (Fig. 6(g). Our estimates of the impact of photoinhibition on daily shoot carbon gain agree with previous studies that showed 2-10% reduction in broadleaved tree species (Ogren and Sjostrom, 1990; Werner et al., 2001; Pearcy et al., 2005; Valladares et al., 2005). The different photoinhibitory limitation on Aday between horizontal leaves and actual shoot leaves can be attributed to two reasons: effect of leaf angle itself and self-shading. Steeper leaf angle contributes to reducing light interception, especially excessive high light of midday that is not necessary for saturating photosynthetic rate. Self-shading can also avoid excess high light and heat load in midday hours and the photosynthesis at shaded part of the leaves might contribute to the whole shoot carbon gain even under high light stresses (Valladares and Pearcy, 1999; Valladares et al. 2005).

In both Betula and Quercus, the difference of temperature conditions did not affect the influence of photoinhibition on photosynthesis, since the present model estimates photoinhibition only from the light absorption. In general, the magnitude of photoinhibition can be influenced by the capacity and activity of photosynthetic and other physiological dissipation of absorbed photons (Osmond, 1994); the lower the photosynthetic activity in midday hours, the larger the photoinhibition since photosynthesis has the largest capacity of energy dissipation, but midday depression of gsw reduces the photosynthetic utilization of absorbed photons (e.g., Osmond and Grace, 1995; Muraoka et al., 2000). Further quantitative evaluation of these effects on canopy photosynthesis should be progressed.

These simulation studies on two canopy tree species with different leaf physiology and shoot architecture provided the species-specific differences in consequences of plant ecophysiology and microenvironment on photosynthetic carbon gain. Although these two species can be grouped into one category of vegetation as "broadleaved deciduous trees'' in satellite remote sensing observations and regional to global-scale models, we should keep in mind that trees with different successional status would perform differently in terms of photosynthetic response to multiple environment at the canopy top, and that the relative dominance of trees with different suc-cessional status in a given forest type would have considerable effect on the ecosystem behavior.

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