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...o be relatively constant when nitrogen content per unit leaf area varies (von Caemmerer & Farquhar 1981; Evans 1983), or across growth irradiance treatments (Sims & Pearcy 1989; Thompson et al. 1992; =-=Wullschleger 1993-=-; Evans 1996). It has also been observed Figure 9. Contour plots of the relative rate of photosynthesis persunit leaf dry mass under (a) low light (200 mmol m-2 s-1) and (b)shigh light (1000 mmol m-2 ...

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...ynthesis model (1982), and mesophyll conductance. The aggregated model has dispensed with this detail to a great degree. We can hypothesize that variables have been discarded for either of two reasons: (1) The variable covaries strongly with another variable—only one of the two is required, or (2) GPP is not sensitive to variation in the parameter, so it can be discarded. As an example of the first case, we would expect 892 MATHEW WILLIAMS ET AL. Ecological Applications Vol. 7, No. 3 GPP to be highly sensitive to variations in the photosynthetic parameters, which vary greatly between species (Wullschleger 1993). However, the aggregation procedure has managed to effectively capture the essential relationship between foliar N concentration and metabolic limits on photosynthesis. The model can be driven with foliar N data, and the more complex facets of metabolic C fixation and their associated variables can be ignored. This is demonstrated by the applicability of the model to ecosystems with very different species composition. In the second case, there are indications that the sensitivity of the fine-scale model to some variables is relatively small. For example, we would expect coniferous forests to ...

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...iables FNR and aR are assumed to be fundamental properties of the enzyme, and we treat them as constants for all plant types. Both FLNR and CNL are known to vary among plant types (Field et al. 1983; =-=Wullschleger 1993-=-), but, as cited above, there is evidence that they vary little with canopy depth for a given plant type. We therefore assume that most of the variation in Vcmax with x will result from variation in S...

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...d Evans (1998) and differ somewhat from the ‘‘Collatz–Bonan’’ values we previously adopted. Because these parameters are rather uncertain, at most one decimal place of accuracy is possible. The lc term will typically be about 2. The last term needed we, sometimes referred to as the ‘‘export-limited rate’’ represents the rate at which triphosphate can be utilized, and so metabolized sugars exported and the initial ribulose reconstituted. It is assumed to be the same as used by Collatz et al. (1991), and Bonan (1995): w 5 0.5 V .e m (10) The numerical factor is known to lie between 1/3 and 1/2. Wullschleger (1993) reviews a wide range of measurements for Vm and triphosphate ‘‘utilization’’ (TPU), giving average values of Vm of 75 mmoles m22 s21 for annuals, 44 mmoles m22 s21 for perennials, and an average triphosphate utilization of 10.1 mmoles m22 s21. Hence, simply averaging the annual and perennial rates and taking we as 3 3 TPU gives we ø 0.5 Vm. Foley et al. (1996) argue for a somewhat smaller value, apparently based on a different averaging approach with the same data. As evident from Eqs. (7)–(8), this term should become rate limiting, when T drops below 58– 108C. A smaller value would raise the...

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...demonstrated considerable improvements to model predictions of carbon fluxes in the Amazon when leaf P was taken into account. Empirically, there is also a strong relationship between Jmax and Vcmax (=-=Wullschleger 1993-=-; Beerling and Quick 1995), and most TBMs simulate Jmax as a linear function of Vcmax. However, this assumption could be erroneous because the correlation between Jmax and Vcmax is likely to be influe...

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...bution between reaction centres, and precise coordination between the production of reductant and ATP from the light reactions and its consumption and subsequent reduction of CO2 in the Calvin cycle (=-=Wullschleger 1993-=-; Walters 2004) ensure homeostasis of photosynthetic processes when faced with changing environmental conditions. Similar responses for light reaction and Calvin cycle gene expression suggest that the...

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...tivity, which includes a temperaturedependent function [dePury andFarquhar, 1997;Chen et al., 1999; Wilson et al., 2000]. [11] Electron transport limited photosynthesis (Aj) is catalyzed by Ribulose-bisphosphate carboxylase-oxygenase (RuBP) enzyme, often called RuBP-limited photosynthesis [Farquhar et al., 1980; Farquhar and von Caemmerer, 1982; de Pury and Farquhar, 1997]: Aj J Ci G* 4:5Ci þ 10:5G* ð3Þ where J is the electron transport rate, calculated from a quadratic equation as a function of effective irradiance (Ie) and the maximum electron transport rate (Jmax). A fixed ratio (2.1; [Wullschleger, 1993]) is usually assumed between Jmax and Vmax even though this ratio can vary with temperature sensitivities of both components. 2.2. Coupled Photosynthesis-Stomatal Conductance Models [12] Many stomatal conductance (gs) models [e.g., Baldocchi et al., 1991; McMurtrie et al., 1992; Sellers et al., 1992; Leuning, 1995; Chen et al., 1999; Oren and Pataki, 2001; Kim et al., 2008] use an empirical equation from Jarvis [1976], which assumes that environmental factors act independently to control stomatal conductance: gs gs:maxf VPDð Þf yð Þf APARð Þf CO2ð Þ ð4Þ where gs.max is the maximum stomatal ...

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...ites in the chloroplast is greater or indicate a low intrinsic allocation rate of foliar N to the photosynthetic apparatus at a given foliar N concentration (Seemann et al., 1987; Lloyd et al., 1992; =-=Wullschleger, 1993-=-; Brown et al., 1996). 4.2. Effects of N addition rates on water-use efficiency of L. chinensis and S. grandis δ13C values of plant foliage, an indicator of long-term WUE, provide a useful measure of ...

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...e RuBP regeneration capacity, which is expressed by Jmax, higher allocation of nitrogen to the responsible component would maintain photosynthetic production under low temperature conditions such as in spring and autumn (cf. Berry and Bjorkman, 1980). The opposite would be also true since the photosynthetic rate under higher temperature conditions in summer is limited by RuBP carboxylation capacity (Vcmax) (see also Hikosaka et al., 1999). If plants change Jmax/Vcmax responding to the seasonal change of temperature condition (i.e., temperature acclimation), simple scaling from Vcmax to Jmax (Wullschleger, 1993) assuming their constant ratio would underestimate Jmax in low temperature seasons. We were not able to test these effects since our present data on Jmax and Vcmax at 25 8C (and thus Jmax25/ Vcmax25) are calculated but not obtained by measuring temperature dependency of A–Ci curves. Future measurements of temperature dependency for each species throughout the season are needed for accurate scaling from leaf level ecophysiology to canopy scale behavior over the season. 5.2. Effects of leaf properties and shoot architecture on productivity at the canopy top Our calculation of light interception ...