Optimal carbon partitioning helps reconcile the apparent divergence between optimal and observed canopy profiles of photosynthetic capacity

Abstract

Summary Photosynthetic capacity per unit irradiance is greater, and the marginal carbon revenue of water (∂A/∂E) is smaller, in shaded leaves than sunlit leaves, apparently contradicting optimization theory. I tested the hypothesis that these patterns arise from optimal carbon partitioning subject to biophysical constraints on leaf water potential. In a whole plant model with two canopy modules, I adjusted carbon partitioning, nitrogen partitioning and leaf water potential to maximize carbon profit or canopy photosynthesis, and recorded how gas exchange parameters compared between shaded and sunlit modules in the optimum. The model predicted that photosynthetic capacity per unit irradiance should be larger, and ∂A/∂E smaller, in shaded modules compared to sunlit modules. This was attributable partly to radiation‐driven differences in evaporative demand, and partly to differences in hydraulic conductance arising from the need to balance marginal returns on stem carbon investment between modules. The model verified, however, that invariance in the marginal carbon revenue of N (∂A/∂N) is in fact optimal. The Cowan–Farquhar optimality solution (invariance of ∂A/∂E) does not apply to spatial variation within a canopy. The resulting variation in carbon–water economy explains differences in capacity per unit irradiance, reconciling optimization theory with observations.

Isohydricity of Two Different Citrus Species under Deficit Irrigation and Reclaimed Water Conditions

“…The water flow reduction through some parts of the roots, which might be hydraulically isolated due to severe water stress, could be related to a reduced release of root ABA into the leaf xylem [86]. This would explain why the lowest ABA values in our study were found with TW-nI treatment of mandarin; however, in recent experiments (although not in citrus) have shown strong stomatal responses to changes in water supply originating primarily in the leaves, but not in the roots [87,88].…”

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

“…Plasticity throughout the canopy may emerge when development of new leaves is influenced by information from adjacent mature leaves or apical meristems and may also acclimate during and after expansion to the current microclimate (Zwieniecki et al ., 2004). Overall, leaf biochemistry, anatomy, and structure may be optimized to local conditions (Niinemets, 2007; Lloyd et al ., 2010; Hikosaka, 2014; Kitao et al ., 2012; Buckley, 2021). However, we lack a cohesive framework for integrating the many differences in leaf traits throughout the canopy.…”

Vertical gradients in photosynthetic physiology diverge at the latitudinal range extremes of white spruce

“…In this study, we focused on the latitudinal relationships between canopy light environments and foliar physiological traits, but intracanopy gradients in other environmental conditions may also impact Vcmax and Jmax measurements (Martin, Hinckley, Meinzer & Sprugel 1999;Zweifel, Böhm & Häsler 2002;Bauerle, Bowden, Wang & Shahba 2009;Bachofen, D'Odorico & Buchmann 2020;Buckley 2021). Perhaps of particular relevance with respect to rubisco carboxylation is the reliance of this process on the enzyme rubisco, whose activity is well-known to be temperature dependent (Yamori & von Caemmerer 2009;Benomar et al 2018).…”

“…Consequently, we conclude that a more mechanistic explanation must exist for the puzzling downregulation of V cmax and J max . While in this study we focused on the latitudinal relationships between canopy light environments and foliar physiological traits, it is possible that intracanopy gradients in other environmental conditions may also impact V cmax and J max measurements (Bachofen et al, 2020;Bauerle et al, 2009;Buckley, 2021;Martin et al, 1999;Zweifel et al, 2002).…”