Construction and maintenance of the optimal photosynthetic systems of the leaf, herbaceous plant and tree: an eco-developmental treatise

Abstract

Background and aims: The paper by Monsi and Saeki in 1953 (Japanese Journal of Botany 14: 22-52) was pioneering not only in mathematical modelling of canopy photosynthesis but also in eco-developmental studies of seasonal changes in leaf canopies.

Scope: Construction and maintenance mechanisms of efficient photosynthetic systems at three different scaling levels--single leaves, herbaceous plants and trees--are reviewed mainly based on the nitrogen optimization theory. First, the nitrogen optimization theory with respect to the canopy and the single leaf is briefly introduced. Secondly, significance of leaf thickness in CO2 diffusion in the leaf and in leaf photosynthesis is discussed. Thirdly, mechanisms of adjustment of photosynthetic properties of the leaf within the herbaceous plant individual throughout its life are discussed. In particular, roles of sugar sensing, redox control and of cytokinin are highlighted. Finally, the development of a tree is considered.

Conclusions: Various mechanisms contribute to construction and maintenance of efficient photosynthetic systems. Molecular backgrounds of these ecologically important mechanisms should be clarified. The construction mechanisms of the tree cannot be explained solely by the nitrogen optimization theory. It is proposed that the pipe model theory in its differential form could be a potential tool in future studies in this research area.

Fig. 1.

Photosynthetic subsystems in a photosynthetic system. For the photosynthetic system of a plant individual, subsystems can be regarded as the individual leaves. For the system of a leaf, subsystems are chloroplasts. Given that the photosynthetic properties of the subsystems are identical and light comes from above, the situation can be imagined in which upper subsystems are light-saturated while lower subsystems are light-limited. Under such conditions, light absorbed by the upper subsystems is wasted (or even harmful), and light is limited in the lower subsystems. In terms of nitrogen economy, this situation is paraphrased as follows. Nitrogen in the photosynthetic enzymes in the upper subsystems is used at maximum velocity, while nitrogen is wasted in the lower subsystems. To maximize the light and nitrogen use efficiencies, the system should be constructed such that all the subsystems reach light saturation at the same time. There are two ways to realize such conditions. One is to homogenize the light environment. The other is to adjust photosynthetic properties of the subsystems to their respective light absorption.

Fig. 2.

Photosynthetic light-response curves measured in the same Glycine max leaf but by irradiation from the different sides. The curve obtained with irradiation from the adaxial side showed a transition from the light-limited to the light-saturated phases much sharper than that obtained with irradiation from the abaxial side. Redrawn from Terashima (1986).

Fig. 3.

Diffusion of CO₂ from the atmosphere to chloroplast stroma. The inset shows two pathways. The diffusion of CO₂ along pathway b is negligible compared with that along pathway a.

Fig. 4.

Internal conductance (g_i) of various leaves plotted against (A) leaf dry mass per area (LMA) and (B) mesophyll surface area. Data from various sources. Open circles, annual herb (tobacco); grey circles, deciduous tree species; grey triangles, Castanea sativa (Mediterranean chestnut); solid circles, evergreen tree species.

Fig. 5.

A model for branch growth and carbon allocation. Areas of circles indicate cumulated light interceptions by the branches. The shaded and solid circles indicate cumulated light interceptions by the branches in the current year and previous year, respectively. Shaded parts of the stems indicate the wood produced in the current year. Solid parts of the stems denote the wood existed in the previous year. An original drawing by K. Sone.