It is widely accepted that the rate of light-saturated net photosynthesis on a leaf area basis (An) attains its maximum at or slightly before full leaf area expansion (for a review, see Šesták 1985). This is the case in many annual crop plants and deciduous tree species. However, there are many other species to which this rule does not apply. For instance, in cocoa (Theobroma cacao, Baker & Hardwick 1973), shade-tolerant species in tropical rainforests (Kursar & Coley 1992a), Syzygium species in tropical rainforests (Woodall, Dodd & Stewart 1998), and in evergreen broad-leaved trees in warm temperate forests (Miyazawa, Satomi & Terashima 1998), An is still low at the time of full leaf area expansion (FLE) and increases for a long time thereafter to attain its peak value. In Castanopsis sieboldii and evergreen Quercus species, An continued to increase for 20–40 d after FLE (Miyazawa et al. 1998).
Kursar & Coley (1992a) found that some shade-tolerant evergreen species in a tropical rainforest invested substantial amount of nitrogen into the leaf after FLE when the leaf became tough enough. Based on the finding, they proposed a hypothesis that delayed greening would be an effective strategy for minimizing loss of resources caused by a removal of young leaves by herbivory (Kursar & Coley 1992a), and referred to the species showing such a slow photosynthetic development as ‘delayed greening’ species to distinguish them from ‘normal greening’ species that shows the peak of An at or slightly before FLE.
In a previous study, we surveyed the published data of leaf development for various plant species and found a clear tendency that maturation of leaf photosynthesis delays with the increase in leaf dry mass per unit area (LMA) of the mature leaf (Miyazawa et al. 1998). Based on this relationship, we pointed out that the difference between normal greening and delayed greening species is not qualitatively distinct but gradual. In other words, delayed greening is a general phenomenon in the species having large LMA. We also pointed out that the delayed greening is not necessarily related to herbivory (Miyazawa et al. 1998).
The leaf longevity and the period taken for photosynthetic maturation increase with the LMA of the mature leaf, and all of these parameters increase in the order of annuals, deciduous trees and evergreen trees (Mooney & Gulmon 1992; Miyazawa et al. 1998). For a first step towards understanding the nature of wide and continuous spectrum, it is useful to compare leaf development between the species from both extremes. One extreme is the annuals and development of the photosynthetic apparatus has been most intensively studied for Phaseolus vulgaris (Čatsk´y, Tichá & Solárová 1976, O'Toole, Ludford & Ozbun 1977; Šesták 1985). Another extreme is the evergreen tree species having leaves of large LMA. However, unlike the annual herbaceous species, functional analyses of leaf development in the evergreen tree species have not been made, with the exception of a few ecological studies (Kursar & Coley 1992a,b; Miyazawa et al. 1998). The aim of the present study is therefore to analyse delayed greening processes in an evergreen tree species in detail.
Ontogenetic changes in photosynthetic capacity of chloroplasts have been already examined for several delayed greening species. These results indicated that Rubisco and nitrogen content on leaf area basis were both very low at FLE, and increased slowly (Kursar & Coley 1992a; Miyazawa et al. 1998). Respiration rate, stomatal conductance, and internal CO2 transfer conductance also change dramatically during leaf development, and thereby inevitably affect net photosynthetic rate. Although the effects of these factors on photosynthesis have been studied for various plant species (Homann 1975; Miranda, Baker & Long 1981; Leech & Baker 1985; Ticháet al. 1985), few studies have been made for the delayed greening species. Thus, the relationship between these factors and An in delayed greening species should be studied.
The LMA increases with leaf area expansion and reaches its maximum value at around FLE in most annual herbs and deciduous trees (Čatsk´y et al. 1976; Šesták 1985; Koike 1990), whereas, in the delayed greening species, LMA continues to increase for a long time after FLE (Jurik 1986; Hanson et al. 1988; Kursar & Coley 1992a; Miyazawa et al. 1998). The LMA values of mature leaves are greater than those of the leaves at FLE by two to threefold (Miyazawa et al. 1998). The increase in LMA suggests that mesophyll thickness and/or cell wall thickness increase after FLE. In Phaseolus vulgaris (Čatsk´y et al. 1976) and Xanthium italicum (Maksymowych 1973), which are annual herbs, the leaf thickness attained its maximum slightly before FLE. The proportion of the intercellular air spaces in X. italicum leaves also reached the maximal value slightly before FLE (Maksymowych 1973). However, the anatomical changes in delayed greening species have not been studied.
Leaf anatomy affects photosynthesis. The surface area of mesophyll cells exposed to the intercellular air spaces on a leaf area basis (Smes or Ames/A) is an important parameter that determines potential area for CO2 diffusion in the liquid phase (Nobel, Zaragoza & Smith 1975; Nobel 1991). If mesophyll thickness increases after FLE, Smes would also increase. If chloroplast development occurs synchronously with that of mesophyll cells, surface area of chloroplast facing the intercellular air spaces on a leaf area basis (Sc) would also increase. Internal CO2 transfer conductance (gi) in mature leaves is strongly correlated with parameters such as Smes and Sc (Evans et al. 1994; Syvertsen et al. 1995; Hanba, Miyazawa & Terashima 1999), as the increase in Smes and Sc after FLE may increase gi, which would lead to a rise in An in delayed greening species. On the other hand, increase in cell wall thickness probably decreases gi through a lengthening of the CO2 diffusion path in the liquid phase in cell walls (Nobel 1991).
In this study, we followed changes in the CO2 gas exchange rate, chlorophyll content and in anatomical properties (chloroplast number on a leaf area basis, Smes and Sc) during leaf development in Castanopsis sieboldii, a delayed greening species and Phaseolus vulgaris, an annual herb. For C. sieboldii the changes in the A–Ci relationships and Rubisco contents during leaf development were further analysed. On the basis of these data, we calculated the potential photosynthetic rate of chloroplasts as well as limitations imposed by respiration, stomatal conductance and gi throughout the leaf development of C. sieboldii.