Extremely thick cell walls and low mesophyll conductance: welcome to the world of ancient living!

Abstract

Mesophyll conductance is thought to be an important photosynthetic limitation in gymnosperms, but they currently constitute the most understudied plant group in regard to the extent to which photosynthesis and intrinsic water use efficiency are limited by mesophyll conductance. A comprehensive analysis of leaf gas exchange, photosynthetic limitations, mesophyll conductance (calculated by three methods previously used for across-species comparisons), and the underlying ultra-anatomical, morphological and chemical traits in 11 gymnosperm species varying in evolutionary history was performed to gain insight into the evolution of structural and physiological controls on photosynthesis at the lower return end of the leaf economics spectrum. Two primitive herbaceous species were included in order to provide greater evolutionary context. Low mesophyll conductance was the main limiting factor of photosynthesis in the majority of species. The strongest sources of limitation were extremely thick mesophyll cell walls, high chloroplast thickness and variation in chloroplast shape and size, and the low exposed surface area of chloroplasts per unit leaf area. In gymnosperms, the negative relationship between net assimilation per mass and leaf mass per area reflected an increased mesophyll cell wall thickness, whereas the easy-to-measure integrative trait of leaf mass per area failed to predict the underlying ultrastructural traits limiting mesophyll conductance.

Keywords: Conifer; LES; LMA; gm; gymnosperm; nitrogen; photosynthesis..

Fig. 1.

(A, B) Photographs (A) and cross-sections (B) of Psilotum nudum (left) and Pinus sylvestris (right) to illustrate different leaf internal anatomies. (C,D) Low (P. nudum, left) and high (P. sylvestris, right) chloroplast area exposed to intercellular airspace (S_c/S). Arrows in (C) indicate sparsely (left) and tightly packed chloroplasts (right). The P. sylvestris cross-section exhibits lobed cells (circled in C).

Fig. 2.

Ultrathin transmission electron microscopy cross-sections showing species with atypical chloroplast shapes and sizes: (A) Selaginella uncinata, (B) Cycas revoluta and (C) Macrozamia riedlei.

Fig. 3.

Leaf dry mass per area (LMA) in relation to (A) leaf density (D_leaf), (B) leaf thickness (T_leaf), (C) mesophyll cell wall thickness (T_cwm), and (D) chloroplast surface area exposed to intercellular airspaces (S_c/S). Each data point corresponds to one species (n=3–5). The non-gymnosperms are marked as open circles, while the only deciduous conifer, Metasequoia glyptostroboides, is marked as a filled square. Excluded species are marked with the species’ name. Error bars show average±SE of all presented species. Note that some standard errors are so small that they are not visible under the data points. Data were fitted by linear regression (all are significant at P<0.05).

Fig. 4.

(A, B) Area-based net assimilation (A_area) in relation to (A) stomatal conductance (g_s) and (B) mesophyll conductance per area (g_m/area). (C) Mass-based photosynthesis (A_mass) in relation to mass-based mesophyll conductance (g_m/mass). (D) The draw-down from the atmosphere to the intercellular airspaces (C_a–C_i) in relation to g_s. (E, F) The draw-down from intercellular airspaces to chloroplasts (C_i–C_c) in relation to (E) g_m/area and (F) g_m/mass. Data in (B) were fitted by non-linear regression of the form y=2.73ln(x)–2.81; data in (D) of the form y=–53ln(x)+389; data in (E) of the form y=–36ln(x)+252; and data in (F) of the form y=–30ln(x)+80. Data are presented as in Fig. 3.

Fig. 5.

Mass- and area-based net assimilation (respectively A_mass and A_area) in relation to (A) mass-based nitrogen content (N_mas) and (B, C) LMA. The data in (B) were fitted with a curvilinear regression of the form y=896x^–0.63. Data presentation and fitting in (A, C) are as in Fig. 3.

Fig. 6.

(A) The influence of chloroplast surface area exposed to intercellular airspaces (S_c/S) on mesophyll conductance (g_m/area). (B–D) The influence of cell wall thickness (T_cwm) on (B) mesophyll conductance per mass (g_m/mass), (C) mesophyll conductance per area (g_m/area), and (D) the draw-down from intercellular airspaces to chloroplasts (C_i–C_c). The data in (B) were fitted with a non-linear regression of the form y=–0.37ln(x)+0.09 and data in (C) of the form y=–26ln(x)+25. Data presentation and fitting are as in Fig. 3.

Fig. 7.

Comparison of g_m/area calculated based on Harley et al. (1992) with (A) g_m/area calculated from anatomical measurements according to Niinemets and Reichstein (2003) and (B) g_m/area calculated based on Ethier and Livingston (2004). Dashed lines represent 1:1 correlation. Data presentation and fitting are as in Fig. 3, but all species are included in the analyses. C. revoluta, C. sempervirens and P. abies were underestimated by the anatomical model.

Fig. 8.

(A) The percentage of assimilation limited by stomatal conductance (l_s), mesophyll conductance (l_m) and biochemistry (l_b). (B) The percentage of mesophyll conductance per area limited by liquid phase components: cell wall, chloroplast, cytoplasm and membrane. Error bars denominate standard errors (n=3–5). The species are in the order of evolutionary age.

Fig. 9.

Correlations of intrinsic water use efficiency (WUE_i) with (A) the share of photosynthesis limited by mesophyll conductance (l_m) and (B) share of photosynthesis limited by stomatal conductance (l_s). Data presentation and fitting are as in Fig. 3.