Crassulacean acid metabolism-cycling in Euphorbia milii

Abstract

Crassulacean acid metabolism (CAM) occurs in many Euphorbiaceae, particularly Euphorbia, a genus with C3 and C4 species as well. With the aim of contributing to our knowledge of the evolution of CAM in this genus, this study examined the possible occurrence of CAM in Euphorbia milii, a species with leaf succulence and drought tolerance suggestive of this carbon fixation pathway. Leaf anatomy consisted of a palisade parenchyma, a spongy parenchyma and a bundle sheath with chloroplasts, which indicates the possible functioning of C2 photosynthesis. No evidence of nocturnal CO2 fixation was found in plants of E. milii either watered or under drought; watered plants had a low nocturnal respiration rate (R). After 12 days without watering, the photosynthetic rate (P N) decreased 85 % and nocturnal R was nearly zero. Nocturnal H(+) accumulation (ΔH(+)) in watered plants was 18 ± 2 (corresponding to malate) and 18 ± 4 (citrate) μmol H(+) (g fresh mass)(-1). Respiratory CO2 recycling through acid synthesis contributed to a night-time water saving of 2 and 86 % in watered plants and plants under drought, respectively. Carbon isotopic composition (δ(13)C) was -25.2 ± 0.7 ‰ in leaves and -24.7 ± 0.1 ‰ in stems. Evidence was found for the operation of weak CAM in E. milii, with statistically significant ΔH(+), no nocturnal CO2 uptake and values of δ(13)C intermediate between C3 and constitutive CAM plants; ΔH(+) was apparently attributable to both malate and citrate. The results suggest that daily malate accumulation results from recycling of part of the nocturnal respiratory CO2, which helps explain the occurrence of an intermediate value of leaf δ(13)C. Euphorbia milii can be considered as a CAM-cycling species. The significance of the operation of CAM-cycling in E. milii lies in water conservation, rather than carbon acquisition. The possible occurrence of C2 photosynthesis merits research.

Keywords: CAM-cycling; citrate; transpiration; water saving; water-use efficiency.

Figure 1.

Cross-sections of the leaf of E. milii. UE, upper epidermis; PP, palisade parenchyma; VB, vascular bundle; BS, bundle sheath; SP, spongy parenchyma; LE, lower epidermis. Arrowheads point at chloroplasts.

Figure 2.

Time course of changes with drought in leaves of E. milii in (A) H⁺ content titrated to pH 7.0 (empty circles, dawn; filled circles, dusk); (B) H⁺ content titrated to pH 8.4 (empty circles, dawn; filled circles, dusk); (C) nocturnal H⁺ accumulation (empty triangles, pH 8.4; filled triangles, pH 7.0), and (D) dawn leaf FM per area (circles) and chlorophyll content (triangles). Values are mean ± SE (n = 12). Different letters indicate significant differences at P < 0.05 after a two-way ANOVA (time under drought × hour of day for each pH in A and B) and a one-way ANOVA (time under drought for each pH in C).

Figure 3.

Response curve of the leaf photosynthetic rate to (A) photosynthetic photon flux density and (B) leaf intercellular CO₂ concentration in watered plants of E. milii. Values are mean ± SE (n = 6). Filled circles, P_N; empty symbols, g_s. Measurements were made at a CO₂ concentration of 380 μmol mol⁻¹ in (A) and a PPFD of 1000 μmol m⁻² s⁻¹ in (B).

Figure 4.

Daily course of the leaf photosynthetic rate, stomatal conductance and transpiration rate in plants of E. milii under drought for 0, 7, 12 and 16 days. Measurements were made at a CO₂ concentration of 380 μmol mol⁻¹, leaf temperature of 24.0 ± 1.0 °C and PPFD of 200 (06:00–10:00 h) and 1000 μmol m⁻² s⁻¹ (10:00–18:00 h). The dark bar on the abscissa indicates the length of the dark period.

Figure 5.

Response curves to leaf intercellular CO₂ concentration of nocturnal respiration rate and stomatal conductance in watered plants of E. milii. Filled symbols, R; empty symbols, g_s. Values are mean ± SE (n = 6).

Figure 6.

Change in nocturnal leaf transpiration rate of plants of E. milii watered and under different degrees of drought as a function of nocturnal respiration rate. Values are data points. The regression line (solid), 95 % confidence intervals (broken lines) and determination coefficient (P < 0.05) are shown.