The capacity for thermal protection of photosynthetic electron transport varies for different monoterpenes in Quercus ilex

Abstract

Heat stress resistance of foliar photosynthetic apparatus was investigated in the Mediterranean monoterpene-emitting evergreen sclerophyll species Quercus ilex. Leaf feeding with fosmidomycin, which is a specific inhibitor of the chloroplastic isoprenoid synthesis pathway, essentially stopped monoterpene emission and resulted in the decrease of the optimum temperature of photosynthetic electron transport from approximately 38 degrees C to approximately 30 degrees C. The heat stress resistance was partly restored by fumigation with 4 to 5 nmol mol(-1) air concentrations of monoterpene alpha-pinene but not with fumigations with monoterpene alcohol alpha-terpineol. Analyses of monoterpene physicochemical characteristics demonstrated that alpha-pinene was primarily distributed to leaf gas and lipid phases, while alpha-terpineol was primarily distributed to leaf aqueous phase. Thus, for a common monoterpene uptake rate, alpha-terpineol is less efficient in stabilizing membrane liquid-crystalline structure and as an antioxidant in plant membranes. Furthermore, alpha-terpineol uptake rate (U) strongly decreased with increasing temperature, while the uptake rates of alpha-pinene increased with increasing temperature, providing a further explanation of the lower efficiency of thermal protection by alpha-terpineol. The temperature-dependent decrease of alpha-terpineol uptake was both due to decreases in stomatal conductance, g(w), and increased volatility of alpha-terpineol at higher temperature that decreased the monoterpene diffusion gradient between the ambient air (F(A)) and leaf (F(I); U = g(w)[F(A) - F(I)]). Model analyses suggested that alpha-pinene reacted within the leaf at higher temperatures, possibly within the lipid phase, thereby avoiding the decrease in diffusion gradient, F(A) - F(I). Thus, these data contribute to the hypothesis of the antioxidative protection of leaf membranes during heat stress by monoterpenes. These data further suggest that fumigation with the relatively low atmospheric concentrations of monoterpenes that are occasionally observed during warm windless days in the Mediterranean canopies may significantly improve the heat tolerance of nonemitting vegetation that grows intermixed with emitting species.

Figure 1.

Temperature dependencies of the emission rates of total monoterpenes (averages ± se) for control leaves (A) and after feeding with 5 μm fosmidomycin for 7 to 8 h (B). The measurements were conducted under a saturating quantum flux density of 1,000 μmol m⁻² s⁻¹. Data were fitted by Equation 5, and the optimum temperature (T_opt) calculated by Equation 6 (T_opt = 37.5°C for A and T_opt = 31.5°C for B).

Figure 2.

Average stomatal conductance to water vapor (g_w; A and D), net assimilation rate (A; B and E), and the rate of photosynthetic electron transport (J_max; Equations 4 and 5; C and F) as a function of temperature for nonfumigated non-fosmidomycin-fed (control in all sections) versus fosmidomycin-fed leaves (A–C) and versus fosmidomycin-fed leaves that were fumigated with either α-pinene or α-terpineol (D–F). In fumigation treatments, the leaves were fumigated with specific monoterpenes at concentrations of 3.5 to 5 nmol mol⁻¹ for 2 h at every temperature, and average stomatal conductance and net assimilation rates were calculated. A and g_w were measured at saturating quantum flux densities of 900 to 1,000 μmol m⁻² s⁻¹ and at ambient CO₂ mole fractions of 380 to 420 μmol mol⁻¹. Data in A, B, D, and E were normalized with respect to the average maximum value for a given treatment (Table I) that was observed at 25°C, except for stomatal conductance for fosmidomycin-fed and α-pinene-fumigated (g_w = 182 mmol m⁻² s⁻¹ at 15°C) or α-terpineol-fumigated (g_w = 107 mmol m⁻² s⁻¹ at 15°C) leaves in D (corresponding control leaves had a g_w of 127 mmol m⁻² s⁻¹ at 15°C). Data in C and F were fitted by Equation 5 and standardized with respect to the value at optimum temperature (Eq. 6; Table I). The average optimum temperatures (T_opt) are reported in Table I. Error bars provide ± se of four replicate experiments.

Figure 3.

Sample graphs of monoterpene uptake measurements with PTR-MS. The fosmidomycin-fed leaves were either fumigated with α-pinene (A) or with α-terpineol (B) at 25°C for 2 h. Fumigation concentration (average ± sd) was 4.5 ± 0.5 nmol mol⁻¹ for α-pinene and 3.9 ± 0.5 nmol mol⁻¹ for α-terpineol. Reference air or the air exiting the leaf chamber passed to PTR-MS (Fig. 6), and the rate of monoterpene uptake was calculated from average differences in monoterpene concentrations in the reference and chamber air (ΔC). The leaf was continuously fumigated, while PTR-MS was switched between the reference line and leaf chamber twice or three times, and sample traces are shown for both monoterpenes. A total of 3.64 cm² leaf area was enclosed in leaf chamber during the α-pinene fumigation and 2.72 cm² during the α-terpineol fumigation.

Figure 4.

The rates of uptake of α-pinene (A) and α-terpineol (B) of Q. ilex leaves as a function of temperature (averages ± se). The fosmidomycin-fed leaves were fumigated with either α-pinene (average ± se fumigation concentration of 4.6 ± 0.8 nmol mol⁻¹) or α-terpineol (3.9 ± 0.7 nmol mol⁻¹), and the rate of uptake determined as in Figure 3. Data were fitted by nonlinear regressions in the form of y = ae^bx. Error bars provide ±se of three experiments.

Figure 5.

Henry's law constant (H_xy, equilibrium gas/liquid-phase partition coefficient; A and B) and equilibrium octanol/water partition coefficient (K_ow; C and D) for α-pinene (A and C) and α-terpineol (B and D) in relation to temperature (averages ± se). Data fitting is as in Figure 4. H_xy (mol monoterpene/mol air [mol monoterpene/mol water]⁻¹) characterizes the monoterpene partitioning between air and water, while K_ow (mol monoterpene/m³ octanol [mol monoterpene/m³ water]⁻¹) characterizes the partitioning between lipid and water. Both H_xy and K_ow were measured gas chromatographically. Data were fitted by nonlinear regressions in the form of y = ae^bx in A and B and in the form of y = alog(bx) in C and D. Error bars show ±se of three separate determinations.

Figure 6.

Schematic overview of the system used for measurements of the rate of monoterpene emission from and uptake by the leaves of Q. ilex. For measurements of the monoterpene emission rates, the air enters into the system from inlet 1, while the second inlet is used for fumigation experiments. The air leaving the chamber is divided between the gas-exchange analyzers of CIRAS-2 to measure CO₂ and water vapor exchange and PTR-MS to measure monoterpene concentrations. The fumigation system was calibrated to provide the air with monoterpene concentration of 5 nmol mol⁻¹. Valves before and after leaf chamber were used to monitor the stability of air concentrations of the added monoterpenes.