Model-based analysis of avoidance of ozone stress by stomatal closure in Siebold's beech (Fagus crenata)

Abstract

Background and aims: Resistance of plants to ozone stress can be classified as either avoidance or tolerance. Avoidance of ozone stress may be explained by decreased stomatal conductance during ozone exposure because stomata are the principal interface for entry of ozone into plants. In this study, a coupled photosynthesis-stomatal model was modified to test whether the presence of ozone can induce avoidance of ozone stress by stomatal closure.

Methods: The response of Siebold's beech (Fagus crenata), a representative deciduous tree species, to ozone was studied in a free-air ozone exposure experiment in Japan. Photosynthesis and stomatal conductance were measured under ambient and elevated ozone. An optimization model of stomata involving water, CO2 and ozone flux was tested using the leaf gas exchange data.

Key results: The data suggest that there are two phases in the avoidance of ozone stress via stomatal closure for Siebold's beech: (1) in early summer ozone influx is efficiently limited by a reduction in stomatal conductance, without any clear effect on photosynthetic capacity; and (2) in late summer and autumn the efficiency of ozone stress avoidance was decreased because the decrease in stomatal conductance was small and accompanied by an ozone-induced decline of photosynthetic capacity.

Conclusions: Ozone-induced stomatal closure in Siebold's beech during early summer reduces ozone influx and allows the maximum photosynthetic capacity to be reached, but is not sufficient in older leaves to protect the photosynthetic system.

Keywords: Fagus crenata; Siebold's beech; Tropospheric ozone; photosynthesis–stomatal model; stomatal closure; stomatal conductance; stress avoidance.

Fig. 1.

Example of diurnal changes of environmental factors (photosynthetic photon flux density, PPFD; air temperature, Temp.; vapour pressure deficit, VPD) and diurnal changes in leaf gas exchange for Siebold's beech grown under ambient and elevated O₃. Data are represented as the mean ± s.e.

Fig. 2.

Relationship between stomatal conductance and leaf-to-air vapour pressure deficit (VPD) for Siebold's beech grown under ambient and elevated O₃ (as indicated in the key). Data were obtained under a PPFD >500 µmol m⁻² s⁻¹. The fitting line denotes the model function g_s = g_ref [l – m ln(VPD)] in ambient (black line) and elevated O₃ (grey line). The determination coefficient (r²) in ambient and elevated O₃ was 0·60 and 0·39 in June, 0·58 and 0·53 in August, and 0·36 and 0·15 in October.

Fig. 3.

Night-time stomatal conductance for Siebold's beech grown under ambient (white bars) and elevated O₃ (grey bars). Data are represented as the mean for five replicated trees ± s.d. The P-value was calculated from the t-test; n.s. denotes non-significant.

Fig. 4.

Relationship between stomatal conductance and the dominant parameter of the photosynthesis–stomatal model 1·6A/ assuming that O₃ stress is zero (k = 0) or an avoidance of O₃ stress via stomatal closure (k = 1·2 × 10⁵ or 1·8 × 10⁵) for Siebold's beech grown under ambient and elevated O₃ (as indicated in the key) in June, August and October 2012. The P-value was calculated from the ANCOVA; n.s. denotes non-significant.