Determinants of stomatal sluggishness in ozone-exposed deciduous tree species

Highlights

• Our knowledge of ozone (O₃) effects on dynamic stomatal response is still limited.

• Determinants of O₃-induced stomatal sluggishness were examined in deciduous tree species in open-top chambers.

• Ozone exposure slowed closing of stomata after leaf cutting.

• Stomatal sluggishness was well explained by stomatal O₃ flux per net photosynthesis.

• Stomatal sluggishness depended both on ozone flux and on the capacity for detoxification or repair.

Abstract

Our knowledge of ozone effects on dynamic stomatal response is still limited, especially in Asian tree species. We thus examined ozone effects on steady-state leaf gas exchange and stomatal dynamics in three common tree species of China (Ailanthus altissima, Fraxinus chinensis and Platanus orientalis). Seedlings were grown and were exposed to three levels of ozone in open-top chambers (42, 69, 100 nmol mol^− 1 daylight average, from 09:00 to 18:00). At steady-state, ozone exposure induced an uncoupling of photosynthesis and stomatal conductance, as the former decreased while the latter did not. Dynamic stomatal response was investigated by cutting the leaf petiole after a steady-state stomatal conductance was reached. Ozone exposure increased stomatal sluggishness, i.e., slowed stomatal response after leaf cutting, in the following order of sensitivity, F. chinensis > A. altissima > P. orientalis. A restriction of stomatal ozone flux reduced the ozone-induced sluggishness in P. orientalis. The ozone-induced impairment of stomatal control was better explained by stomatal ozone flux per net photosynthesis rather than by stomatal ozone flux only. This suggests that ozone injury to stomatal control depends both on the amount of ozone entering a leaf and on the capacity for biochemical detoxification or repair. Leaf mass per area and the density of stomata did not affect stomatal sluggishness.

Introduction

Tropospheric ozone (O₃) is recognized as a significant phytotoxic air pollutant and greenhouse gas (Bytnerowicz et al., 2007, Serengil et al., 2011). Ozone concentrations have been increasing in the northern hemisphere since the pre-industrial age (Akimoto, 2003, Vingarzan, 2004). Especially in East Asian countries, further increases in O₃ concentrations are predicted throughout this century because of rapid economic growth (Ohara et al., 2007, Yamaji et al., 2008).

The phytotoxic nature of O₃ may cause adverse effects on physiological and biochemical processes in tree species (Karnosky et al., 2003, Matyssek and Sandermann, 2003, Ashmore, 2005, Paoletti, 2007). There is still little information on effects of O₃ on native plant species of Asia (Royal Society, 2008). Many studies on European and North American species reported that O₃ may reduce carbon assimilation, and limit the growth of trees (e.g., Wittig et al., 2007, Wittig et al., 2009). On the other hand, the effect of O₃ on stomatal conductance is not straightforward (Mansfield, 1998, Paoletti and Grulke, 2005). Ozone has been reported to induce stomatal closure (Wittig et al., 2007). However, slower or less efficient stomatal control may occur, especially a weaker ability to close stomata, referred to as “O₃-induced stomatal sluggishness” (Paoletti, 2005, Mills et al., 2009, Paoletti et al., 2009, Paoletti and Grulke, 2010, Hoshika et al., 2012a, Hoshika et al., 2013a, Hoshika et al., 2013b, Dumont et al., 2013). Sluggishness may occur because O₃ reduces stomatal sensitivity to abscisic acid (ABA) (Mills et al., 2009). This loss of stomatal response to ABA is related to O₃-induced ethylene emission (Wilkinson and Davies, 2010). Although our knowledge of the mechanism is still limited, such a reduced stomatal control may impair an efficient water use for plants (Sun et al., 2012).

Stomatal O₃ flux is a crucial factor for the assessment of O₃ effects, because stomata are the principal interface for entry of O₃ into a leaf (Omasa et al., 2002, Karlsson et al., 2007, Mills et al., 2010). Stomatal response to environmental stimuli may be related to leaf anatomical traits such as stomatal density (e.g., Drake et al., 2013). Small stomata and their general association with high density of stomata provide the capacity for rapid response in the stomatal conductance of a leaf (Aasamaa et al., 2002, Heatherington and Woodward, 2003, Drake et al., 2013). This implies that fast response of stomata in leaves with high stomatal density and/or lower stomatal conductance during O₃ exposure may result in less diffusion of O₃ into a leaf, and may lead to a decrease in O₃-induced injury (Pääkkönen et al., 1995).

Foliar O₃ injury may also depend on the available resource for repair or biochemical O₃ detoxification of a leaf (e.g., Tausz et al., 2007, Paoletti et al., 2008). Wieser et al. (2002) suggested that antioxidative capacity increased with increasing leaf mass per area (LMA). Inherent LMA may thus be roughly related to sensitivity to O₃ stress (Bussotti, 2008, Zhang et al., 2012). Also Musselman and Minnick (2000) suggested that plant tolerance to O₃ stress may depend on its photosynthetic capacity because detoxification and repair require energy (Noctor and Foyer, 1998). The sensitivity to foliar O₃ injury may thus be explained by the ratio of stomatal O₃ flux to net photosynthesis (Fredericksen et al., 1996, Kolb and Matyssek, 2001), indicating a balance between O₃ exposure of mesophyll cells and availability of photosynthates for repair or detoxification.

In this study, we examined O₃ effects on steady-state leaf gas exchange and dynamic stomatal response under severe water stress imposed by cutting a leaf in three tree species that are common in China. The objective of the study was to test whether the degree of O₃-induced injury to stomatal control was related to stomatal density, LMA, stomatal O₃ flux or the ratio of O₃ flux to net photosynthesis.