Stomata are the pores on a leaf surface controlling gas exchanges, mainly CO2 and water vapor, between the atmosphere and plants (Woodward 1987; Hetherington and Woodward 2003), and thus regulate carbon and water cycles in various ecosystems (Franks and Beerling 2009; Haworth et al. 2010; Taylor et al. 2012). Globally, the gas exchanges between leaf surface and the atmosphere are massive at c. 440 × 1015 g CO2 per year through photosynthesis and 32 × 1018 g H2O per year through leaf transpiration (Ciais et al. 1997; Hetherington and Woodward 2003; Lake and Woodward 2008). Plant leaves usually optimize their gas exchange by altering stomatal pore openness, stomatal aperture size, stomatal frequency (stomatal density and stomatal index), and stomatal distribution pattern, which are regulated by both environmental factors (Lake et al. 2002; Hetherington and Woodward 2003; Schlüter et al. 2003; Casson and Gray 2008; Lake and Woodward 2008; Franks and Beerling 2009) and genetic signals (Bergmann 2004; Liang et al. 2005; Shpak et al. 2005; Hara et al. 2007; Lampard et al. 2008; Hunt and Gray 2009; Hunt et al. 2010; Kondo et al. 2010; Sugano et al. 2010). In connection with the studies of photosynthetic acclimation under climate warming, stomatal response is of special interest, because stomatal traits set the limit for maximum stomatal conductance for gas exchange and thus has a potential to affect carbon gain and water use efficiency (Beerling 1997; Luomala et al. 2005).
Leaf maximum stomatal conductance has been widely used to quantify gas exchange efficiency, which is dependent on stomatal aperture size and shape, frequency, and distribution pattern (Buckley et al. 1997; Hetherington and Woodward 2003; Franks and Beerling 2009; Franks et al. 2009). Usually plants respond quickly to short-term environmental changes by changing the openness of the stomatal pore, a response also known as stomatal movement (Sharkey and Raschke 1981; Kwak et al. 2001; Guo et al. 2003; Young et al. 2006; Shimazaki et al. 2007; Shang et al. 2009). Many studies have shown that stomatal movement is controlled by light (Humble and Hsiao 1970; Sharkey and Raschke 1981; Kwak et al. 2001; Takemiya et al. 2006), CO2 concentration (Ogawa 1979; Young et al. 2006; Lammertsma et al. 2011), temperature (Honour et al. 1995; Feller 2006; Reynolds-Henne et al. 2010), drought stress (Guo et al. 2003; Klein et al. 2004), air humidity (Lange et al. 1971; Schulze et al. 1974), and ultraviolet light (Herčík 1964; Eisinger et al. 2000). In addition to responses to short-term environmental changes through stomatal movement, long-term (decadal) environmental changes such as climate warming may also affect individual stomatal aperture size, stomatal frequency, and stomatal distribution pattern (Anderson and Brisk 1990; Lammertsma et al. 2011).
So far, no consistent conclusions have been drawn on the effect of warming on stomatal traits in the literature. Most studies found that warming had little effect on stomatal density and stomatal index (Apple et al. 2000; Hovenden 2001; Kouwenberg et al. 2007; Fraser et al. 2009), while other studies found that warming could decrease stomatal density (Beerling and Chaloner 1993) and index (Ferris et al. 1996) or increase stomatal density (Reddy et al. 1998; Xu et al. 2009) and stomatal index (Xu and Zhou 2005). In addition, warming could also change individual stomatal aperture size and shape (Ferris et al. 1996; Zuo et al. 2005; Zhang et al. 2010). For example, Ferris et al. (1996) found that experimental warming substantially increased the stomatal aperture length of a perennial ryegrass (Lolium perenne). By contrast, a more recent study reported that warming significantly decreased stomatal aperture length of four alpine meadow species including Thalictrum alpinum, Kobresia humilis, Gentiana straminea, Elymus nutans in the Qinghai-Tibetan plateau, China (Zhang et al. 2010).
In addition to the number, size and shape of stomata on the leaf surface, warming may also alter the spatial distribution pattern of stomata through cell division and cell differentiation (Croxdale 1998, 2000; Berger and Altmann 2000; Shpak et al. 2005) which are regulated by genetic signals (Nadeau and Sack 2002; Bergermann et al. 2004; Juarez et al. 2004; Shpak et al. 2005; Wang et al. 2007; Hunt et al. 2010) and environmental factors (Wang et al. 2007; Casson and Gray 2008) during stomatal development stages. The pattern of stomatal distribution is highly variable among species and recent advances in genetic studies have found that a number of genes, such as SDD1, EPF1, the putative receptors TMM, and the ERECTA-gene family, are involved in the determination of stomatal spacing (Nadeau and Sack 2002; Hunt et al. 2010). The spatial variation of stomata can be characterized at multiple scales, such as the adaxial versus abaxial surface, variations among different leaf sections, and the association/aggregation of individual stomata on a single leaf surface. Earlier studies have reported that stomatal density significantly differed between the adaxial and abaxial surfaces (Ciha and Brun 1975; Green et al. 1990; Ferris et al. 1996, 2002; Croxdale 1998, 2000; Reddy et al. 1998). Meanwhile, the distribution of stomata between leaf surfaces is associated with acclimation and adaptation to environmental factors such as temperature, water stress, light exposure, and CO2 concentration (Parkhurst 1978; Mott et al. 1982; Ceulemans et al. 1995; Smith et al. 1998; Ferris et al. 2002; Driscoll et al. 2006; Soares et al. 2008). Moreover, the changes in the adaxial/abaxial ratio of stomata may also alter leaf function such as photosynthesis, because the stomata in the adaxial and abaxial leaf surfaces feature specific responses to environmental stresses such as CO2 and temperature, thus result in the changes in leaf photosynthesis. Previous studies have found that growth at high CO2 altered the regulation of photosynthesis on the adaxial and abaxial leaf surfaces of maize (Zea mays) (Driscoll et al. 2006) and Paspalum dilatatum (Soares et al. 2008) due to the changes in adaxial/abaxial ratio of stomata between leaf surfaces. In addition, the spatial variation of stomatal distribution was also seen among different leaf sections, such as the leaf tip, middle, and base section (Salisbury 1927; Sharma and Dunn 1969; Tichá 1982; Smith et al. 1989; Ferris et al. 1996; Zacchini et al. 1997; Stancato et al. 1999; Xu et al. 2009). However, several previous studies investigated stomatal features only collecting samples at the middle section of the abaxial or abaxial leaf surface (Beerling and Chaloner 1993; Hovenden 2001; Xu and Zhou 2005; Kouwenberg et al. 2007).
There are three photosynthetic pathways in terrestrial plants including C3, C4, and crassulacean acid metabolism (CAM). Globally, most plant species use the C3 photosynthetic pathway, which is characterized by a low photosynthetic efficiency, because the process is compromised by photorespiration (Osborne and Freckleton 2009). However, C4 pathway represents evolutionary advancements over the ancestral C3 pathway (Ehleringer et al. 1997) due to high rates of photosynthesis and efficient use of water and nitrogen (Wang et al. 2009). It is noted that the performance of each pathway is significantly influenced by environmental conditions such as temperature (Ehleringer et al. 1997). Given the morphological and biochemical innovation, C4 plants are proposed to better adapt to warming conditions than their C3 counterparts (Dwyer et al. 2007; Sage and Kubien 2007). Maize (Zea mays L.) is an economically important food crop, which also uses C4 photosynthetic pathway. So far, most studies mainly focused on the responses of leaf photosynthesis of maize plants to nitrogen (Muchow and Sinclair 1994; Correia et al. 2005), drought stress (Dwyer et al. 1992; Earl and Davis 2003), CO2 concentration (Driscoll et al. 2006; Leakey et al. 2006), and salt stress (Khodary 2004; Sheng et al. 2008). To our knowledge, however, few studies have been reported investigating warming effects on the adaxial/abaxial ratio, the variation of stomata on different leaf sections, and the stomatal distribution pattern on single leaf surfaces of maize plants. The objectives of the current study are to examine warming effects on: (1) stomatal frequency; (2) stomatal aperture size; and (3) stomatal distribution pattern in maize leaves through a field warming experiment in northern China.