Photosynthetic responses to altitude : an explanation based on optimality principles

Ecophysiologists have long been fascinated by the photosynthetic behaviour of alpine plants, which often have to withstand extreme environmental pressures (Gale, 1972; Friend & Woodward, 1990; Körner, 2003, 2007; Shi et al., 2006). About 8% of the world's land surface is above 1500 m altitude (Körner, 2007). High altitudes can be climatically unusual, often with (for example) low temperatures, strong winds, and now high rates of warming (Körner, 2003; Pepin & Lundquist, 2008; Rangwala & Miller, 2012). Moreover, the low atmospheric pressure provides a set of environmental conditions unique on Earth (Table 1). There has been extensive speculation about altitudinal effects on photosynthesis and, in particular, how to account for the puzzling – but consistently observed – tendencies towards higher carbon dioxide (CO2) drawdown (low ratio of leaf-internal to ambient CO2 partial pressures (ci : ca; hereafter, χ), resulting in low carbon isotope discrimination) and higher carboxylation capacity (Vcmax) with increasing altitude (Gale, 1972; Körner & Diemer, 1987; Friend et al., 1989; Terashima et al., 1995; Bresson et al., 2009; Zhu et al., 2010). At first glance, it might be expected that CO2 assimilation rates would be reduced at high altitudes due to the low partial pressure of CO2 (Friend & Woodward, 1990). However, actual measured photosynthetic rates are usually as high as, or even higher than, those at low altitudes (Mächler & Nösberger, 1977; Körner & Diemer, 1987; Cordell et al., 1999; Shi et al., 2006).