Dynamics of Tissue Heat Tolerance and Thermotolerance of PS II in Alpine Plants
At first sight heat may not be expected to be an environmental constraint of significant importance in alpine environments, as low atmospheric temperatures are among the well-known common features of the alpine macroclimate (see Körner 2003). Although atm
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Dynamics of Tissue Heat Tolerance and Thermotolerance of PS II in Alpine Plants Gilbert Neuner and Othmar Buchner
6.1
Heat Strain in Alpine Environments was found to heat to temperatures of up to 80 C
At first sight heat may not be expected to be an environmental constraint of significant importance in alpine environments, as low atmospheric temperatures are among the well-known common features of the alpine macroclimate (see K€ orner 2003). Although atmospheric temperatures are low, alpine plants – due to their small, prostrate growth form – often grow very close to the soil surface and can be surrounded by bare soil, causing a decoupling from ambient air temperature. In addition, the decoupling effect is promoted by an appropriate protection from cooling winds, a favourable slope, and exposure to the usually increased solar irradiation at high altitudes. However, the major determinant of plant temperature in alpine habitats is plant stature (see Salisbury and Spomer 1964; K€ orner and Cochrane 1983; K€ orner and Larcher 1988). This effect of plant stature on daily leaf temperature courses is shown in Fig. 6.1a for four species with contrasting growth forms growing close together at an alpine site (2,200 m). Infrared imaging reveals the small scale scattering of temperatures at alpine sites where temperature differences of more than 30 C can occur within centimetres (Fig. 6.1b). Compared to plants, the bare soil surface is exposed to even more extreme temperatures. It is usually the coldest during the night but exposed to considerable daytime heating. South-facing bare, dark, raw humus
G. Neuner (*) O. Buchner Institute of Botany, University of Innsbruck, Innsbruck, Austria e-mail: [email protected]
(Turner 1958). Soil temperatures drop rapidly with soil depth. A comparison of maximum leaf temperatures of six alpine plant species of diverse growth forms determined at three alpine sites (1,950, 2,200 and 2,600 m) throughout three successive growing periods (1998, 1999, 2000) reveals the frequency of heat events (Fig. 6.2). Across species, daily maximum leaf temperatures exceed 30 C quite frequently (13%; Fig. 6.3). 30 C is the temperature threshold for the onset of heat hardening in Silene acaulis (Neuner et al. 2000; >32 C in higher plants: Alexandrov 1977). 42 C is the lowest heat killing temperature of the most susceptible alpine plant species (Pisek and Kemnitzer 1968; Larcher and Wagner 1976; Smillie and Nott 1979; Gauslaa 1984) which is still surpassed on about 1% of days. Overheating and heat stress appeared to be particularly pronounced in compact cushion plants, with cushions being 15–24.5 C warmer than the air on clear days (Salisbury and Spomer 1964; K€orner and De Moraes 1979; Gauslaa 1984; Larcher and Wagner 2009). Under drought conditions, S. paniculata leaves were up to 36 C warmer than the air (Neuner et al. 1999). Although heating above ambient temperature in a cold climate can be advantageous for carbon uptake, growth, and reproductive processes, the heat-trapping stature ma
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