Introduction

‘Summer every day and winter every night’ (Hedberg 1964) is a brief but succinct characterization of the tropical alpine climate, pointing to the fact that the amplitude of the daily temperature oscillation by far exceeds that of the monthly mean values. Although cloudiness exerts a mitigating effect on the daily temperature extremes during the rainy seasons, nocturnal frost may occur throughout almost all of the year at altitudes above 4000 m. Therefore tropical alpine plants must maintain mechanisms of permanent frost hardiness which differ considerably from those providing the overwintering plants of temperate climates with seasonal frost resistance. Whereas, for example, in Norway spruce the frost-hardy state is characterized by a high proportion of unsaturated fatty acids in the membrane lipids (Senser 1982), by a shift from photosynthetic starch formation to the production of sucrose and its galactosides (Kandler et al 1979), by a reduced capability of photosynthetic electron transport (Senser & Beck 1979) and by a suspension of growth activity, tropical alpine plants must combine physiological features providing frost resistance with continuously high rates of photosynthesis and growth.