Leaf cold acclimation and freezing injury in C(3) and C(4) grasses of the Mongolian Plateau

The scarcity of C(4) plants in cool climates is usually attributed to their lower photosynthetic efficiency than C(3) species at low temperatures. However, a lower freezing resistance may also decrease the competitive advantage of C(4) plants by reducing canopy duration, especially in continental steppe grasslands, where a short, hot growing season is bracketed by frost events. This paper reports an experimental test of the hypothesis that cold acclimation is negligible in C(4) grasses, leading to greater frost damage than in C(3) species. The experiments exposed six C(3) and three C(4) Mongolian steppe grasses to 20 d chilling or control pre-treatments, followed by a high-light freezing event. Leaf resistance to freezing injury was independent of photosynthetic type. Three C(3) species showed constitutive freezing resistance characterized by <20% leaf mortality, associated with high photosynthetic carbon fixation and electron transport rates and low leaf osmotic potential. One freezing-sensitive C(4) species showed the expected pattern of chilling-induced damage to photosynthesis and >95% leaf mortality after the freezing event. However, three C(3) and two C(4) species displayed a cold acclimation response, showing significant decreases in osmotic potential and photosynthesis after exposure to chilling, and a 30–72% reduction of leaf freezing injury. This result suggested that down-regulation of osmotic potential may be involved in the cold acclimation process, and demonstrated that there is no inherent barrier to the development of cold acclimation in C(4) species from this ecosystem. Cold acclimation via osmoregulation represents a previously undescribed mechanism to explain the persistence of C(4) plants in cool climates.