C 4 photosynthesis at low temperatures

. C 4 plants grown in optimum conditions are, by comparison to C 3 , capable of higher maximum dry‐matter yields and greater efficiencies of water and nitrogen use, yet they are rare outside the subtropics. Both latitudinal and altitudinal limits of C 4 distributions correlate most closely with a mean minimum temperature of 8‐10°C during the period of active growth. The possibility that the C 4 process is inherently incapable of functioning at low temperatures is examined. The reversible effects of chilling on the quantum efficiency of C 4 photosynthesis and the functioning of the individual steps in the C 4 cycle are examined. Chilling also produces an irreversible loss of capacity to assimilate CO 2 which is directly proportional to the light received during chilling. It is suggested that the reversible reduction in capacity to assimilate CO 2 and the lack of an alternative pathway for the utilization of lightgenerated reducing power may make C 4 species more prone to chilling‐dependent photoinhibition. Laboratory studies and limited field observations suggest that this damage would be most likely to occur during photosynthetic induction at the temperatures and light levels encountered on clear, cool mornings during the spring and early summer in cool climates. Even those C 4 species occurring naturally in cool climates do not appear fully capable of tolerating these conditions; indeed their growth patterns suggest that they may be adapted by avoiding ‘rather than enduring’ such conditions.