An additional problem is that, unlike some crops, tomatoes require a fairly long period of enrichment each day (8-10 h) in order to show significant yield increases (Willits and Peet,1989). Acclimation of photosynthesis has been noted by besford (1993) and yelle et al.(1989,1990). Behboudian and lai (1994) found tomatoes Co2 enriched at 22 c did not grow any more rapidly than non-enriched plants in most of the enrichment treatment. At 25 c , there was a growth response overall, but only if enrichment continued for at least 8 h. Four hours of enrichment actually decreased growth significantly compared to the ambient co2 treatment. Exposing plants to high co2 at 22 c for half the week was also ineffective in terms of increasing growth. They suggested that lower translocation rates in Co2 enriched plant meant that photosynthates accumulated in leaves rather than contributing to leaf expansion. Co2 enriched leaves had higher rates of dark respiration,possibly because of the greater presence of photosynthates. At 25 c , higher translocation rates may have improved the plant's ability to use leaf reserves in growth.