Abstract: The flue-cured tobacco of NC89、LJ851、LJ911 grow rapidly, consume water a lot in the vigorous growing stage. The level of relative soil water content had better be kept at 75%~85%. The fall of photosynthesis rate at noon is brought by stomatal limitation. When relative soil water is below 75%, the cause of photosynthesis rate drop is non-stomatal limitation. The non-stomatal limitation play dominant role in decline of photosynthesis while relative soil water content is below 65% which could do harm to photosynthesis organ.

Key words: Flue-cured tobacco, water stress, photosynthesis rate, stomatal limitation, non-stomatal limitation