An immediate, marked response to small amounts of rainfall occurs in Opuntia basilaris , despite previous drought conditions. The effect of rainfall is upon plant water potential, which is the single most important parameter influencing stomatal opening, CO 2 assimilation, and organic acid synthesis. Nocturnal stomatal opening is initiated following rainfall, and stomata remain open during the daytime. Decreasing stomatal and mesophyll resistances correlate with increasing rates of nocturnal assimilation of 14 CO 2 . Photosynthetic rates of 14 CO 2 assimilation are low, despite high plant water potentials and low stomatal diffusion resistances. The decreased mesophyll resistances and increased rates of nocturnal 14 CO 2 assimilation correlate with the increases of nocturnal efficiency of water use and CO 2 assimilation. The diurnal efficiency of water use and CO 2 assimilation is lower than the nocturnal gas exchange efficiency values.
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Szarek et al. (1975) studied this question.