The effect of water stress on the mossDicranum polysetumSw. was studied in the growth chamber, and water potential (psi;) was found to vary as much as 15ensp;atm between the tip of the plant and areas 2 to 4ensp;cm within the cushion, which suggests that very little water is transported from the base of the stem to its tip. With an infrared gas analyzer it was determined that photosynthesis ceases at minus;21 to minus;23ensp;atm and the moss remains dormant until it is rewetted. Upon rewetting, there is a sharp burst of respiration that doubles the normal CO2output. This burst reached its maximum level of 4ensp;mg CO2ensp;gminus;1ensp;hminus;1within 15ensp;min and was of short duration. Full photosynthetic rate was restored in 8ensp;h.
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