Control of water movement by stem chilling

Abstract

Apparatus is described whereby a portion of the stem of a young tree of Pinus radiata D. Don could be maintained at a temperature close to freezing point, while the transpiration of the upper part of the plant was measured. The freezing point of the stem was shown to be about -2°C, and while the stem temperature was kept above this point transpiration continued normaiiy; freezing the stem resulted in a decrease in transpiration, after a short lag period, and within 3 hr transpiration was almost undetectable. When the stem temperature was maintained at just below the freezing point for 7 days, the leaf water deficit of the shoot rose from 10 to 47 per cent., but returned to normal values within 4 days after freezing was stopped; the plant showed no signs of damage due to freezing. Field trials, using solid carbon dioxide as a refrigerant, showed that growth was reduced considerably on a branch chilled to -4°C for 7 days, but needles remained alive and appeared healthy. Lower temperatures caused damage to needles and twigs.