Temperature Distribution and Freezing of Young Tea Plants in a Tea Garden in Winter

Abstract

Relationship between temperature distribution and freezing of a young tea plant (Camellia sinensis L., cultivar ‘Yabukita’, two years old) was investigated in a field condition. Freezing of the plant was detected by temperature rize with fine thermocouples (0.1mm in diameter) inserted in the stems and attached to the leaves.
The lowest nighttime temperature of the plant (plant hight: 65cm) was observed on leaves at about 35cm above ground. The leaf temperature was 3-4°C lower than the air temperature at 1.5m hight (Ta₁₅₀) when wind speed U₁₅₀ and net radiation Rn were below 0.5m/s and -60W/m², respectively. Freezing of plant body was found to spread to all shoots through trunk in a relatively short period of from several to a dozen minutes. A boundary of frozen and unfrozen part was at the lowest part of trunk in a straw mulch layer.
Relatively wet conditions when dew-point temperatures around leaves at about 35cm height were above -2°C, freezings did not occur until leaf temperatures fell below about -2°C in spite of adequate dew formation. Conditions when dew-points were -2 to -4°C, freezing temperatures of the leaves were changed with and nearly equall to the dew-points. More dry conditions when dew-points were below -4°C, freezings occurred at almost constant temperatures of -4°C, and they seemed not to be related to the dew-points. In these conditions, freezings might occur first in stem tissues without dew formation on leaves.