Abstract
To clarify the effect of sunshine in early morning upon the spring frost damage, the critical thawing rate causing damage and the thawing rates of buds exposed to sun beams immediately after sunrise in the field were investigated using 5 year old Todo fir (Abies sachalinensis MAST.)
In the Todo fir immediately before unfolding, no damage was observed both in buds and stems, when thawed slowly in the air at 10 to 15°C (thawing rate: 0.6°C/min to 10.8°C/min) after freezing at -4°C for 1 to 2 hours, while the buds and stems were seriously damaged when thawed rapidly by immersion into the water at 5°C (thawing rate: 4.4°C/ min) or exposure to sun beams at 14 o'clock (2.4°C/min). It was noted however that when the frozen buds at -4°C were kept in the air at 0°C for 45 minutes before being rapidly rewarmed, they suffered hardly any damage.
In the field, the thawing rates of the buds exposed to sun beams immediately after sunrise were within 0.5°C/min, which was not great enough to cause damage.
From these facts, it may reasonably be considered that sun beams in early morning have hardly any harmful influence on the frozen buds and stems of a Todo fir in a spring frosty morning.
