METHODS FOR THE DETERMINATION OF FREEZING RESISTANCE IN TOMATO LEAVES

The use of a thermogradient plate for freezing leaf disks is described. It is pointed out that an effective length of 35cm ensures that a linear gradient is restored 10 to 15 minutes after a change in the set temperature at one of the margins. The variations in temperature across the plate are small. The plate has room for 15 rows of 15 leaf disks with diameter up to 2cm. Good thermal contact between the leaf disks and the plate is achieved by covering the plate with a thin layer of petroleum jelly and pressing the leaf disks firmly against the plate. An electrical conductivity assay is used for estimating the freezing injury. The method allowed very precise determination of the freeze killing point.

Supercooling points were measured by means of fine thermocouples taped to a sheet of thin plastic film fastened to a frame. A leaf disk was fixed on each thermocouple by means of petroleum jelly. The frame was placed inside a plastic box partly filled with glass beads to increase heat capacity, and the whole setup placed in a deep freezer. The temperature was monitored with a scanning recorder at 0.5 min intervals.

Leaves of four varieties grown in a greenhouse showed no difference in freeze killing temperature. Hardening plants for two or three days at 2 or 5°C resulted in only 0.2 to 0.3° lowering of the freeze killing temperature. An experiment with different tomato species indicated a variation of 0.5°C in freeze killing temperature.

There was no difference in supercooling temperature of the four varieties tested. Wet leaves supercooled to about -7° while dry leaves supercooled to -12°.

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