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Frosted Soybeans?!

Mechanism

Low temperatures injure plants primarily by inducing ice formation between or within cells. The water that surrounds the plant cells freezes first (at about 0 C), while the water within the cell contains dissolved substances that depress the freezing point of water by several degrees. When the water around the cells becomes ice, water vapour moves out of the cells and into the spaces around the cell, where it becomes ice. The reduced water content of the cells depresses further the freezing point of the intracellular water. This can continue to a point without damaging the cell, but below a certain point, ice crystals form within the cell, disrupt the cell membrane, and cause injury to the cell.

Late-Season Frost Injury

Studies indicate that soybeans are easily injured by frost until they reach physiological maturity or R7 stage. Prior to this stage, soybeans will be injured both for grain and seed purposes. Soybean reproductive development can be divided into eight stages (Table 1).

Freezing during earlier development (the green pod stage, R6) will result in a severely damaged bean with a greenish "candied" appearance. Even moderately frosted beans with a greenish colour and slightly wrinkled seedcoat are considered damaged soybeans and may be discounted. The seed will eventually dry down with a wrinkled seedcoat: germination will be severely affected. The Canadian Grain Commission classifies frost damaged soybeans as those "soybeans whose cotyledons, when cut, are green or greenish-brown in colour with a glassy wax-like appearance". Table 2 shows the pertinent grading standards for frosted soybeans and corn.

Adapted from Canadian Grain Commission

Adapted from Canadian Grain Commission

An early frost can significantly reduce seed yield. See Table 3.

Seed quality can be impacted by frost, both with reductions in seed germination and seed vigour. (Table 4).

Seed producers and growers should be especially cautious about using soybean seedlots that have been frosted before maturity. Although some of the severely wrinkled and shrunken seed can be eliminated during cleaning, slightly injured seed may remain which would be expected
to have less seedling vigour, storability and field performance capacity.

Frosted plants will reach harvest maturity earlier but seed moisture will be equal to non-frosted plants. Seed protein should not be affected by frost. Oil concentration will be lower if the frost occurred before the R6 stage.

Points to consider when assessing a field:

1. If all the seed has turned yellow (physiologically mature) there are no yield or quality impacts due to frost. R7 fields that have not completely turned yellow may have green beans that will remain green at harvest. Yield impact is minimal. (0-5% reduction)
2. Yellow or brown pods should be opened to determine if the seed is detached from the pod. If the beans have not detached from the white membrane inside the pod the beans will stay green. If the seed has detached from the pod the seed should turn yellow over time.
3. If all the pods were green before the frost a large percentage of the seed will remain green even after dry-down. (frosted pods may turn black due to frost)
4. Even if the stem is still green, once the temperature gets below -2 C essentially no translocation occurs from the stem to the pods. The majority of the seed will stay green.

   Table 4. Effect of freezing temperature on the standard germination of seed

   Temp. Celsius	Exposure Time	Percent Germination
   		Green	Yellow	Brown
   Control		12.2	84.2	83.2
   -2	1 Hr	7.2	----	----
   	2	12.2	78.5	----
   	4	9.2	72.8	----
   	8	10.0	79.0	----
   	16	9.0	74.0	----
   	32	----	75.8	----
   -7	1	7.0	----	----
   	2	0.8	63.0	77.2
   	4	0.2	61.0	82.5
   	8	0.0	50.8	82.5
   	16	0.0	34.2	80.8
   	32	---	10.2	58.0
   -12	1	0.0	42.8	73.8
   	2	0.0	34.2	77.8
   	4	0.0	23.5	58.0