WHAT HAPPENS TO SOYBEANS WHEN THEY GET FROSTED!

 Various areas in the province received frost in September and there continues to be concern the impact subsequent frost will have on the soybean crop. The extent of injury will depend on the stage of development as well as the length of exposure to frost.

 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.  Furthermore, 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 which is attained at the R7 stage. Frost after physiological maturity (maximum dry weight) generally does not damage soybean plants if pods remain intact. 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).

 Table 1 – Stage of development descriptions for soybeans.

 R1 (Beginning flower) – One open flower on any node on main stem.

R2 (Full flower) – Open flower at one of the two uppermost nodes.

R3 (Beginning pod) – Green 0.5cm (1/4″) long pod at one of the                                              four upper nodes.

R4 (Full pod) – Green pod 2cm (3/4″) long at one of the four upper                       nodes.

R5 (Beginning seed) – Beans beginning to develop 0.25cm (1/8″) seed                                    in at one of the four upper nodes.

R6 (Full seed) – Green seed fills pod cavity at one of the four                                           uppermost nodes.

R7 (Beginning maturity) – One normal pod on main stem has reached its mature colour (brown or tan); 50% of leaves yellow.

R8 (Full maturity) – 95% of pods are mature brown colour.  Harvest           moisture is reached within 1-2 weeks.

Freezing during earlier development (the green pod stage, R6) will result in a severely damaged bean with a greenish “candied” appearance.  But even moderately frosted beans with a greenish colour and slightly wrinkled seedcoat are considered as damaged soybeans and will be discounted if present in excess of limits (3%) for No. 2 Canada soybeans.  The seed will eventually dry down with a wrinkled seedcoat and 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”.

Besides affecting seed quality, an early frost can also significantly reduce seed yield.  See table #3.

 Table 3 – Soybean Yield Response to Freeze Damage

 Growth Stage                                                   Yield Reduction

 R4      Full Pod                                                            70-80%

R5      Beginning Seed                                                50-70%

R6      Full Seed                                                          15-30%

R7      Beginning Maturity                                            0-5%

R8      Full Maturity                                                     0%

 What is the impact on seed quality?  It has been determined that temperatures required to cause reductions in seed germination and vigour decrease as seed maturation progresses (Table 4).

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

 Temp.           Exposure                           PERCENT GERMINATION

 C                 Time                       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

 Therefore, 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 should have the same level of seed moisture as non-frosted soybeans. Seed protein should not be affected by frost, but the oil concentration will be lower if the frost occurred before the seed grew to full size. In soybeans grown for seed, germination and vigour may be reduced in cases where the seed contained more than 55 percent moisture at the time of the frost.

 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.  However since even R7 fields have not completely turned yellow some green beans will remain green at harvest.  Yield impact is minimal. (0-5% Yield reduction)

2)     If some pods had turned yellow or brown before the frost these pods should be opened up 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 will likely turn yellow if given enough 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.

Albert Tenuta, Field Crop Plant Pathologist, OMAFRA, Ridgetown

and

Horst Bohner, Soybean Specialist, OMAFRA, Stratford

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