Factors Affecting Plant Survival From Flooded Soils

Extended periods of saturated soils will affects crop growth and yield, but the extent of the damage will depend on the duration of the flooding, type of crop, air and water temperatures just after the time of flooding and even the plant growth stage. All these factors make it really difficult to predict how extensive the damage will be until sufficient time has passed and accurate observations can be taken.

HOW EXCESS MOISTURE EFFECTS PLANTS
Germinating seeds and plant roots require oxygen in the soil for survival. Research has shown that oxygen concentration can be close to zero after 24 hours in flooded soils. Without oxygen, these living plant tissues cannot breathe (respire) and support critical life functions such as root and shoot growth as well as nutrient uptake. When soil oxygen levels drop too low in the soil, plant respiration changes to a pathway similar to fermentation. While some life-sustaining energy is produced during fermentation, energy production is reduced by up to 95 percent. In a nutshell, the main effect of low oxygen is drastically reduced metabolism, which can reduce yield and, if long enough in duration, cause death to a portion or the entire plant.

FACTORS THAT INFLUENCE PLANT SURVIVAL
Duration of Saturated Soils
The longer the duration of flooding and/or water-logged conditions, the greater the chance is for injury. Duration of flooding is important because many of the effects of low oxygen on plants are reversible if the duration is not too long. Long durations allow for increased oxygen depletion and the build up of harmful gases like carbon dioxide.

Under flooded conditions, 36 to 48 hours is often the tolerable limit for plants. Under water-logged conditions, crop tolerance and adaptation is plant species dependent. Most annual crops can tolerate wet soil conditions for 3 to 7 days. In terms of relative crop tolerance to excess water :

• cereal crops: oats > wheat > barley
• oilseed crops: canola > sunflower > flax
• pulse crops: fababeans > soybeans > field beans > peas

Of all these crops emerged fababeans and soybeans tolerate excess water the best, differences between crop types is a lot smaller when the flooding or saturated soils occurs before the crop emerges - when the seed is germinating.

Temperature – Both Water & Air
Temperature affects the speed of plant respiration. The warmer the temperature, the faster the plant respires, the quicker oxygen is depleted and the sooner the fermentation process begins. Cooler temperatures and cloudy conditions prolong survival and reduce the amount of damage from flooding, due both to slower growth rates and that more oxygen will dissolve in cool water than in warm water.
With emerged plants that survive the flooded soils, cold, wet weather favors disease development. Under these severe soil conditions seed treatments can only provide limited protection as the slow growing root systems allow soil-born pathogens greater opportunity to cause damage.
 
Stage of Plant Growth
Fields seeded just before the heavy rains should have better survival rates than earlier seeded fields which hadn’t emerged before the soils were flooded. This is because germination is a three step process - absorption of water (imibibition), enzyme activation and ending with emergence of the root from the seed coat. Seeds in the first stage of germination (imbibition) have very low rates of respiration which will allow them to survive longer than seeds that are actively respiring as they develop roots. Saturated soil conditions may also retard or prevent germination because oxygen concentration is too low around the seed.

For emerged crops, young plants are more susceptible to damage for two reasons. First of all, the growing point for many crops are at or below the soil surface and therefore is directly subject to the stress of oxygen depleted conditions. Secondly, young plants are in the process of trying to successfully establish a vigorous root system. Stunting or death of roots by oxygen-depletion can be a major stress for a plant that is not yet fully established. On the flip side however, a younger plant has a smaller demand for oxygen and may be able to survive better.

PLANTS SURVIVE EXCESS MOISTURE - MAY NOT BE OUT OF THE WOODS YET
Even if seeds and/or plants survive excess moisture conditions, the effects from those conditions could last after waters recede and soils dry up. One concern for plants that have yet to emerge is soil crusting which may result after the soil dries after a rain (especially those high in clay). The crust can physically impede growth of the emerging plant and therefore prevent emergence.

For emerged crops subjected to excess moisture during the early vegetative stages can slow or impair future root development. The main tap root of canola or soybean plants are often killed after sitting in saturated soil conditions and all new lateral roots grow close to the soil surface. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Lengthy periods of wet soil conditions can also favor the development of disease.

FINAL NOTES:
To determine plant survival, it is best to check the color of the growing point and look for new leaf growth five days after water drains from the field. A living growing point will be white and cream colored, while a dead growing point is dark and spongy in texture. What is the outcome or yield of the water-logged areas of the fields? The answer will depend on all the factors listed above plus Mother Nature and her plans in the upcoming days.
 

WHAT ABOUT THE EFFECT OF FLOODING ON SOYBEAN INOCULANTS?
How long can commercial soybean inoculants survive under saturated soil conditions? The Bradyrhizobium japonicum bacteria that are responsible for forming nodules on the soybean roots and fixing nitrogen for the plant fixation are dynamic and their numbers will increase and decrease in the soil depending upon how harsh the soil environment is. We suspect that Bradyrhizobium don’t live long after the oxygen has been depleted from the soil.

For growers using an on-seed inoculant – either liquid or peat, flooded soils can have a detrimental effect on the number of nodules found on the main plant root (crown nodules). This is because Bradyrhizobium bacteria don’t move in the soil and they can only enter soybean plants through root tips. When only an on seed inoculant is used, the only root that the bacteria can only enter through is the root that emerges from seed coat. However if the soil is flooded for too long, most of the bacteria that was around the seed will be killed and not enough bacteria will be able to enter the root tip which can result in poor crown nodulation. The bacteria numbers will quickly increase after the soil dries out and becomes oxygenated once again but since the bacteria cannot move very far in the soil, the soybean root tip will have grown too far away to allow nodulation to occur.

If the soybeans had germinated before the soil flooded, the amount of crown nodulation might be better if the bacteria had a chance to infect the emerged root before the soil became saturated. However any factor that causes stress on the soybean plant can have a negative impact on nodulation so even if the bacteria had a chance to enter into the root, crown nodulation may still be poor. The impact of flooding won’t be known until the soybeans reach the 2nd to 3rd trifoliate stage and the plant roots are examined for nodules.

For growers using a granular inoculant (where bacteria is placed in the seed row) then nodulation on lateral roots should be unaffected by the flooded soil. This is because by the time soybean roots grow into the soil profile where the granular inoculant was placed, the bacteria numbers would have had a chance to build up their numbers enough to result in effective nodulation on lateral roots. This would be a similar situation where a farmer seeded soybeans into a field that had previously grown well nodulated soybeans, and the bacteria have become naturalized to the field. Current thought is that a field will have had to have grown at least 2 successful soybean crops to fall into this category.

This difference between on seed and granular inoculants is the reason why MAFRI has always suggested that farmers growing soybeans on heavy clay soils in the Red River Valley include both an on seed and granular inoculant when growing soybeans for the first or even second time in a field.

For further information, contact your GO representative.

From the June 2010 edition of the CROPS E-NEWS (subscribe)