Heat Stress and Blank Pods in Canola

Should I scout for blank pods in my canola crop?

It is a good idea to examine flowers for heat blast while scouting canola fields for diseases, insects, and weeds. Flowers that open with shortened stamens that do not extend beyond the petals is a sign of heat damage. Lighter coloured petals can indicate that a flower is drying up and will not produce a full pod (Figure 1). Take note of fields with symptoms of heat stress at the beginning of flowering. Pod development may be impacted in these fields, but flowering may recover to allow pod development higher on the plant.

It is important to note that typically 45 to 60 percent of flowers on a canola plant do not develop into productive pods.¹ Canola overproduces flowers from beginning bloom at the bottom of the plant and working up the stem. Often, the topmost flowers are not needed, and young pods fail to enlarge and drop from the plant.

How do high temperatures affect flowering and pod formation in canola?

Canola is a cool-season plant, and temperatures of 28 to 30°C during the day and 16°C at night are considered hot enough to cause heat stress from bud to mid-flowering stages.² High temperatures for even a few days can cause imbalanced plant hormones. The natural processes can breakdown between the stigma (female part of the flower) and any pollen grains (male) that have fallen on it. Pollen tubes are unable to develop and pollen grains are unable to move from the stigma surface to the unfertilized egg and develop into seeds. Pod formation and seed set cease without these processes leading up to successful fertilization.³

Heat injury to pollen and female parts of the flower is related to gene expression. Genes affecting cell wall organization and cellular transport are down-regulated during heat events. Pollen health and integrity are studied for the possibility of acclimation (gradual increases to higher temperatures) and heat shock (sudden exposure to extreme heat). Pollen survival can go from 90 to 25 percent with four hours of high temperatures. Successful pollen tube germination drops from 62 percent to 10 percent with four hours of high temperatures.⁴

Pod formation is affected by severe plant stress which can also be caused by extremely low temperatures or drought during flowering. Frost damage in canola at flowering appears as aborted flowers or poorly filled pods.⁵ Drought during flowering appears as aborted flowers and reduced pod set.⁶ To identify the cause of deformed or missing pods review weather conditions and field history. Did frost occur during flowering? Did drought occur during flowering? Did extreme heat occur during flowering?

What if conditions improve and temperatures cool down?

Plants can recover with lower temperatures and better moisture conditions; however, depending on the growth stage, it could be a week before hormones rebalance and pod formation returns. Flowering typically lasts two to three weeks under normal conditions. Ideal conditions for flowering would include good moisture and temperatures between 13 and 22°C.¹

Canola may not have any yield penalty if conditions improve by early- to mid-flower stage.² Ideal conditions occurring and declining over the three-week flowering period can result in plants aborting and preserving flowers off and on. Stems then appear with pods at the bottom and top, but no pod development in the middle (Figure 3).

What if hot temperatures continue and rains are variable?

Growth stage plays a role in plant response to the severity of heat stress. There is an increased sensitivity to heat stress at early flowering compared to pod fill. Heat stress during pod fill and the resulting hormone imbalance can cause formed seeds to germinate in the pod.

Drought stress exacerbates heat stress, with heat damage occurring at lower temperatures during moisture stress. If drought stress continues, later developing flowers are dropped from the top of the main raceme and branches. By doing this, plants can put remaining energy into preserving the lower pods.

Rains often bring cooler days or nighttime temperatures in addition to drought relief. Cool nights help canola plants recover from hot daytime temperatures; whereas warm nights (16°C) cause more flowers to abort.² Conversely, too much heat with waterlogged soils can reduce yield potential. A heavy rain leaving soil waterlogged for three days or more at flowering can cause reductions in pods per branch and seeds per pod.¹ Rains that are timely and moderate help fill seeds but won’t increase the number of seeds after the bloom stage.⁷

Are there any remedies farmers can use to help preserve canola yield potential during heat stress?

Unfortunately, the best recovery for this canola crop is in the hands of Mother Nature. Timely rains and cooler temperatures, even cooler evenings, are needed to help the crop along. Nutrient applications that include boron and sulfur have been tested as deficiencies of these nutrients can lead to empty pods. However, a positive yield response from these nutrient applications has not consistently been proven during heat stress at flowering.³

Canola varieties are being examined for their tolerance to heat stress. This will be important for future canola crops on the Canadian prairies if hot, dry weather prevails in the summer. Because variety differences can affect this year’s crop, it is worth noting which canola product held up this summer and had the best yield at harvest. Soil differences will also be a factor as water-holding capacity often goes together with heat and drought stress survival.

Consider selecting multiple canola varieties with different maturities and managing planting dates accordingly. Planting the early maturing canola variety first and the later maturing variety last can help spread out flowering timing. This can help reduce risk of all canola on the farm experiencing a heat wave (or heat dome of 2021) during mid-flower (Figure 4).

Can drought-stressed canola be used or marketed for other uses?

Canola is a palatable feed for livestock. Forage canola should have a nutrient analysis performed as it is expected to have higher crude protein and energy levels if cut during the early podded stage rather than later when lower leaves drop. Forage varieties of canola have nutrient compositions close to alfalfa with crude protein of 12 to 16 percent and digestible nutrients of 55 to 60 percent.⁸

Knowledge of sulfur and nitrate levels is always important when introducing a new forage to a livestock diet. Plants should be dried to 16 to 18 percent moisture if baled. Moistures of about 65 percent are recommended for ensiled canola.⁸ Keep in mind this new forage should be blended with other feed allowing livestock to become accustomed to eating it.