Can we heat-proof canola?

Canadian research shows that day time highs of 27°C and above can start to disrupt pollination and seed production in flowering canola. Warm nights can add to the yield loss.

Canola doesn’t like heat 

Many studies show hot temperatures at flowering reduce canola yield. Agriculture and Agri-Food Canada (AAFC) research scientist Malcolm Morrison ran experiments showing that B. napus grown in a hot growth chamber produced short plump pods that did not contain seeds. Morrison’s experiments show that canola yield loss starts at around 29.5°C, and that pollen tubes (female) grew best at temperatures of 20°C to 23°C.  

More recently, AAFC researchers Baoluo Ma and Wei Wu compared two temperature treatments: maximum 23°C and minimum 17°C, which was the control, and maximum 27°C and minimum 24°C. Ma’s high temperature treatment reduced canola seed yield by 34 per cent. Ma found that heat stress reduced pollen viability and its germination on the stigma, resulting in pollination failure. This heat stress also reduced seed set and caused missing pods. 

Raju Soolanayakanahally, in an AAFC Saskatoon greenhouse study on the effects of heat and drought on canola performance, found that high day-time temperatures led to stunted flowers, shrunken stamens and shrivelled petals. The published report noted that heat reduced photosynthetic activity in general and disrupted many steps in the pod and seed process. Specifically, it altered the gametogenesis (of pollen and ovules in flowers), embryo sac differentiation, fertilization and post-fertilization structures, such as the growth of the endosperm and the embryo. The result was a 43 per cent decrease in the number of pods and an 85 per cent reduction in seed yield when compared to canola plants grown under more moderate temperatures. 

What can canola growers do about it?

The following five actions sound reasonable, but none has a proven track record.

Early seeding 

Status: Relatively easy to implement, but it comes with trade-offs

Seeding early has long been the recommended practice to make sure canola isn’t flowering during the hottest days of the year. This can work to some extent, given that weather averages show that early July is cooler than late July. Seeding any time in the first half of May usually means canola starts to flower around the first week of July. But we can get hot days in June, and early seeding has other risks – including frost and slow growth that can increase flea beetle damage. 

Genetics 

Status: Nothing commercial at this time

Malcolm Morrison showed considerable genetic differences in a growth-chamber comparison of 47 cultivars from 2011 to 2013. Chad Koscielny, in his University of Manitoba PhD thesis, also showed differences in heat tolerance among hybrid parent lines. Both studies suggest that seed companies could improve canola heat tolerance by selecting for the trait from among existing lines. 

Koscielny is now North America Canola Breeding Lead for Corteva, the company that breeds and market Pioneer Hi-Bred and Brevant canola seed. “Heat tolerance at flowering is an area we’re focused on,” he says, “but it will be a few years before we have anything commercial.” 

University of Manitoba plant physiologist Claudio Stasolla has looked into heat stress genetics. In the article, “The Brassica napus phytoglobin 1 (BnPgb1) mitigates the decrease in plant fertility resulting from high temperature stress,” published in the Journal of Plant Physiology in 2024, Stasolla described his study to determine if over-expression of B. napus Pgb1 (BnPgb1) “confers thermotolerance to plants exposed to moderate heat stress during the reproductive stage through enhancement of antioxidant responses.” 

To test this notion, Stasolla and his co-investigators selected B. napus lines that over-expressed or down-regulated BnPgb1. They then exposed these lines to daily heat stress cycles during the initial stages of flower development. The study concluded that BnPgb1 plays an important role in B. napus fertility and pollen viability under heat stress.  

Stasolla generated transgenic plants to over-express the Pgb gene and therefore over-produce the Pgb protein. The trait could lead to canola plants with higher tolerance to heat stress during flower fertilization and seed formation, Stasolla says. He adds that some current cultivars and experimental lines might have naturally higher BnPgb1. Breeders could also select for some degree of higher expression. 

Cytokinin 

Status: Not ready to recommend it as a best practice

University of Manitoba researcher Mark Belmonte has done growth chamber studies looking at the effect of cytokinin applications on canola and wheat. For canola, he applied the hormone at sclerotinia fungicide timing (10 to 50 per cent flower) right before cranking up the growth chamber temperatures to 30°C days and 16°C evenings. He compared results to canola plants grown at 22°C. 

Cytokinin is a naturally-occurring plant hormone with five primary functions: 

• Cell division 
• Above-ground growth 
• Sugar transportation 

• Promotes chlorophyll synthesis 

• Promotes stomata opening 

In a heat event, cytokinin levels tend to drop in canola plants, Belmonte says. Spraying on cytokinin just before the heat event is intended to replenish cytokinin levels in plant reproductive structures, which could allow for greater pollination and early cell division in new seeds. Overall seed set improves when compared to plants without the added cytokinin, he says. 

Corteva’s X-Cyte is one commercially available cytokinin spray. Corteva claims it protects yield potential by reducing flower abortion and pod loss during times of excess heat, and improves overall crop resilience by delaying premature senescence caused by heat loss. 

Belmonte notes that soil moisture is essential to cytokinin efficacy. “If the plant has limited water, cytokinin provides no benefit,” he says. Though cytokinin sprays act at the point on contact in the flowers (and is not taken up through the roots), soil moisture is essential to maintain crop health. “This underscores the difference between heat blast and drought,” Belmonte says. These products don’t work if the crop is drought-stressed. But with healthy plants and high yield potential, cytokinin can help protect yield in a heat wave.  

Benefits of a cytokinin spray last about a week, perhaps as much as two weeks, Belmonte says. If a heat wave continues, a second application may help to protect the later flowers.  

Corteva provides the following application tips: 

• apply cytokinin just before the forecast heat event, targeting flowers. 

• commercial cytokinin products can go in a tank mix with fungicides 

• apply the cytokinin-based blend at 1/2 litre per acre.  

• use the same water rate as recommended for sclerotinia stem rot fungicide 

Nutrients, including boron 

Status: Boron has been examined for years and shows no consistent benefit

In general, a canola crop with access to balanced and adequate nutrition – either from soil reserves or fertilizer – will be healthier and more resilient to stress.  

Canola with adequate sulphur and potassium are also shown to have higher drought tolerance, but the major issue with heat at flowering – from a management perspective – is not related to drought management. It is the heat effect on flowering, and the major economic cost of that heat, specifically, is higher in a crop experiencing generally good growing conditions and high yield potential.  

Boron is the one nutrient often connected with heat stress tolerance in canola. 

Boron contributes to many plant functions, including pollination and seed reproduction. Canola has higher boron demand than other crops, yet deficiencies of this essential micronutrient are not that common on the Prairies. They can occur in light soils with low organic matter, high pH (8+) and excess moisture.  

Ontario studies from 2007 to 2012 tested the hypothesis that foliar boron applied to heat-stressed canola at early flower with reduce flower abortion and increase pod retention. Laxhman Ramsahoi, a University of Guelph Masters student, conducted the 2009 to 2012 studies and shared results in a 2013 thesis report. Ramsahoi tested liquid boron at rates of 0.4 lb./ac. and 0.8 lb./ac. of actual boron, and borax at 0.6 lb./ac. of actual boron. 

The years had quite different weather – 2009 was cool with favourite canola-growing conditions, 2010 was warmer with evenly distributed moisture, 2011 had hot weather through July and 2012 had a long hot, dry period from mid June to the end of July. 

Averaged across all nine sites that produced data, foliar boron provided no significant benefit. But it did produce a significant benefit – a five to six per cent yield increase compared to checks – in fields with heat stress at flowering. 

“These results support the hypothesis that foliar boron application can help mitigate heat stress effects in spring canola specifically by reducing heat-induced pod abortions,” Ramsahai concluded. In the absence of heat stress at the early reproductive stage, foliar boron had no significant effect. 

Hugh Earl, University of Guelph professor and Ramsahai’s advisor, adds: “Even when boron is effective at increasing yields, the benefit tends to be small.”

Belmonte is not willing to give up on boron – mostly because a study published in 2024 shows that boron applied at flowering can also treat sclerotinia stem rot. The study compared various boron rates against treatments with boscalid fungicide and untreated checks. Boron applied alone reduced the number of S. sclerotiorum-forming lesions by 87 per cent compared to an untreated control. The study showed that boron can directly antagonize S. Sclerotiorum spores and also primes the B. napus plant defence response. 

Belmonte would like to do more field studies on fungicide with added cytokinin and boron, and various combinations. Canola with high yield potential is mostly likely to provide an economic return from cytokinin, Belmonte says. That same high-yielding canola may also benefit from sclerotinia stem rot protection. The problem, from an on-farm research perspective, is that a tank mix of fungicide, cytokinin and boron stacks three treatments. A proper experiment would look at all three separately plus all the possible combinations to see which actually worked best. 

Misting 

Status: It can work, but isn’t practical

Misting crops at flowering has shown to work in rice. A recent Chinese study (Jiang et al, 2020) showed yield increases of around 13 per cent for rice misted with water during a heat wave. At least one company is developing a fleet of misting drones to cool high-value crops.  

“Evaporative cooling is a real thing and it works,” says Malcolm Morrison, “but what is the infrastructure needed to mist an entire section of canola? Mist must be very fine. Droplets tend to magnify solar radiation and heat the pod prior to cooling.” 

Morrison adds that frequent misting is a “great way” to increase diseases such as sclerotinia stem rot. 

“So, this is a case of it working in principle, but not really offering a solution that is affordable on a large scale,” he says. “It is much better to spend the research effort selecting for heat stress tolerance genetically.”