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Agronomy update: Correlating root length to pea and lentil yield

Plant breeding has typically focused on yield and key agronomic traits when making variety selections. But plant breeders are also interested in using other tools to better help improve variety selection. The use of physiological breeding is one of those tools, and can complement conventional breeding by characterizing other agronomic traits that contribute to yield, such as root length and rooting depth. Physiological breeding provides breeders with the ability to choose crosses based on these other traits, which can be more reliable than selecting crosses based on yield alone. 

To see if physiological breeding would work in pea and lentil varietal development, Maryse Bourgault, the Western Grains Research Foundation Integrated Agronomy Research Chair in the department of plant sciences at the University of Saskatchewan, conducted research into whether pea and lentil rooting patterns could be used as a selection tool. She was joined by other colleagues at the university, along with others at Montana State University and Washington State University.

The objectives of this study were to determine how much variability there is in total root length and rooting depth in different cultivars of field pea and lentil, and to determine if root growth, particularly at depth, improves the performance of these cultivars under semi-arid environments. 

The research was conducted over three years, from 2017 to 2019, at the Northern Agricultural Research Center of Montana State University near Havre, Montana, on very deep, well-drained fine to fine-loamy Aridic Argiustolls soil with moderately slow to slow permeability. The research site has been managed under no-till for more than 20 years with a diverse crop rotation. Long-term (1916-2018) average annual rainfall was 12.6 inches (319.8 mm), with eight inches (202.9 mm) falling during the growing season from April through August. All three years had below-average rainfall during the research.

The field pea and lentil cultivars that were evaluated were primarily from North Dakota State University, Washington State, and the Crop Development Centre at the University of Saskatchewan. A few advanced breeding lines were also included in 2018 and 2019. Twenty-nine field pea cultivars were evaluated along with 25 lentil cultivars. The cultivars were planted in late April, and managed according to accepted agronomic practices.

Mini-rhizotrons were used to image root growth, allowing non-destructive, field-based measurements. A soil core was removed and the mini-rhizotrons – clear acrylic tubes 3.5 feet long by three inches in diameter (1.05 m long x 7.5 cm) – were inserted into the soil. At flowering and physiological maturity, root images were captured with a CI-600 In Situ Root Imager at depths of approximately 0 to 6, 6 to 12, 12 to 18, and 18 to 24 inches ± 2 inches (0-15, 15-30, 30-45, and 45-60 cm ± 5 cm).

Above-ground measurements were taken at flowering and maturity. At flowering, height was recorded and biomass samples were collected and analyzed for green leaves, dry leaves, stems and reproductive structures. At maturity, full biomass dry weight was measured, along with pod numbers and grain yield. Harvest Index (HI) was calculated as the ration of yield weight to biomass weight.

There was significant variability in total root length among the field pea cultivars. Total root length ranged from approximately 118 inches (3,000 mm) up to 354 inches (9,000 mm). But one pea cultivar, Carousel, had only 43 inches (1,100 mm) of total root length. 

The proportion of pea root length below 12 inches also varied significantly between varieties. One variety, LG Koda, had about 20 per cent of total root length below 12 inches, while others ranged from 32 per cent up to 80 per cent of total root length below 12 inches. 

Logically, cultivars with greater total root length and more roots below 12 inches should yield more because they could access more soil moisture and nutrients. However, the highest-yielding pea cultivars were ranked intermediate for total root length, root length at depth, or the proportion of root length at depth. 

For lentil, total root length, root length at depth, and the proportion of root length at depth also varied significantly by cultivar. Total root length ranged from around 80 inches (2,000 mm) to 335 inches (8,500 mm) among cultivars.

The proportion of lentil root length captured below 12 inches ranged from 22 per cent up to 70 per cent. 

Total lentil root length and root length found below 12 inches were well correlated with above-ground biomass and grain yield in lentil. 

Overall, the researchers concluded that using lentil biomass and yield as selection criteria, as is currently done, would be just as effective as selecting breeding lines based rooting characteristics, and that using lentil root characteristics as an additional tool would not be beneficial.

For pea, rooting characteristics were not found to be a reliable tool for cultivar selection.

While the research did not find a benefit to using pea and lentil root characteristics as a breeding tool, the research was worthwhile in validating current breeding approaches.  

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.