Correlating root length to pea and lentil yield

Pea and lentil root lengths and depths were assessed as a breeding tool for cultivar selection. In field pea, the highest yielding cultivars were intermediary in both total root length and the proportion of root length found below 12 inches (30 cm) in the soil. In lentil, total root length and root length below 12 inches was well correlated with yield.

Plant breeding has typically focussed on yield and key agronomic traits when making variety selections. The use of physiological breeding can complement conventional breeding by characterizing other traits that contribute to yield, such as root length and rooting depth.

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 located 11 km from 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 over 20 years with a diverse crop rotation. Long-term (1916–2018) average annual rainfall was 12.6 inches (319.8 mm), with 8 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 x 3 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.

Field pea results were varied

There was significant variability in total root length among the field pea cultivars. Total root length ranged from approximately 118 inches (3000 mm) up to 354 inches (9000 mm). But one pea cultivar, Carousel, only had 43 inches (1100 mm) of total root length.

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

Lentil rooting patterns varied as well

Total root length, root length at depth, and the proportion of root length at depth also varied significantly by lentil cultivar. Total root length ranged from around 80 inches (2000 mm) to 335 inches (8500 mm) among cultivars.

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

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

The researchers concluded that using biomass and yield as selection criteria, as is currently done, would be just as effective as selecting breeding lines based rooting characteristics. For pea, rooting characteristics were not found to be a reliable tool for cultivar selection.

Funding was provided by Montana State University and the USA Dry Pea and Lentil Council.

Bourgault, M., Lamb, P., McPhee, K., McGee, R. J., Vandenberg, A., & Warkentin, T. (2022). Genotypic variability in root length in pea (Pisum sativum L.) and lentil (Lens culinaris Medik.) cultivars in a semi-arid environment based on mini-rhizotron image capture. The Plant Phenome Journal, 5,e20037.

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