Establishing Lethal and Growth Reduction Doses of Fast Neutron Irradiated Cowpea [Vigna Unguiculata (L.) Walp.]

Technical Summary: Establishing Lethal and Growth Reduction Doses of Fast Neutron Irradiated Cowpea

Problem Statement
Cowpea (Vigna unguiculata L. Walp.) is a critical staple legume in sub-Saharan Africa, valued for its nutritional profile and adaptability to marginal environments. However, crop improvement is constrained by narrow genetic variability, susceptibility to pests and diseases, and the limitations of conventional breeding. While mutation breeding offers a strategy to induce heritable genetic changes, the efficacy of this approach relies heavily on determining optimal mutagen doses. Specifically, there is a lack of established benchmarks for fast neutron irradiation—a high linear energy transfer (LET) mutagen—in cowpea. Without precise data on the median lethal dose (LD₅₀) and median growth reduction dose (GR₅₀), breeding programs risk using doses that are either too low to induce mutations or too high, causing excessive mortality and growth impairment.

Methodology
The study investigated two improved cowpea varieties, SAMPEA 21 and SAMPEA 20T, to establish their radiosensitivity to fast neutron irradiation.

• Experimental Design: Uniform seeds were irradiated at the Centre for Energy Research and Training (CERT), Ahmadu Bello University, Zaria, using fast neutrons. Treatments involved varying exposure durations (2 to 20 seconds) corresponding to specific doses ranging from 0.00 to 16.40 µSv.
• Varieties: SAMPEA 21 (released 2022, resistant to Striga and bacterial blight) and SAMPEA 20T (released 2019, resistant to legume pod borer).
• Growth and Evaluation: The M₁ generation was raised in a Randomized Complete Block Design (RCBD) with two replications at Kaduna State University. Data were collected on seed germination, seedling survival, and vegetative growth parameters: shoot length, stem girth, number of leaves, leaf area, and number of branches.
• Statistical Analysis: Data were analyzed using Analysis of Variance (ANOVA) with Duncan Multiple Range Test (DMRT) for mean separation. Probit regression analysis and linear dose-response modeling were employed to calculate LD₅₀, LD₂₅, LD₇₅, and GR₅₀ values for each variety and growth parameter.

Key Results
The study revealed a progressive, dose-dependent decline in germination, survival, and all measured growth parameters for both varieties.

• Radiosensitivity Differences: SAMPEA 21 demonstrated significantly higher radiosensitivity than SAMPEA 20T.
  • LD₅₀ Values: The median lethal dose was estimated at 1.70 µSv for SAMPEA 21 and 5.38 µSv for SAMPEA 20T.
  • Survival: Survival rates dropped to zero at doses ≥ 7.82 µSv for SAMPEA 21 and ≥ 12.00 µSv for SAMPEA 20T.
• Growth Reduction (GR₅₀):
  • For SAMPEA 21, GR₅₀ values ranged from 1.20 µSv (number of branches) to 1.82 µSv (leaf area). The number of branches and leaf area were identified as the most sensitive growth parameters.
  • For SAMPEA 20T, GR₅₀ values were higher, ranging from 4.85 µSv (shoot length) to 8.64 µSv (number of leaves), indicating greater tolerance.
• Model Fit: The regression models exhibited high coefficients of determination (R² = 0.5713–0.9753), confirming strong dose-response relationships. Notably, SAMPEA 21 showed higher R² values (0.93–0.97) compared to SAMPEA 20T (0.57–0.77), suggesting more uniform responses in the former.

Significance and Claims
The paper establishes essential baseline data for optimizing mutation breeding protocols in cowpea using fast neutrons. The primary significance lies in providing specific LD₅₀ and GR₅₀ benchmarks that allow breeders to select sub-lethal doses capable of maximizing genetic variability while minimizing plant mortality.

• Genotype-Specific Optimization: The study underscores that a "one-size-fits-all" approach is ineffective; SAMPEA 21 requires significantly lower doses (1.50–1.82 µSv) compared to SAMPEA 20T (5.00–8.50 µSv) to achieve similar mutagenic effects without lethal damage.
• Trait Sensitivity: The findings highlight that different growth traits respond differently to radiation. For instance, branching is highly sensitive, while shoot length shows moderate tolerance. This differential sensitivity informs the selection of doses based on specific breeding objectives (e.g., inducing branching vs. general yield improvement).
• Efficiency: By utilizing the established regression equations, breeders can predict growth reduction at untested doses, thereby reducing the time and resources spent on empirical trial-and-error in dose optimization.

The authors conclude that moderate doses of fast neutron irradiation, specifically those aligned with the established LD₅₀ and GR₅₀ values, are suitable for cowpea improvement programs, provided that dose selection is tailored to the specific genotype's radiosensitivity.