Shade Avoidance Restricts Soybean Breeding Progress and Increases Herbivore Susceptibility.

Area of Science:

• Plant science
• Agricultural science
• Crop breeding

Background:

• High planting densities cause light competition, inducing shade avoidance responses like stem elongation in crops.
• These responses, while adaptive in nature, can decrease crop yield and increase stress susceptibility in agricultural settings.

Purpose of the Study:

• To investigate how soybean breeding over the last century has influenced shade avoidance responses and their associated trade-offs.
• To determine if modern soybean cultivars exhibit altered shade avoidance compared to older varieties.

Main Methods:

• Twenty-one Canadian soybean cultivars (1922-2018) were grown under controlled and shade-avoidance-inducing light conditions.
• Shade-avoidance conditions were created by altering the reflected light spectrum, not reducing overall light intensity.

Main Results:

• Plants under shade-avoidance light showed increased stem elongation and greater thrips damage, indicating reduced defense.
• More recent soybean cultivars exhibited higher susceptibility to thrips damage than older ones.
• Yield gains in soybean breeding were primarily due to increased seed biomass allocation and reduced branching, but these gains were diminished under shade-inducing light.

Conclusions:

• Soybean breeding has enhanced yield and adapted morphology for high-density planting but has not overcome persistent shade avoidance.
• Shade avoidance responses limit further yield progress and increase susceptibility to herbivores like thrips.
• Developing soybean varieties with reduced sensitivity to neighbor light cues could improve yield and resilience in high-density plantings.