Rapeseed (Brassica napus L.) is an important global oilseed crop and a major source of vegetable oil. Increasing planting density can boost yield potential and make mechanized harvesting easier, but packing plants too closely intensifies competition for light and resources, resulting in weaker stems, reduced mechanical strength, and a higher risk of lodging — when stalks bend or break under their own weight. High-density lodging has traditionally been blamed on shading and resource competition, but how exactly high density disrupts the coordination between stem elongation and structural reinforcement has remained poorly understood.

Researchers from the Rapeseed Cultivation Physiology Team at Huazhong Agricultural University have now provided new insight into the question. Their study, published in The Crop Journal, reveals that high-density planting triggers early activation of gibberellin (GA), a plant growth hormone, accelerating stem elongation while delaying structural reinforcement.

“We found that high-density planting does not simply restrict stem development. Instead, it alters the timing of GA activation, causing stem elongation to occur ahead of structural reinforcement,” says corresponding author Associate Professor Jing Wang. “This temporal mismatch provides new insight into why stems become weaker under high-density conditions.”

A Timing Problem, Not Just a Crowding Problem

In two-year field trials with the conventional cultivar ZS11 and hybrid cultivar HYZ50, the team compared low-density (3.0 × 10⁵ plants per hectare) and high-density (6.0 × 10⁵ plants per hectare) planting. Stem elongation in both cultivars occurred mainly from budding to initial flowering, and high-density planting further increased that period’s contribution to final plant height while reducing stem mechanical strength. Analysis of 243 accessions further showed that a greater proportion of height gain during this stage was negatively correlated with mature stem bending strength, particularly under high-density conditions.

Transcriptomic and physiological analyses showed that high density triggered earlier GA activation, followed by rapid stem elongation, while cellulose and lignin accumulation and mechanical reinforcement lagged behind. “High-density plants also showed reduced cellulose and lignin contents, lower cellulose crystallinity, and weaker stem bending strength,” Wang says.

Testing the Fix

To verify GA’s role, the researchers applied the GA biosynthesis inhibitors uniconazole and DPC under field conditions. Uniconazole reduced endogenous GA levels, restrained excessive stem elongation, and promoted cellulose and lignin accumulation, improving stem bending strength and tissue density. Importantly, it also improved yield under high-density conditions, increasing per-plant yield by 9.4% in ZS11 and 10.8% in HYZ50, and yield per unit area by 7.0% and 6.6%, respectively.

“The improvement in both stem strength and yield following GA regulation is particularly encouraging,” Wang says. “Optimizing GA dynamics during critical developmental stages may help achieve a better balance between plant architecture, lodging resistance, and yield under high-density cultivation.”

The study was published in The Crop Journal (DOI: 10.1016/j.cj.2026.06.019). The journal is published by KeAi, established by Elsevier and China Science Publishing & Media Ltd.