Critical periods for the expression of vegetative and reproductive plasticity in maize crops

Context or problem

Across the world, maize (Zea mays L.) production has expanded into marginal environments where low densities are used. Low densities increase the resource offer pl⁻¹ and promote vegetative (tillering) and reproductive (prolificacy, tillering) plasticity mechanisms, with strong differences among genotypes. However, the periods of the cycle when mechanisms are defined vary, and seasonal stresses may hinder tiller emission or growth, as well as kernel setting on multiple ears of the main shoot and/or tillers.

Objective or research question

To define critical periods for the expression of vegetative-reproductive plasticity in low-density maize crops.

Methods

We analyzed the impact of growth reductions through shading (70 % reduced incident radiation) throughout different stages of the cycle (S₁: V₃-V₇; S₂: V₇-V₁₃; S₃: V₁₃-R₁ and S₄: R₁-R₂) versus a non-shaded control on crop grain yield and its components for four maize hybrids presenting different plasticity mechanisms under potential growth conditions (phenotypes): i) tillering, ii) prolific, iii) prolific+tillering, and iv) non-tillering non-prolific (“flex”) cultivated at 3 pl m⁻². Four field experiments (Exp₁, Exp₂, Exp₃, Exp₄) were conducted during two growing seasons.

Results

Under the control treatment, all plastic phenotypes reached a higher grain yield m⁻² than the flex, but both the tillering and prolific+tillering phenotypes had the highest grain yields. Differences in kernel number m⁻² among treatments were very strong in all phenotypes, with the prolific and prolific+tillering being relatively more stable among shading treatments. S₁ reduced tiller emission, while S₂ reduced tiller growth. The greatest reductions in kernel number m⁻² were always observed with S₃ and S₄ but affected different components depending on the phenotype: kernel set in apical ears of the main shoot, reduced expression of prolificacy and/or promotion of tiller sterility.

Conclusions

Maize phenotypes cultivated at low density present different critical periods for the contribution of plasticity mechanisms to crop grain yield. Particularly for tillering phenotypes, it would be relevant to sustain high plant growth rates from early vegetative stages to promote tiller emission and during subsequent growth stages to promote tiller fertility. By contrast, the critical period for the prolific phenotype was similar to that of the flex phenotype.

Implications or significance

The findings of this work regarding the expression of plasticity mechanisms in maize as a response to growth reductions throughout the cycle should be considered when selecting the adequate phenotype for marginal environments with different timings of resource restrictions.