Growth Regulator and Nitrogen Combo Lifts Yields in Late-Sown Wheat

Late-sown wheat is one of the quiet headaches of South Asian agriculture. Across the Indo-Gangetic Plains, which occupy nearly 61 percent of India’s wheat area and supply roughly 67 percent of national production, farmers routinely plant wheat after the recommended window because the preceding rice harvest runs late. The crop then compresses its entire life cycle into fewer days, and the grain filling period shrinks from about 47 days to as little as 34. Less time to fill grains means less time to accumulate the assimilates that ultimately become yield. A new two-year field study from Punjab Agricultural University in Ludhiana, published in Frontiers in Agronomy, suggests that a familiar plant growth regulator, applied at the right stages and paired with slightly enriched nitrogen, can claw back a meaningful share of that lost productivity.

The research, led by Kavita Devi and colleagues, tested the anti-gibberellin compound mepiquat chloride on late-sown wheat cultivar PBW 752, sown on December 11, 2023, and December 10, 2024, at the university’s Student’s Research Farm. The team crossed three nitrogen levels, corresponding to 100, 125 and 150 percent of the recommended dose (125, 156.25 and 187.50 kg N per hectare), with seven growth-regulation regimes: an untreated control, and foliar sprays of mepiquat chloride at 750 or 1000 ppm applied at maximum tillering (40 to 45 days after sowing), at the flag leaf stage (60 to 70 days after sowing), or at both stages. The 21 treatment combinations were laid out in a split-plot design with four replications, giving a statistically robust picture of how each factor behaved across two contrasting seasons, one wetter and cooler (138.5 mm of rain) and one drier and hotter (75.0 mm).

The physiological measurements tell the clearest mechanistic story. Using a portable infrared gas analyzer on flag leaves at booting and physiological maturity, the researchers tracked chlorophyll content, stomatal conductance, transpiration and net photosynthetic rate. All four traits rose with nitrogen supply, peaking at 150 percent of the recommended dose, which fits nitrogen’s role as a structural component of chlorophyll and of the photosynthetic machinery itself. But the more striking result came from the growth regulator. Dual-stage application of mepiquat chloride at 750 ppm, at maximum tillering plus flag leaf, consistently produced the highest chlorophyll retention and gas-exchange values of any treatment, statistically matching the corresponding 1000 ppm regime. The untreated plants sat at the bottom of the range in both years.

Why would a growth retardant improve photosynthesis? The authors explain it through canopy architecture. Mepiquat chloride suppresses gibberellin biosynthesis, checking internode elongation and producing a more compact plant. Within that tighter canopy, radiation penetrates to deeper leaf layers, mutual shading declines, and the photosynthetically active area persists longer into the reproductive phase. Delayed chlorophyll degradation follows, sustaining the source of assimilates during grain filling. This mechanism is well documented in cotton, where mepiquat chloride treatments have lifted net photosynthetic rates by 3.4 to 23.2 percent over untreated controls, but its interactive effects with nitrogen in wheat had been barely explored for north-western India until now.

The consequences show up most clearly in how the plant distributes its resources. At physiological maturity, the researchers sampled plants from defined row lengths, separated them into stems, leaves and grains, and oven-dried each fraction to constant weight. Under higher nitrogen, grain dry matter climbed from 43.3 and 45.6 quintals per hectare at 100 percent of the recommended dose to 49.9 and 51.3 quintals at 150 percent, with the proportion of biomass partitioned to grains rising from roughly 42.2 to 43.6 percent. The dual-stage 750 ppm spray was even more effective at redirecting assimilates: grain dry matter reached 49.8 and 51.1 quintals per hectare, 14.5 and 13.1 percent above the untreated control, with grain share of biomass peaking at about 44.5 percent. Notably, the spray simultaneously reduced stem and leaf biomass, evidence that suppressing excess vegetative growth releases photosynthate that would otherwise be locked into structural tissue.

Those shifts in partitioning translated into a bigger and better-supplied sink. Sink capacity, calculated as grains per square meter multiplied by mean individual grain weight, reached 52.0 and 57.3 quintals per hectare under the highest nitrogen level, 36.8 and 29.6 percent above the standard dose. The dual-stage 750 ppm treatment achieved comparable sink values at 50.8 and 55.2 quintals per hectare, alongside spike weights of 1.73 and 1.85 grams, roughly 41 percent heavier than unsprayed plants, and thousand-grain weights of 37.1 and 38.6 grams. Heavier spikes reflect improved spikelet survival and grain set, while plumper kernels indicate that developing grains received an adequate assimilate supply throughout the compressed filling window.

Grain yield, the bottom line, followed the same pattern in both seasons. Raising nitrogen from 100 to 125 percent of the recommended dose lifted yields from 43.7 and 45.7 quintals per hectare to 48.1 and 49.1 quintals, gains of 10.1 and 7.4 percent. Going to 150 percent added a further margin, reaching 50.4 and 51.9 quintals, or 15.3 and 13.6 percent above the control, although the two enriched levels were statistically indistinguishable from each other. The growth regulator told a parallel story: dual-stage 750 ppm application produced 50.2 and 51.8 quintals per hectare, about 14 percent above the untreated control and statistically at par with the 1000 ppm double spray. The single-stage sprays at either concentration consistently lagged behind the double application.

Perhaps the most revealing number is what happened to straw. Untreated plants produced the most straw; the dual-sprayed plants produced the least, around 8 to 9 percent below the control. Yet their harvest index, the fraction of total biomass converted into grain, hit 41.2 percent in both years against 35.7 and 36.1 percent for the control. In other words, mepiquat chloride did not make the crop bigger; it made the crop better at shipping its carbon budget into the grain rather than into stems and leaves. That distinction matters agronomically, because it means the yield gain comes from redirected assimilate flow rather than from extra input-driven biomass, a more sustainable route to higher productivity.

The study’s multivariate statistics reinforced the coherence of the story. Across 168 experimental-unit observations and eight variables, the first principal component alone explained 72.3 percent of the variation and was dominated by chlorophyll content, stomatal conductance, net photosynthetic rate, spike weight and grain yield, all loading in the same direction. Pearson correlations echoed this: net photosynthetic rate at maturity correlated with spike weight at r = 0.91, and chlorophyll content with grain yield at r = 0.77. Hierarchical clustering grouped all of the highest-nitrogen, dual-spray combinations together, while the low-nitrogen, unsprayed treatments fell into a separate, poorly performing cluster. Yield, in this system, emerged from the combined behavior of a whole physiological suite rather than from any single trait.

The authors are careful about scope. The trial involved a single cultivar at one site, and the team did not quantify nitrogen-use efficiency, nitrogen losses, greenhouse gas emissions or economic returns, all of which matter before farmers are advised to push nitrogen above the recommended dose. Still, the practical headline is clear and encouraging: under delayed sowing, 125 percent of the recommended nitrogen performed as well as 150 percent, and a modest 750 ppm spray of mepiquat chloride at two well-chosen growth stages matched the effect of a stronger dose. For the rice-wheat rotations of north-western India, where late planting is a structural reality rather than an occasional mishap, that combination offers a low-cost, physiologically elegant way to protect grain filling when the calendar will not cooperate. Future work across genotypes and environments, with biochemical and molecular probing of the underlying mechanisms, will determine how widely the recipe can be applied.

Subject of Research: Effects of mepiquat chloride and nitrogen fertilization on physiology, assimilate partitioning and grain yield of late-sown wheat

Article Title: Optimizing physiological performance, assimilate partitioning and productivity of late-sown wheat (Triticum aestivum L.) through mepiquat chloride application under nitrogen enrichment

Article References: Devi, K., Deol, J. S., Sharma, R., Kaul, A., Kumar, A., & Sharma, R. (2026). Optimizing physiological performance, assimilate partitioning and productivity of late-sown wheat (Triticum aestivum L.) through mepiquat chloride application under nitrogen enrichment. Frontiers in Agronomy, 8, Article 1924091. https://doi.org/10.3389/fagro.2026.1924091

Image Credits: AI Generated

DOI: 10.3389/fagro.2026.1924091

Keywords: late-sown wheat, mepiquat chloride, nitrogen fertilization, grain filling, assimilate partitioning, sink capacity, photosynthesis, harvest index, plant growth regulator, Indo-Gangetic Plains, Punjab Agricultural University, grain yield