Smaller, more densely packed leaf pores linked to higher winter wheat yields

Looking closely at the flag leaf

The flag leaf, the uppermost leaf on a wheat shoot, supplies products of photosynthesis to developing grain. Its stomata admit carbon dioxide while allowing water vapour to escape. Their numbers and dimensions set part of the leaf’s potential for gas exchange, but their opening also matters.

Elena Ivandi and colleagues examined these traits in 25 winter wheat varieties at Jõgeva, Estonia, during the 2023–2025 growing seasons. Their findings are reported in a bioRxiv preprint that has not been peer reviewed.

The varieties grew in three replicate plots each, without fungicide treatment. At the end of heading, when flag leaves were fully expanded, researchers measured stomatal conductance on both leaf surfaces. This measures how readily water vapour passes through the stomata. They then made leaf impressions to count stomata and measure their dimensions under a microscope, and compared the traits with harvested grain yield.

An association, not a yield recipe

Across the combined three-year dataset, higher stomatal density on either leaf surface was associated with higher grain yield. Shorter guard cells—the cells controlling each pore, used here as a measure of stomatal size—were also associated with higher yields.

The pattern was not equally consistent every year. Upper-surface density was positively associated with yield in 2024, while lower-surface density showed that relationship in 2024 and 2025. Smaller upper-surface stomata were associated with higher yield in two of the three years; the lower-surface size relationship appeared only when years were combined.

These distinctions matter. The study compared existing varieties and naturally varying growing seasons; it did not experimentally alter stomatal density or size. It therefore cannot establish that breeding for those traits alone would raise yield.

One proposed explanation is that smaller stomata could adjust their opening more quickly as conditions change. However, the researchers did not measure opening and closing speeds, and the paper notes that smaller stomata are not necessarily faster in wheat. Improved responsiveness remains a hypothesis, not a demonstrated mechanism here.

Two surfaces, different behaviour

Winter wheat had more stomata per unit area on the upper flag-leaf surface than on the underside. Although densities changed between years, the ratio between the two surfaces remained relatively stable. That suggests their development adjusted in a coordinated way.

Actual conductance told a different story. Upper- and lower-surface measurements were not significantly correlated within individual years, although they were correlated across the pooled dataset. The authors suggest that the surfaces may respond partly independently, or experience different conditions at measurement time.

Conductance was also an inconsistent guide to yield. Upper-surface and total conductance were positively associated with yield in 2023, but negatively associated with yield when all three years were combined. A measurement taken around heading cannot capture gas exchange over an entire growing season.

Weather complicates the picture

The lowest yields occurred in 2025, a particularly wet year. Lodging—plants falling over—affected several varieties, and two could not be harvested. The authors attribute much of that year’s poor performance to heavy rain and lodging, which may have reduced light reaching leaves and hindered grain filling.

Disease severity was negatively associated with yield in every year. Higher stomatal density, however, was not accompanied by a general increase in disease severity. This does not establish that dense stomata protect plants; other differences between varieties could outweigh their influence.

The strong contrasts between years also caution against treating relationships in pooled data as universal biological rules.

Removing a leaf tests another question

The team also removed flag leaves from selected plants. In winter wheat, removal significantly reduced grain weight per main-shoot spike only in 2025, by approximately 18%. It did not significantly change grain number in any year, and the pooled grain-weight effect was not significant.

A comparison involving ten spring wheat varieties in 2025 found larger effects: approximately 24% less grain weight and 6.5% fewer grains per spike. These were spike-level measurements, not whole-field yield losses. Other leaves or stored stem reserves might compensate in winter wheat, but that explanation was not directly tested.

Overall, the study identifies promising associations under real field conditions, rather than a ready-made breeding prescription. Testing across more locations and environments, alongside direct measurements of stomatal responses, would help establish whether smaller, denser stomata reliably contribute to higher winter wheat yields.