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Wheat is one of the most important food crops in Europe, but its yields have stagnated in this millennium. Plant biologists from the University of Tartu, in collaboration with researchers from the METK – Centre of Estonian Rural Research and Knowledge, have discovered that wheat genotypes with higher stomatal density on the adaxial leaf side show lower yields in the field.
Wheat is a major food crop in Europe, and maintaining and possibly increasing its yields in the face of climate change is a major challenge for breeders and farmers. Plants absorb photosynthetic carbon dioxide from the air and lose transpirational water through small openings, or stomata, on the leaf surfaces.
Thus, the traits associated with stomata have high breeding potential. Changing the density, distribution between leaf surfaces or openness of the stomata during breeding makes it possible to create varieties with better drought tolerance. Stomatal traits can also be used to breed varieties with a higher yield potential under more humid conditions.
However, in order to successfully use stomatal traits in breeding, it is first important to understand how different stomatal patterns affect plant growth, yield and stress tolerance.
On most plants, the stomata are only on the abaxial (lower) side of the leaf. If stomataare present on both sides of the leaf, they are usually much more numerous on the lower than on the upper surface. Exceptions are many grasses, including crops, which often have more stomata on the top side of the leaf.
Since most plants have stomata mainly on the abaxial leaf side, research on the development and functioning of stomata has so far mostly focused on the abaxial side of the leaf.
Several studies have shown that the stomatal density has a significant effect on the drought tolerance and growth of plants. However, very little is known about whether and how stomata on the upper side of the leaf affect plant growth, stress tolerance and yield. In addition, many of the studies on stomatal traits and plant growth or yield have been carried out under controlled laboratory conditions, which differ from the great variation of abiotic conditions encountered in the field.
A look at the field
In collaboration with researchers from the Centre of Estonian Rural Research and Knowledge, we investigated the relationship between stomatal traits of spring wheat flag leaves and yield under field conditions. The results of a study published in the scientific journal New Phytologist showed that in the flag leaves of spring wheat, there are always more stomata on the upper than on the lower side of the leaf. More CO2 can potentially enter the leaf through upper stomata, but also more water can be lost, particularly in the conditions promoting stomatal openness.
Field trials carried out over two years showed that genotypess with a higher adaxial stomatal density of the flag leaf had a lower grain yield. However, no correlation was found between the abaxial stomatal density and the yield. The study shows that the adaxial stomata, which have been largely unexplored so far, can play a very important role in plant yield development and deserve more attention.
Why did wheat genotypes with a higher adaxial stomatal density density yield less? One possible explanation may be greater water loss through the stomata of leaf upper surface: it is exposed to stronger light and drier air.
The field trials were carried out in two drier and warmer than long-time average years, when the potential water loss through adaxial stomata could have been large. However, based on previous studies, one would expect that in more humid years, plants with higher adaxial stomatal density may produce higher yields because they are better able to take up carbon dioxide for photosynthesis.
The published research shows that the stomatal density of spring wheat, and in particular the adaxial stomatal density, is a trait showing significant genotypic variation, which could be potentially useful in breeding. Further research is needed to understand why plants with more adaxial stomata of the leaf have lower yields, how this depends on environmental conditions and whether other crops show the same effect.
Author: Hanna Hõrak ja Ebe Merilo. This article was originally published on the the Estonian Public Broadcasting online news portal.
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