It is not often that a shelved hypothesis from the nineteenth century is proven and the associated results are announced on the front page of the renowned journal "Science" - presented with a blade of grass.
The question is why the strip-shaped, vertical leaves of the grass look so completely different from the horizontal leaves of an oak or beech.
Can the classic three-way division of an oak leaf into leaf base, leaf stalk and leaf blade also be found in grasses, or do their leaves only have one or the other of these three structures?
This question goes back centuries.
The first answer comes from the Swiss botanist Augustin-Pyrame de Candolle.
In 1827 he suggested that the tubular leaf sheath attached to the knot of the blade of grass, which merges at the upper end into the leaf blade protruding from the blade, is morphologically a petiole.
This hypothesis, which was also supported by other botanists at the time, was then rejected again.
Today the leaf sheath of the grass is considered the morphological equivalent of the leaf base.
Because the vascular bundles in the petioles always run parallel, and this is also the case in the leaf blade of the grass, the hypothesis was also raised for a while that both the leaf sheath and the leaf blade of the grass are equivalents of the leaf stalk.
Corn, wheat and rice are also grasses
Both points of view are wrong. De Candolle's hypothesis is correct. This has now been shown by a research group led by Annis Richardson and Enrico Coen from the John Innes Center in Norwich, England, using computer models that visualize the morphological consequences of the gene activities measured in the laboratory during leaf development. The results suggest that the leaf sheath of the grasses corresponds to the petiole of the dicotyledonous plants and the leaf blade of the grasses is the equivalent of the horizontal leaf blade of the dicotyledonous plants.
Leaves arise in so-called growth zones. In the dicotyledonous plants, to which many trees, shrubs and flowering plants belong, the main growth zone is at the upper end of the stem axis, in the monocotyledonous grasses this is above the nodes in the protection of the leaf sheath. Grass therefore always grows in several places at the same time, which explains its rapid growth. The researchers led by Richardson and Coen have now been able to show that the monocotyledonous and dicotyledonous plants use similar developmental genes in the formation of the foliage, but that they come to remarkably different morphological results due to a different spatial and temporal choreography.
Is this knowledge more than a botanical petitesse?
The results are relevant for plant breeding because all important cereal crops are grasses, such as maize, wheat or rice.
The number and shape of the leaves have a direct influence on the productivity of the plants and thus on their yield through photosynthesis.
So if you want to optimize the yield through the formation of further or different leaves, you will certainly look very carefully at the results of Richardson and Coen.