Longer growing seasons promote stronger leaves
Diverse accessions of the warm-season bioenergy grass Panicum virgatum growing in Texas
Strong leaves provide many advantages to plants like slowing herbivores and preventing environmental damage. These leaves can live longer and contribute to plant growth by producing sugars during photosynthesis over a longer period. There are also costs to producing strong leaves. They often require more resources and take longer to grow than weaker leaves. They have advantages in environments where possessing long-lived leaves outweighs the costs of producing them, such as places with long growing seasons. Habitats with long, cold winters, need to produce leaves that can survive for many months. In that environment, it's more advantageous for the plant to produce an abundance of weaker leaves than just a few strong leaves.
To put this idea to the test, scientists grew hundreds of switchgrass (Panicum virgatum) plants, a widespread grass, together at a location in central Texas. They used a common garden design to separate genetic effects from environmental ones. When plants grow in the same environment, differences in traits are more likely to stem from differences in genetics than if the plants were grown in different environments. The scientists measured leaf dimensions and leaf strength and found that plants with stronger leaves grew larger. This suggests that natural selection favors plants with strong leaves at this field site. They also found that plants originating in habitats with longer growing seasons had stronger leaves, partly because these leaves were thicker and contained more leaf mass over the entire leaf area. Several of the traits responsible for leaf strength in switchgrass were controlled by the same genomic regions as leaf strength, which suggests that growing season length is an important force driving leaf evolution.