Is there a maximum length of strap-like leaves for submerged angiosperms?

Leaves are critical plant organs for photosynthesis. In addition, leaf length is an important indicator of leaf size. Although previous studies have revealed the maximum length of leaves from terrestrial plants, as well as their limiting factors, these issues remain relatively unexplored for leaves of submerged macrophytes. Based on meta-analysis of 48 species of aquatic plants, we found that the strap-shaped leaves of submerged macrophytes were particularly long, with a maximum length among studied species of 300 cm (e.g., Vallisneria australis). To explore the role of light in regulating maximum leaf length, we first calculated the suitable growth zone (SGZ) for submerged macrophytes according to light conditions. Results showed that the vertical range of the SGZ was approximately twice that of the maximum leaf length mentioned above. This suggests that light was not the only factor limiting maximum leaf length. In addition to hydraulic conditions and water depth, the short lifespan and slow leaf growth rate also contributed to the limitation of maximum leaf length. Correlation analysis between maximum leaf length and ramet density also revealed a potential trade-off between the two traits in submerged macrophytes. Thus, in addition to light conditions, various other factors limited leaf elongation and thus regulated the maximum length of submerged strap-like leaves.