Submerged plants face a fundamental challenge because the physical and chemical properties of water differ vastly from air. To survive entirely underwater, their leaves have developed unique characteristics that allow for efficient gas exchange, nutrient uptake, and structural integrity in a dense, flowing medium.
Adaptations of the Leaf Surface
The submerged leaf surface is adapted to absorb substances directly from the surrounding water. A thick, waxy outer layer, or cuticle, is unnecessary because the plant does not need to conserve moisture. Instead, the cuticle is either extremely thin or entirely absent, which significantly reduces the barrier for dissolved gases and nutrients to pass into the leaf cells. This thin surface enables the plant to efficiently absorb compounds like carbon dioxide and mineral nutrients.
For gas exchange, the specialized pores known as stomata are typically absent or non-functional on submerged leaves. The entire epidermal layer acts as the primary surface for gas exchange because gases diffuse much more slowly in water than in air. The epidermal cells of submerged leaves often contain chloroplasts. This placement allows the surface layer itself to maximize the capture of light and perform photosynthesis directly at the point of gas and light entry.
Hydrodynamic Leaf Shapes and Flexibility
The physical form of submerged leaves is optimized to minimize resistance and damage from water movement. Many species possess finely divided, thread-like, or highly dissected leaves. This feather-like structure is a hydrodynamic advantage, as it reduces the drag force exerted by water currents, preventing the leaves from being torn or uprooted. The increased surface area is also beneficial for maximizing the uptake of gases and nutrients from the water.
Other submerged plants develop long, narrow, ribbon-like or linear leaves that are thin and flexible. This morphology allows the leaf to bend and flow with the water current. The water provides external support, which eliminates the need for strong, rigid tissues like lignin. High flexibility prevents mechanical damage from turbulence or flow.
Internal Structures for Gas Management
Submerged leaves feature extensive internal air spaces, collectively known as aerenchyma. This spongy tissue forms large, continuous air channels within the leaves and stems. The air-filled cavities provide essential buoyancy, helping the plant maintain an upright position in the water column to reach available sunlight.
Aerenchyma creates a low-resistance pathway for gas transport throughout the entire plant. Oxygen produced in the leaves during photosynthesis is channeled through the aerenchyma to the submerged parts of the plant, including the roots in oxygen-poor sediments. This internal aeration sustains aerobic respiration in the roots.
The internal plumbing for water transport is significantly modified in submerged leaves. Since submerged leaves absorb water directly across their surface, the need for a complex internal water transport system is greatly reduced. Consequently, the xylem tissue, which conducts water, is often poorly developed. While water transport is minimal, the phloem, which transports sugars produced during photosynthesis, remains an important component for nutrient distribution throughout the plant.