Construction costs and mesostructure of leaves in hydrophytes

Construction costs (CC) and parameters of leaf structure (specific leaf weight, dry matter content, volume of photosynthesizing cells, and the number of cells per leaf area unit) were determined for 19 species of aquatic higher plants. The CC of 1 g dry matter varied from 0.98 g glucose in Lemna gibba L. to 1.48 g glucose in Nuphar pumila (Timm) DC. and Potamogeton natans L. The CC of leaf area unit varied to a greater extent than the CC of 1 g dry wt (from 10 to 97 g glucose/m²) and depended on the type of mesophyll structure. In leaves of hydrophytes with dorsoventral mesophyll structure, the CC of 1 m² leaf area was 3–9 times larger than in leaves with homogeneous structure. Variations in CC of 1 m² leaf area in hydrophytes were affected insignificantly (by 2% only) by variations of CC per 1 g dry wt and were mainly determined (by 82%) by changes in specific leaf weight. Two-factor analysis of variance has shown that the CC of 1 g dry wt in hydrophytes depended on the attachment of plants to the sediment: the CC was 1.2 times larger in rooted hydrophytes than in free floating plants. The second factor (the extent of submergence) potentiated the effect of rooting on CC. Reliable differences were found between the leaf CC for hydrophytes belonging to four groups distinguished by the extent of their contact with water and sediment. In a group series: rooted hydrophytes with floating leaves → submerged rooted hydrophytes → free floating submerged hydrophytes → free floating surface inhabiting hydrophytes, the CC of 1 g dry wt decreased by 1.3 times. Path analysis has shown that this trend was due to the increase in photosynthesizing cell volume and to reduction in number of cells per leaf area unit, which caused the decrease in dry matter content. The decrease in the content of leaf dry matter was accompanied by changes in its chemical composition: the content of carbon and nitrogen decreased. This led to a consistent decrease in leaf CC expressed per 1 g dry wt upon the increase in extent of plant hydrophilicity.