HETEROPHYLLY IN PLANTS - O.M. NEDUKHA - 2011

CHAPTER ONE. MORPHOLOGICAL AND STRUCTURAL-FUNCTIONAL CHARACTERISTICS OF VEGETATIVE ORGANS IN HETEROPHYLLOUS PLANTS

1.5. PLANT GROWTH CHARACTERISTICS

Submerged and emerged leaves of many hydrophyte species vary significantly not only in shape but also in growth characteristics. In some species, submerged leaves appear early in spring (April - May). Slightly later (14 - 30 days later), emerged leaves begin to grow, while the submerged leaves age and die off. This applies to such species as the broad-leaved Water parsnip (Cicuta virosa), as well as numerous arrowhead species (Sagittaria). In heterophyllous species of the family Nymphaeaceae (such as yellow pond-lily, snow-white water lily, etc.), submerged basal leaves can grow and function during the winter, especially in the southern regions of Europe, Asia, and North America.

The submerged leaves of Proserpinaca palustris can also grow in both summer and winter (under a short photoperiod of up to 8 hours), with leaf shape varying accordingly: heavily dissected leaves are formed in winter. Only in spring, when day length increases significantly (up to 16 hours), does vertical stem growth and lateral branching begin, and leaf Morphology changes from highly dissected to entire with serrated blade margins. Internode elongation and the induction of flower development occur simultaneously in P. palustris plants (Davis, 1967; Kane, Albert, 1999). On the submerged stems of P. palustris, even under prolonged bright sunlight, newly formed submerged leaves retain their dissected shape (Schmidt, Millington, 1968), which helps them withstand water resistance and pressure both in standing water and in rivers with constant currents and waves (Tsukaya, 2006). The number of emerged leaves in P. palustris depends on the number of immature leaves in the bud. When the leaf primordium reaches a size of 500–600 µm, five pairs of successive lobes are formed through Cell Division along the marginal edge of the meristem. At this stage, morphological differences become apparent, leading to differential cell division patterns in the blade lobes and leaf axils. This morphological feature of emerged leaf formation becomes a defining structural characteristic (Schmidt, Millington, 1968).

Submerged macrophyte species are characterized by diverse strategies that enable them to grow and avoid mechanical damage during rapid river currents. They possess specifically structured boundary layers that reduce water flow velocity around the leaf blade (Sand-Jensen, Mebus, 1996; Sand-Jensen, Pedersen, 1999). Another feature that allows them to overcome the pressure of rapid currents is an increase in SHOOT plasticity, which provides stem mobility and reduces mechanical damage to submerged Organs under high water flow velocities (Usherwood et al., 1997).

According to T. Vogel (Vogel, 1994), plasticity is defined as the stem's ability to undergo substantial changes in Anatomical Structure in response to increased water velocity, alongside a decrease in stem rigidity and cross-sectional area. Differences in stem rigidity are related to its diameter and the architecture of its structural elements.

The study demonstrated that current velocity can significantly affect growth parameters. Using Veronica anagallis-aquatica L. as a model homophyllous plant (having leaves uniform in shape) that can grow both in shallow water with slow currents and in deep, unshaded river habitats with rapid currents, researchers showed a strong dependence of stem growth parameters on current velocity. When V. anagallis-aquatica grows in slow-flowing rivers, it develops both submerged and emerged forms of leaves and stems, whereas under rapid currents, the plant exhibits exclusively submerged forms of leaves and stems (Boeger, Poulson, 2003).

An examination of V. anagallis-aquatica plants growing at a depth of about 58 cm (± 1.0%) revealed that they possessed both emerged and submerged leaves. Plants with submerged leaves grew in areas with high current velocity (19 cm/s) and depths up to 66 cm, whereas plants with both emerged and submerged leaves grew in areas with low current velocity (2 cm/s) at a depth of 58 cm (Boeger, Poulson, 2003). It was established that the surface area of submerged leaves at low current velocity was 27% larger than that of submerged leaves at high current velocity. However, the surface areas of emerged and submerged leaves at low current velocity were practically identical (Table 1.5.1). The dry mass and density of emerged leaves were significantly greater than those of submerged leaves under both low and high current velocities. Submerged leaves growing in rapid water currents were found to have a greater mass and density compared to those growing in slow currents (Table 1.5.1). These researchers confirmed that the dry mass and density of emerged leaves were considerably higher than those of submerged leaves (regardless of current velocity). Furthermore, submerged leaves growing under high current velocity had a significantly greater dry mass and density than those growing under low current velocity, indicating that current velocity profoundly affects these parameters (Boeger, Poulson, 2003).

In addition, the Anatomical structure of the leaf blades differed: emerged leaves possessed a three-layered palisade parenchyma and a four-layered spongy parenchyma. Meanwhile, in submerged leaves (regardless of current velocity), both palisade parenchyma and Stomata were absent, and the photosynthetic parenchyma consisted of numerous (seven to nine) cell layers whose dimensions were significantly smaller compared to the spongy parenchyma Cells of emerged leaves.

All Cells of the submerged leaves were isodiametric in shape, whereas the palisade cells of emerged leaves were elongated (Boeger, 1992; 1994; Boeger, Poulson, 2003).

Differences were also observed in the Anatomical Characteristics of the stems. Plants grown above water had longer stems with a larger diameter than submerged plants. In submerged plants grown under high and low current velocities, stem lengths were 48% and 43% shorter, respectively, than in emerged plants. The number of internodes in the stems of submerged plants was lower regardless of current velocity (Boeger, Poulson, 2003). Regarding the anatomical CHARACTERISTICS OF THE first five apical internodes, the xylem and phloem in all three samples were poorly developed and showed no significant differences among themselves. However, regardless of current velocity, the stem pith was occupied by aerenchyma extending continuously between adjacent internodes. A difference was found in the radius of the air cavities, which were larger in emerged stems. All these structural changes in leaves and stem diameters were accompanied by a 1.7- to 2.9-fold higher chlorophyll content in emerged leaves compared to submerged leaves (from both high- and low-velocity environments).

TABLE 1.5.1. Structural characteristics of leaves and stems of Veronica anagallis-aquatica grown in three different habitats: emerged at low current velocity, and submerged at low and high current velocities (Boeger, Poulson, 2003)

Thus, the leaf structure of V. anagallis-aquatica is similar to that of submerged and emerged leaves in other aquatic plant species, indicating a high degree of phenotypic leaf plasticity in this species (Sculthorpe, 1967; Sand-Jensen, Frost-Christensen, 1999). The structure of emerged leaves resembles that of sun-type leaves growing under high illumination (Givnish, 1987; Chazdon, Kaufmann, 1993). Leaves developing under high light intensities typically form a multi-layered palisade parenchyma, which efficiently distributes directional light within the leaf interior and optimizes photosynthetic rates (Donahue, 1991; Vogelmann, 1994). Structurally, submerged leaves—regardless of whether they grow in fast or slow currents—resemble the shade leaves of terrestrial plants. Their cell shape and minimal leaf blade thickness are adaptations to low photosynthetic rates, as maintaining a low photon flux density can be efficiently utilized only by thin leaf blades (Thompson et al., 1992). Studies of shade leaves in Thermopsis montana suggest that spongy mesophyll parenchyma in such leaves enhances Light absorption by intensifying internal light scattering (Donahue, 1991; Vogelmann, 1994). In terrestrial plants under high light intensity, leaf area decreases to conserve water and reduce Transpiration (Taiz, Zeiger, 1998).

However, the emerged leaves of V. anagallis-aquatica are similar in size to the submerged leaves of specimens growing in low-current habitats. Differences exist between the surface areas of terrestrial and aquatic plants, which is evidently related to differences in light intensity in water versus on land (Boeger, Poulson, 2003). It is well known that the leaves of terrestrial plants are exposed to high light intensities, often under conditions of water deficit (Maberly, Spence, 1989).

The presence of stomata in the emerged leaves of V. anagallis-aquatica Supports this hypothesis. Under conditions of water deficit and very high illumination, the leaves of terrestrial plants generally become thicker and amphistomatic. Stomata on both leaf surfaces are typically found in plants with high photosynthetic capacity and short distances between photosynthetic cells and the epidermis, which facilitates CO₂ uptake (Parkhurst, 1978; Mott, Michaelson, 1991).

The large leaf surface area of submerged plants in slow currents is comparable to that of V. anagallis-aquatica individuals growing in very rapid currents. This may be driven by the necessity for an expanded leaf surface area to facilitate gas exchange. The surface of submerged leaves possesses

optimal functional capacity for the diffusion of gases dissolved in the surrounding water layers. As researchers note, the CO₂ concentration around the leaf is rapidly depleted during Photosynthesis (Madsen, Breinholt, 1995). Consequently, only a thin leaf can respond to a limited inward flux of CO₂, where low carbon dioxide availability correlates with reduced photosynthetic performance (Black et al., 1981). High water current velocities induce morphological and anatomical changes in submerged V. anagallis-aquatica plants. Their leaf blades may undergo slight mechanical stress, and small, thin leaves observed in submerged plants help reduce hydraulic resistance (Fox, 1996; Schutten, Davy, 2000). Although submerged leaves growing in fast currents have a greater dry mass than those in slow currents, the mechanical action of the current does not destroy them. It is likely that their cells become more compact and smaller, analogously to terrestrial plant leaves exposed to mechanical stress such as wind (Lecoeur et al., 1995; Lu, Neumann, 1998).

Differences in specific leaf area (SLA) are associated with anatomical variations among leaves. Emerged leaves exhibit greater blade thickness and dry mass (Niinemets, 1999). The author observed no statistically significant difference in SLA between emerged and submerged leaves. A similar phenomenon was reported by H. Lichtenthaler for sun leaves of terrestrial plants (Lichtenthaler et al., 1981). For aquatic plant leaves grown at high current velocities, SLA differs statistically from that of plants grown at low velocities due to their higher dry mass and smaller leaf area.

Differences in leaf density are also related to anatomical structure. It is well established that emerged leaves have greater blade thickness and dry mass compared to submerged ones, leading to increased leaf density (Niinemets, 1999). In terrestrial plants, leaf density increases alongside greater thickness and reduced area, which is characteristic of sun-adapted leaves (Marques et al., 2000). The morphological differences described in the stems of V. anagallis-aquatica—such as increased stem length and internode diameter in emerged plants—are a consequence of their floating habit. This leads to an expansion of cortical and pith aerenchyma and The Development of medullary cavities in the apical and intermediate internodes. Similar Anatomical Features have been noted by other researchers in other plant species (Boeger, 1994; Usherwood et al., 1997). The absence of a central cavity in the basal internodes of V. anagallis-aquatica stems can be explained by the fact that the lowest internodes are anchored and undergo minimal displacement under water currents (Usherwood et al., 1997).

Regarding the submerged stems of V. anagallis-aquatica, high current velocity acts as a mechanical stress factor that shapes stem structure, resulting in shortened stem length and reduced internodal diameter. This compact plant size reflects an adaptive need for increased flexibility under high flow rates, thereby conferring resistance to mechanical Damage caused by rapid currents (Schutten, Davy, 2000). Submerged specimens of speedwell in slow-current environments possess stem structures similar to those of plants growing in quiescent waters, where stem aerenchyma ensures buoyancy and branching.

The lower chlorophyll content per unit area in submerged leaves compared to emerged ones correlates with the SLA values of submerged speedwell leaves and other species (Nielsen, Sand-Jensen, 1989; Nielsen, 1993). Furthermore, submerged leaves are thinner (Nielsen, Sand-Jensen, 1989) under both high and low flow velocities. Variations in chlorophyll content among different leaf types of speedwell were also modulated by light availability, with emerged leaves receiving light levels comparable to typical sun leaves. Hydrophyte emerged leaves are known for high photosynthetic rates driven by elevated chlorophyll content (Nielsen, Sand-Jensen, 1989; Nielsen, 1993), high Rubisco protein content and activity (Beer et al., 1991), and enhanced CO₂ diffusion access (Madsen, Breinholt, 1995; Sand-Jensen, Frost-Christensen, 1999). Differences observed in submerged leaves under high current velocity may be attributed to stress-induced effects of water flow (Madsen et al., 1993), potentially altering CO₂ diffusion and mitigating boundary-layer resistance (Madsen, Søndergaard, 1983; Madsen et al., 1993; Carr et al., 1997). In our case, emerged leaves exhibit elevated photosynthetic rates and higher chlorophyll levels, analogous to sun-type leaves characterized by high Rubisco activity and elevated levels of other photosynthetic Enzymes (Salvucci, Bowes, 1982; Maberly, Spence, 1989; Beer et al., 1991).

Thus, the research findings suggest a close interrelation between environmental factors—such as CO₂ concentration, hydraulic stress, and light availability—and the morphological, anatomical, and physiological parameters of V. anagallis-aquatica. The observed structural Variability in leaves enables this ecological form to function successfully across varying water velocities and submergence depths.

The triggers of leaf dimorphism in aquatic plants may also involve changes in cellular turgor pressure and cell expansion mediated by Abscisic acid (Deschamp, Cook, 1983; Lin et al., 2005).

M.E. Kane demonstrated that adding ABA to the aqueous medium supporting the growth of the hydrophyte Myriophyllum heterophyllum Michx. induced the development of aerial leaves during the vegetative-juvenile phase. These leaves differed from submersed ones by having shorter epidermal cells, stomata, and a thicker cuticle. The onset of these transformations was accompanied by significant shifts in cellular turgor (Kane, 1984).

In the aquatic plant Callitriche sp., heterophilly is manifested in distinct leaf morphologies. The authors suggest that cell turgor changes during this metamorphosis act as a biophysical and mechanical driver influencing leaf shape Selection under controlled conditions that simulate the plant's natural environment (Deschamp, Cooke, 1983; Lin et al., 2005).