PLANT HETEROPHYLLY - O.M. NEDUKHA - 2011

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

1.9. FUNCTIONAL CHARACTERISTICS OF LEAVES

1.9.4. Plants with C₃ and C₄ Photosynthetic Pathways

Emergent leaves of higher aquatic plants exhibit the C₃ pathway of Photosynthesis. It is well established that The rate of photosynthesis depends on the CO₂ concentration in METABOLISM/14.html">Chloroplasts and The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase, which fixes CO₂ (von Caemmerer, Quick, 2000). The carboxylase activity of Rubisco is known to be inhibited by oxygen, whereas oxygenase-induced activity leads to energy-consuming Photorespiration. To achieve efficient CO₂ fixation, energy must be used economically. Therefore, leaves need to increase CO₂ conductance from the environment to the chloroplasts. Since leaf thickness in C₃ plants is a critical factor for CO₂ conductance and diffusion, a sufficient lamina thickness and a "long" pathway for Carbon dioxide transport—including both apoplastic and cytoplasmic routes—are key factors for light (emergent and floating) leaves (Nobel, 1999; Uehlein et al., 2003).

In the hydrophyte *Egeria densa*, photosynthesis operates via the C₄ pathway, even though the leaves lack bundle sheath Cells (Casati et al., 2000). The induction of Isoenzymes characteristic of both C₃ and C₄ terrestrial species has been demonstrated in the leaves of this species. Specifically, the induction of an NADH-malic enzyme with a molecular mass of 72 kDa (typical of C₃ plants) alongside the kinetic and regulatory properties of another enzyme, PEPC, revealed that certain modifications of the latter enzyme cause an increase in V_max and affinity for malate, which is typical of terrestrial plants with C₄ photosynthesis (Casati et al., 2000).

Species of the genera *Ranunculus*, *Ceratophyllum*, and *Myriophyllum* belong to C₃ plants (Salvucci, Bowes, 1982; Reiskind et al., 1997) and exhibit a low rate of photorespiration, with Carbonic anhydrase activity detected in both the chloroplast stroma and the apoplast (Majeau, Coleman, 1991). Although species of the genera *Hydrilla*, *Elodea*, and *Egeria* possess the C₄ photosynthetic pathway, their thin leaves are characterized by the absence of Kranz anatomy and the lack of key carboxylation-decarboxylation Enzymes (Reiskind et al., 1997). In such leaves, photosynthetic activity correlates with cytosolic carbonic anhydrase activity, exclusively in the light.

Investigating the leaves of *Eleocharis vivipara*, Japanese researcher O. Ueno discovered that a C₄-like leaf blade Structure develops under terrestrial growth conditions, whereas submerged growth of this freshwater reed promotes only The Development of a C₃-type photosynthetic apparatus lacking Kranz anatomy (Ueno, 1998). When the plants were transferred to emergent conditions, however, the C₄ pathway developed. If the plants were kept submerged, but the Water contained 5 mM ABA, then in the leaves

emerging to the surface and in newly induced submerged leaves, Kranz anatomy and C₄ photosynthesis also developed. These findings indicate the regulatory role of the phytohormone ABA in the differentiation and activation of C₄ photosynthetic enzymes (Brown, 1975; Nelson, Langdale, 1992).

The C₄ pathway requires coordination of biochemical Functions between two distinct types of photosynthetic cells and the expression of enzymes involved in this type of photosynthesis (Hatch, 1987; Furbank, Taylor, 1995). Phosphoenolpyruvate carboxylase and Pyruvate orthophosphate dikinase have been shown to be localized in mesophyll cells, whereas NAD-malic enzyme and ribulose-1,5-bisphosphate carboxylase are located in Kranz cells. In contrast, C₃ plants possess only a single type of photosynthetic cells—mesophyll cells—while bundle sheath cells are poorly developed and contain few Organelles (Brown, Hattersley, 1989).

Recently, the potential for a transition from the C₄ to the C₃ type has been discovered in species of the genera *Flaveria* and *Moricandia* (Edwards, Ku, 1987; McGonigle, Nelson, 1995; Rawsthorne, 1992), as well as in *Hydrilla verticillata* (Bowes, Salvucci, 1989). However, regarding the latter species (*H. verticillata*), it should be emphasized that this transition was not accompanied by structural Changes in the photosynthetic Tissues. Conversely, other researchers studying the leaves of the freshwater reed *Eleocharis vivipara* (Ueno et al., 1988; Ueno, 1996a, 1996b) revealed significant structural changes. The terrestrial form of the freshwater reed exhibited the Biochemical characteristics of C₄ plants and featured Kranz anatomy, whereas the submerged forms possessed the biochemical properties of C₃ plants and lacked Kranz anatomy. Thus, the freshwater reed is a particularly attractive model for such studies, as its leaves demonstrate both genetic links and connections at the level of cellular differentiation (Agarie et al., 1997).