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

1.9.1. The role of CO₂ assimilation

The contrasting environments experienced by aerial versus submerged leaves induce functional changes in photosynthetic Cells, specifically: a significant alteration in CO₂ assimilation, Photorespiration, and reserve carbohydrate synthesis. Photosynthetic METABOLISM, along with The activity of RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) and PEPC (phosphoenolpyruvate carboxylase), has been investigated in numerous species of higher aquatic plants, including heterophyllous ones (Sagittaria sagittifolia, Nuphar lutea, Ranunculus sceleratus, Polygonum amphibium, etc.) (Nekrasova et al., 2003). The authors demonstrated that submerged leaves differed from aerial and floating leaves by a lower potential photosynthetic rate—averaging 2.2 mg CO₂/(dm²·h)—a lower fraction I protein content, and a negligible activity of RuBisCO and PEPC, averaging 10 and 1 mg/(dm²·h), respectively. The activity of these Enzymes was 2–4 times lower in submerged leaves compared to aerial and floating leaves (Nekrasova et al., 2003). According to the researchers, these changes occur As a result of submerged leaf adaptation to Temperature fluctuations and reduced light levels in the Water Column.

Aerial leaves of heterophyllous aquatic plants can be classified as sun-type leaves that develop under direct sunlight (Terashima et al., 2001). Such leaves are known to feature a substantial blade thickness and an extended pathway for CO₂ diffusion. Carbon dioxide penetrates from the environment into the chloroplast stroma through The Cell wall, Plasma Membrane, Cytosol, and chloroplast envelope.

Membrane Aquaporins and The Cell wall are involved in CO₂ transport. The latter does not limit CO₂ transport via the apoplast (Nobel, 1999). Researchers have shown that aquaporin 1 from Nicotiana tabacum L. (NtAQP1) participates in CO₂ transport (Uehlein et al., 2003). It is well established that The rate of Photosynthesis depends on the CO₂ concentration within Chloroplasts, whereas the affinity of CO₂ for the CO₂-fixing enzyme RuBisCO is very low (von Caemmerer, Quick, 2000; Kosakivska, 2003).

As the thickness of sun-type leaves increases, so do the reserves of the enzyme RuBisCO. According to I. Terashima's model, with an increase in leaf blade thickness, RuBisCO reserves increase per unit of mesophyll area, which is necessary to maintain photosynthesis at an optimal level, since the photosynthetic rate per unit of leaf area initially increases and then gradually declines (Terashima et al., 2001). Significant blade thickness makes it possible to maintain an optimal level of RuBisCO. An increase in leaf thickness is also accompanied by an expansion of the chloroplast surface area (for CO₂ storage) (Terashima et al., 2006).

When terrestrial plants are submerged, profound changes occur in their photosynthesis. Submerged plants exhibit an increased assimilation rate and a reduced CO₂ content; consequently, the rate of underwater photorespiration should be lower in acclimated plants compared to those unacclimated to submergence. The regulatory mechanisms induced by the transition from terrestrial to submerged conditions in submergence-acclimated leaves are controlled in specific ways, as described for heterophyllous aquatic plants (Mommer, Visser, 2005). This is manifested in the morphological and functional plasticity of photosynthetic leaf cells, because photosynthetic products (oxygen and CARBOHYDRATES) alleviate submergence stress—specifically through stem elongation and an increase in aerenchyma volume, which helps prevent oxygen deficiency in submerged Organs (Mommer, Visser, 2005).

Measurements of photosynthetic rates and related parameters have revealed similarities in photosynthetic rates between aerial and submerged leaves (Maberly, Spence, 1989; Nielsen, Sand-Jensen, 1993); however, their morphologies differed markedly, as each morphological leaf type is finely tuned to photomorphogenesis. According to other researchers, the submerged leaves of hydrophytes are characterized by a high rate of photorespiration (Maberly, Spence, 1989; Jahnke et al., 1991), which may lead to the loss of assimilated carbon. At the same time, certain aquatic plants, such as Elodea nuttallii (Planch.) and E. densa, are unaffected by high photorespiration rates because photosynthetic efficiency in their leaves decreases only slightly (Van et al., 1976; Salvucci, Bowes, 1981a, b), which is apparently associated with C₄-type metabolism (Hough, 1974; Bowes et al., 2002). Data from Lloyd et al. (1977) demonstrated that when the oxygen concentration in water is doubled, photosynthesis drops by 50%, thereby enhancing photorespiration.

A comparative study of aerial (terrestrial) and submerged leaves of Eleocharis vivipara (Ueno, 1998) showed that aerial leaves exhibited C₄-type photosynthesis, whereas submerged leaves operated via the C₃-type. Upon ABA application, newly formed submerged leaves switched from C₃ to C₄ photosynthesis. Immunocytochemical analysis (using gold-labeled Antibodies against the large subunit of RuBisCO, PEPC, and Pyruvate phosphate dikinase [PPDK]) proved that the chloroplasts of the three cell types in ABA-induced new leaves contained high levels of the RuBisCO large subunit (Ueno, 1998). The author demonstrated that aerial

leaves, compared to submerged ones, accumulated more label in PEPC within bundle sheath cells relative to mesophyll cells, and in NAD-malic enzyme within the Mitochondria of Kranz cells. Dense labeling was detected for PEPC and PPDK enzymes in leaves formed under The Influence of ABA, exceeding the levels found in submerged leaves. Labeling density in chloroplasts was also higher in bundle sheath parenchymal cells than in mesophyll chloroplasts. Furthermore, labeling was observed in the cytosol of Kranz cells. In ABA-induced leaves, mesophyll cells and bundle sheath parenchymal cells contained PPDK label similar to aerial leaves; however, PPDK was also present in the cytosol of Kranz cells. Biochemical analysis of leaves formed under ABA Treatment revealed that enzyme activities in the leaf blades were higher than in submerged leaves: 3.4- to 3.8-fold higher for NAD-malic enzyme, PEPC, and PPDK, and 1.6-fold higher for RuBisCO (Ueno, 1998).

It was established that in aerial leaves of Eleocharis retroflexa, the activities of RuBisCO, PEPC, and pyruvate kinase were higher compared to submerged leaves. In contrast, the activities of enzymes such as aspartate aminotransferase and Alanine aminotransferase were significantly elevated in submerged leaves. Concurrent autoradiographic analysis using radiolabeled carbon (NaH¹⁴CO₃) revealed that in submerged leaves, the label was primarily incorporated into malate (52%) and aspartate (37%), with only 7% entering sugar phosphates. Thus, incorporation occurred predominantly into C₄ components (Ueno et al., 1988; 1998). Consequently, the researchers proved that the aerial leaves of terrestrial forms of E. retroflexa belong to C₄ plants, whereas the submerged leaves of this species can be designated as C₄-like. In other words, the submerged leaves of E. retroflexa are characterized by a C₄-like type of photosynthesis, similar to other aquatic plants of this family, and are capable of assimilating carbon underwater via the C₄ pathway (Ueno et al., 1988; Uchino et al., 1995).