PLANT HETEROPHYLLY - O.M. NEDUKHA - 2011

CHAPTER TWO. MECHANISMS OF HETEROPHYLLY MANIFESTATION IN PLANTS

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2.1. EXOGENOUS FACTORS INFLUENCING THE FUNCTIONING OF PHOTOSYNTHETIC CELLS IN HETEROPHYLLOUS PLANTS

2.1.2. Carbon Nutrition Substrates

The carbon dioxide content in Water affects leaf formation in heterophyllous plants. Investigating The impact of natural variations on The Development of different leaf types in Nuphar variegata, J. Titus and P. Sullivan demonstrated that low CO₂ concentrations primarily induce the development of floating leaves compared to submerged ones (Titus, Sullivan, 2001). This aligns with other observations showing that a high Abundance of submerged leaves forms under conditions of elevated CO₂ concentrations in lakes (Bristow, 1969). Enriching soil and water with CO₂ also promotes the development of floating leaves; both factors accelerate Plant GROWTH AND DEVELOPMENT. Measurements of internal oxygen concentration in the petioles of submerged Rumex palustris plants (Rijnders et al., 2000; Mommer et al., 2004) revealed an increase in oxygen concentration in the light under limited dissolved CO₂ availability in water during flooding. Although carbon dioxide dissolves in water 28 times better than oxygen, it is characterized by a low diffusion rate. Consequently, submerged leaves increase their surface area for optimal CO₂ absorption—the average CO₂ content in water ranges from 3 to 100 µM (Mommer, Visser, 2005)—and possess a very thin cuticle in epidermal Cells for optimal gas exchange with the aquatic environment (Rascio et al., 1999), as demonstrated in Ranunculus aquatilis (Bruni et al., 1996) and Elodea nuttallii (Jones et al., 2000).

It is known that while some higher aquatic plants have adapted to assimilate exclusively CO₂, others utilize both CO₂ and HCO₃^- forms of inorganic carbon dissolved in water (Allen, Spence, 1981; Bowes, Salvucci, 1989; Prins, Elzenga, 1989; Madsen, 1993; Newman, Raven, 1993; Raven, 1994; Rascio et al., 1998; 1999). Therefore, the photosynthetic apparatus of hydrophytes depends on The ratio of CO₂ to HCO₃^- (Smith, Walker, 1981; Madsen, Maberly, 1991). CO₂ has a high solubility in water but a low diffusion coefficient, which for water is 1.7 × 10^-6 m²/s at 20 ^oC (Smith, Walker, 1981; Madsen, Maberly, 1991). In hydrophyte leaves, the uptake system for the HCO₃^- form of inorganic carbon is actively operating; mediated by the enzyme Carbonic anhydrase (EC 4.2.1.1), it catalyzes The conversion of HCO₃^- into CO₂ According to the following reaction:

It has been established that this reaction releases hydroxyl ions into the surrounding environment close to the leaf. This leads to rapid Changes in the pH of the aquatic environment and the balance between CO₂ and HCO₃^-.

The utilization of HCO₃^- acts as a carbon-concentrating mechanism often coupled with C₄-type METABOLISM, which is characteristic of Hydrilla verticillata (Holaday, Bowes, 1980; Magnin et al., 1997; Reiskind et al., 1997), Elodea canadensis (Elzenga, Prins, 1989), and E. densa (Browse et al., 1979; Casati et al., 2000). This type of metabolism is distinguished by differences in carboxylating Enzymes within bundle-sheath cells in terrestrial plants (Magnin et al., 1997). Meanwhile, aquatic plants lack bundle sheaths, and differences exist solely in the localization of carboxylation enzymes between C₃ and C₄ types (Reiskind et al., 1997; Casati et al., 2000).

Another pathway, alternative to the carboxylation pathway, is CAM (Crassulacean Acid Metabolism) Photosynthesis. In this type of photosynthesis, CO₂ assimilation and The Calvin Cycle are separated not spatially as in C₄ plants, but temporally. Malate accumulates in vacuoles at night, and the Calvin cycle operates during the day. This mechanism allows for maximum water conservation; however, it is less efficient than the C₄ and C₃ types. CAM photosynthesis has been described in Lobelia dortmanna L. and Littorella uniflora (Robe, Griffiths, 1992; 1998; Madsen, Maberly, 1991). These species can utilize CO₂ from the surrounding sediment, where CO₂ is produced As a result of microbial Respiration (Pedersen, Sand-Jensen, 1997; Pedersen et al., 2006). Carbon dioxide diffuses from the soil into the interior of the roots, and then the gas moves up along a concentration gradient into the stem and leaf aerenchyma, which are capable of fixing low levels of carbon dioxide for optimal photosynthesis.

It has been established that CO₂ can freely cross the Plasmalemma of cells, whereas HCO₃^- cannot. Therefore, in hydrophytes, the conversion of HCO₃^- into CO₂ occurs in the apoplast, or HCO₃^- transport is carried out across the plasmalemma via transporters identified in cyanobacterial and algal cells (Badger, Price, 1992; Raven, 1994; 1996; Beer, 1998; Andria et al., 1999; Kaplan, Reinhold, 1999; Moroney, Somanchi, 1999). In cyanobacteria and Algae, Active Transport of HCO₃^- occurs in parallel with H⁺ symport, which has also been demonstrated in cells of Elodea nuttallii (Eighmy et al., 1991; Fagerberg et al., 1991).

Polarization has been detected on the leaves of higher aquatic plants: the conversion of HCO₃^- into CO₂ takes place on the lower leaf surface, whereas H⁺ ion efflux occurs on the upper leaf surface; these processes mainly take place in the light (Miedema, Prins, 1992). Carbonic anhydrase activity has been detected in Ranunculus penicillatus Dumort. (Newman, Raven, 1993), R. trichophyllus Chaix (Rascio et al., 1999), and Ceratophyllum demersum (Rascio et al., 1998), whereas in other species, such as Potamogeton lucens (Staal et al., 1989) and Elodea canadensis, this enzyme does not play a significant role (Rascio, 2002).

At the same time, studies on polarized plants like Potamogeton lucens and several Elodea species have shown that the polar uptake of HCO₃^- and the release of hydroxyls are accompanied by cation Transport from the lower leaf surface to the upper one. This transport is accompanied by the generation of a specific electrical potential, making the upper side of the leaf negatively charged relative to the lower side (Miedema et al., 1980; Miedema, Prins, 1992). In characean algae, a potential difference of 7 mV between the two streams has been recorded, compared to 40 mV in P. lucens. HCO₃^- moves along The Cell wall, yet it cannot be transported directly across the cytoplasmic membrane. This occurs exclusively via cotransport with hydrogen ions and through the action of H⁺-ATPase. The presence of a proton pump leads to the extrusion of H⁺ and acidification of the apoplast, which facilitates the conversion of HCO₃^- into CO₂ and the efflux of hydroxyls from the upper epidermis (Miedema, Prins, 1992; Rascio, 2002). Studies of carbon assimilation during photosynthesis using radiolabeled ¹⁴CO₂ and H¹⁴CO₃ in submerged leaves of P. lucens demonstrated that carbon dioxide saturation is achieved using both CO₂ and HCO₃^- 12 minutes after the onset of illumination (Lucas et al., 1978). The authors suggested that the uptake of HCO₃^- into leaves may be coordinated in some way with ion transport.