HETEROPHYLLY IN PLANTS - O.M. NEDUKHA - 2011

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

1.7. LEAF EPIDERMIS IN HETEROPHYLLOUS PLANTS

1.7.1. Stomata

The epidermal Cells of floating leaves in hydrophytes contain Stomata, much like the leaves of terrestrial plants. Transpiration and gas exchange, essential for Respiration and Photosynthesis, take place through the stomatal pore. Stomatal transpiration and gas exchange are regulated by the opening and closing of the stomatal pore, a process performed by the guard Cells of the stomatal apparatus. The guard cells of stomata in floating hydrophyte leaves function similarly to those in terrestrial plants. To reduce stomatal transpiration or limit carbon dioxide uptake, the stomata partially or completely close.

The Mechanism of guard Cell closure is attributed to the action of Abscisic acid and the activation of specific Ion Channels (Lemtiri, 1996; MacRobbi, 1998; Blatt, 2000). Upon receiving a stomatal closure signal from the epidermal cells, neighboring mesophyll cells utilize cyclic ADP-ribose (cADPR) to induce the expression of the Gene responsible for ABA synthesis (Wu et al., 1977). Researchers have demonstrated that during stomatal closure, Ca²⁺ channels in The Plasma Membrane of guard cells open, causing an instantaneous surge in ionized calcium ([Ca²⁺]_cyt) levels within the Cytoplasm. This shift alters cellular turgor pressure, ultimately leading to stomatal closure (Lecki et al., 1998; Grabov, Blatt, 1998; 1999). An increase in cytoplasmic calcium can also be triggered by a rise in the plasma Membrane Potential (Grabov, Blatt, 1999), the magnitude of which is directly influenced by ABA (Grabov, Blatt, 1998). Consequently, the indirect closure of stomata driven by elevated [Ca²⁺]_cyt levels in guard cells occurs As a result of:

1) cADPR-dependent influx of ionized calcium from cytoplasmic stores;

2) influx of free calcium from the apoplast via ABA-dependent opening of calcium channels in the plasma membrane.

In the first scenario, calcium is released from vacuoles, METABOLISM/14.html">Chloroplasts, and other Organelles via slow-vacuolar (SV) calcium channels, which are activated by Inositol 1,4,5-trisphosphate produced upon the activation of phospholipase C (Blatt, 2000). According to the model proposed by M. Blatt, ABA induces the concentration-dependent modulation (activation or inactivation) of Three types of ion channels in the membranes: Two Types of K⁺ channels oriented outward (toward the apoplast, I_k,out) and inward (toward the cytoplasm, I_k,in), as well as slowly activating chloride channels of the Plasmalemma and tonoplast, and potentially H⁺-ATPases. In this process, ABA binds to a specific plasmalemma receptor (X). Such binding can activate a G protein (G_α) and trigger the activation of phospholipase C (PLC), leading to the Hydrolysis of phosphatidylinositol 4,5-bisphosphate to generate inositol 1,4,5-trisphosphate and release Calcium Ions from intracellular stores. The elevated calcium concentration also activates the H⁺-ATPase, increases cytoplasmic pH, and activates protein phosphatase (PP) and protein kinase (PK) (Blatt, 2000). During stomatal opening, the potassium ion content in guard cells increases due to an influx from cells adjacent to the stomatal guard cells.

In typical leaf cells, fluctuations in cytoplasmic ionized calcium occur within milliseconds, whereas in guard cells, such fluctuations are considerably longer, lasting 10–15 minutes. ABA at a concentration of 10 nM can extend calcium oscillation times up to 30 minutes (MacAinsh et al., 1995; Webb et al., 1996). M. Blatt suggests that calcium released from intracellular stores may enter vesicles that fuse with the cytoplasm, thereby altering the volume of the guard cells. Calcium exiting the vesicles stimulates the opening of plasma membrane chloride channels, leading to membrane depolarization. As a result, chloride channels close while potassium channels open, and the membrane repolarizes while cytoplasmic calcium levels decrease (Blatt, 2000). It has been established that two Proteins participate in the fusion of vesicles with the plasma membrane: syntaxin and soluble N-ethylmaleimide-sensitive factor attachment protein, which anchor to a protein receptor on the plasma membrane

(Hanson et al., 1997; Martin, 1997; Jahn, Sudhof, 1999; Blatt et al., 2000). It has been proven that ABA specifically induces the expression of these aforementioned proteins. Syntaxin belongs to a group of integral proteins that form the core of the molecular Structure of mobile vesicles in stomatal guard cells (Leuman et al., 1999). Thus, wave-like oscillations in cytoplasmic calcium concentration, integrated with the events described above, alter the membrane potential of stomatal guard cells during their operation.

In flooded plants of Carya illinoensis (Wangenh.) K. Koch and Vaccinium ashei J. M. Reade, a significant decline in stomatal functionality and a reduction in leaf stomatal aperture have been observed (Davies, Flore, 1986, a; 1986, b; Smith, Ager, 1988).