HETEROPHYLLY IN PLANTS - O.M. NEDUKHA - 2011

INTRODUCTION

Heterophilly is observed in a wide range of vascular plants. The phenomenon of heterophilly refers to The formation of leaves of different shapes on a single stem, accompanied by Changes in the Structural and functional Organization of both leaves and stems. Changes in leaf shape are a manifestation of growth plasticity across all Levels of organization, starting from altered directions of Cell Division down to subcellular and molecular modifications.

Such changes are most pronounced in higher aquatic plants, where PARTS OF THE leaves and stems are submerged in Water while others emerge above the surface. This Variability in leaf shape and the structural-functional ORGANIZATION OF THE Cells within this organ manifests as heterophilly. Plants growing in water or on land are in constant interaction with other species in the biocenosis and are exposed to various environmental factors: Temperature, soil and air humidity, wind, soil composition, gaseous environment, etc. The potential resistance of plants during ontogeny and their physiological responses to external factors are reflected in phenotypic plasticity, driven by corresponding genotype adjustments in reaction to the environment (Kordium et al., 2003). Perspectives on THE ORIGIN OF heterophilly vary. Some authors believe that heterophilly most commonly appears as the expression of a stable genetic program distinguishing juvenile leaves from mature ones (heteroblastic leaf development) (Briggs, Walters, 1984; Kerstetter, Poethig, 1998). Other researchers maintain that heterophilly arises under extreme conditions when the environment surrounding different parts of the plant changes abruptly (Kerstetter, Poethig, 1998). In such cases, the primary external signal triggers alterations in Gene Expression, leading to shifts in METABOLISM and growth rates (Bailey-Serres, Voesenek, 2008).

Recently, highly interesting research findings regarding heterophilly have begun to emerge, particularly concerning The impact of soil flooding or drainage on plants and the mechanisms of plant adaptation to these factors. Higher aquatic plants (hydrophytes) have become model organisms for studying heterophilly. Investigating hydrophyte biology is crucial because the partial or complete submergence of terrestrial plants, driven by climate change, not only exerts detrimental effects on their GROWTH AND DEVELOPMENT but can also lead to plant death due to oxygen deprivation. This applies to both wild species and crops growing along riverbanks. Certain species possess The ability to survive under such conditions, exhibiting a specific tolerance to flooding. One of the main challenges for living organisms underwater is the low concentration of dissolved oxygen, which diffuses poorly in water despite having a very high diffusion coefficient. Gas diffusion in water is 10,000 times greater than in air (Armstrong et al., 1994). Anaerobic Metabolism in cells under anoxia causes significant shifts in the Energy balance of Cells and Tissues, which can also result in the depletion of tissue carbohydrate reserves and the accumulation of toxic substances and their Fermentation products, subsequently leading to cellular damage and death (Drew, 1997).

Plant sensitivity to flooding contributes to a decline in species richness and their distribution within flooded ecosystems (Bailey-Serres, Voesenek, 2008). The distribution of flood-tolerant plants is restricted on the high banks of rivers and lakes. Such species predominantly grow on gently sloping shores where flooding occurs frequently, with plant distribution aligning along the vertical flood gradient (Mommer, Visser, 2005).

Prolonged flooding is characterized by numerous detrimental factors, the main ones being changes in plant-environment gas exchange and the accumulation of toxic substances in the muddy soil surrounding The ROOT System. Flooding significantly impairs gas exchange between the plant and its environment, affecting oxygen, carbon dioxide, and Ethylene exchange. The internal oxygen concentration in the plant decreases, limiting aerobic Respiration; the concentration of ethylene increases around the plant; and oxidized salts accumulate in the soil, sometimes reaching toxic concentrations (Bailey-Serres, Voesenek, 2008). Despite this, flooded plants develop numerous mechanisms to mitigate the negative effects of flooding, including metabolic and morphological plasticity aimed at the most efficient utilization of oxygen and carbon dioxide (Armstrong et al., 1994; Vartapetian et al., 1970). This is man-

ifested in the elongation of stems, petioles, and leaf blades (Voesenek et al., 2003), the formation and elongation of aerenchyma in stems and roots (Visser et al., 1996; Jackson, Armstrong, 1999), and changes in leaf shape, area, and epidermal Structure. The transition and spread of aquatic plants onto land, and vice versa, leads to readaptation to altered living conditions in both plant Structure and function.

Phenotypic variability plays a vital role in organismal ADAPTATION TO ENVIRONMENTAL conditions. A particular case of leaf shape variability is plant heterophilly. Quite high organ plasticity is observed in higher aquatic plants, specifically manifested as heterophilly: leaves exposed to air (above the water surface) are rounded and thick, whereas leaves submerged in water become elongated or dissected, thin, and almost devoid of Stomata.

Heterophilly is characterized by anatomical and structural differences between submerged and emergent Organs, as well as changes in their functioning, namely:

✵ changes in the shape, area, and STRUCTURE OF THE leaf blade;

✵ reorganizations of leaf and stem cell ultrastructure, with poorly developed mechanical tissues;

✵ presence of Chloroplasts in the epidermis of submerged leaves;

✵ changes in gas diffusion (O₂ and CO₂);

✵ absence or reduction of stomata in submerged leaves;

✵ formation of specialized roots designed for oxygen uptake and plant support (feathery roots, often lacking root hairs);

✵ shifts in the phytohormonal balance;

✵ acceleration of Photosynthesis;

✵ changes at the molecular-genetic level.

In this monograph, the primary focus is on the structural and functional differences and Specific features of submerged and emergent leaves in heterophyllous plants, as well as the cellular mechanisms involved in the manifestation of heterophilly that help the plant function normally both under and above water.