HETEROPHYLLY IN PLANTS - O.N. NEDUKHA - 2011

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

1.6. ANATOMICAL STRUCTURE OF LEAVES AND STEMS IN HETEROPHYLLOUS PLANTS

1.6.4. Plant Stems

The cellular Structure of stems in fully submerged plants has been only partially investigated. Structural modifications in various stem Tissues of flooded plants exemplify the phenotypic plasticity of this organ (Kende et al., 1998; Mommer et al., 2006, a, b; Voesenek et al., 2006). The primary stress factor for aquatic plants is impaired gas exchange, leading to internal stem Hypoxia (Jackson, Armstrong, 1999; Rijnders et al., 2000). This internal oxygen deficit can trigger accelerated stem elongation, which helps push the leaves out of the Water and accelerates their contact with the air (Voesenek et al., 2004). Elongated air cavities within submerged stems and roots enhance the diffusion of atmospheric oxygen through the aerenchyma of the emergent stem into the submerged Organs (Armstrong, 1979).

Recent studies utilizing the model system Rumex palustris have identified novel aspects of stem plasticity induced by flooding (Mommer et al., 2007). Complete stem submergence has been shown to induce a series of changes at the leaf structure level. The thickness of the blade, cuticle, and epidermal Cell walls decreases in acclimatized leaves that developed underwater compared to non-acclimatized ones (Mommer et al., 2005). Such plasticity is a consequence of enhanced gas exchange between the leaves and the aquatic environment after 10 days of flooding (Mommer et al., 2006, b), driven by increased oxygen content within the stem once it emerged above the water surface (Mommer et al., 2004).

Japanese researchers (Sato et al., 2002) studied stem elongation in Potamogeton distinctus developing from buds on the roots of plants that had overwintered under anaerobic submerged conditions. Stem elongation was accompanied by

a decrease in the dry mass of its segments and the depletion of starch reserves from the amyloplasts of stem Cells, whereas the total content of Amino Acids in the stem (Alanine, valine, leucine, isoleucine) increased. During the Cytology/cytology/16.html">Early stages of elongation, lactate content in the stem also rose. Furthermore, the authors demonstrated that under anoxic conditions, radiolabeled carbon (¹⁴C-glucose) was incorporated into ethanol, with the label also detected in alanine. This established that reserve starch is utilized as a substrate for post-winter growth exclusively under anoxia. This finding was corroborated by light Microscopy of cross-sections of P. distinctus (L.) A. Benn., which showed that starch disappeared from the amyloplasts within three days of the onset of stem elongation (Sato et al., 2002).

By analogy with heteromorphic leaves, the Anatomical Features of emergent and submerged stems in heterophyllous plants also differ. For instance, emergent stems of Ludwigia repens differ from submerged ones in having a reduced aerenchyma volume, a thickened cuticle, higher Cellulose and sugar contents, and lower contents of Lignin, hemicellulose, and starch (Table 1.6.4.1) (Little, 2003).

TABLE 1.6.4.1. Characteristics of submerged and emergent stems of Ludwigia repens (Little, 2003)

Studies on the EFFECT OF WATER current velocity on Stem Morphology and Photosynthesis in Veronica anagallis-aquatica L. (Scrophulariaceae), growing naturally in high-current conditions in the Batiste River (USA), demonstrated that flow velocity significantly affects anatomical traits (Boeger, Poulson, 2003). In V. anagallis-aquatica, some leaves emerge above the water, while others remain submerged. The authors found that emergent stems were longer, had a larger diameter, and a greater dry mass compared to submerged stems, whereas the internodes of emergent stems were more compact than those of submerged ones (Boeger, Poulson, 2003).

The phytohormone Ethylene may act as a signal for the rapid elongation of submerged stems (Malone, Ridge, 1983; Ridge, 1987). Ethylene is produced via an oxygen-dependent pathway, and its endogenous concentration is determined primarily by the rates of its synthesis and diffusion. Both processes are critical for plant functioning under water. Several Biosynthesis genes responsible for encoding 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO) are known to be upregulated during flooding (Rieu et al., 2005; Vriezen et al., 1999), whereas the outward diffusion of ethylene into the surrounding environment is inhibited. As a result, its endogenous concentration increases. Ethylene is retained as long as oxygen continues to diffuse from the water into the submerged stem (Mommer et al., 2004).