HETEROPHYLLY IN PLANTS - O.N. NEDUKHA - 2011

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

1.9. FUNCTIONAL CHARACTERISTICS OF LEAVES

1.9.2. The role of photosynthetic pigments

The aquatic environment significantly influences the pigment content of leaves, particularly affecting the Organs of higher aquatic plants that are submerged in Water. Their assimilatory apparatus is characterized by low photosynthetic activity and a hygromorphic blade Structure (Ronzhina & Pyankov, 2001). Given that the quantitative and qualitative parameters of the pigment complex are dynamic and vary depending on light intensity and quality, the Structural Features of leaves and METABOLISM/14.html">Chloroplasts, as well as the number of chloroplasts per unit surface area, many researchers view these indicators as one of the

key mechanisms of plant adaptation. Specifically, this involves a decrease in the chlorophyll (a/b) ratio, which is typical of shade-tolerant plants compared to sun-loving ones (Lubimenko, 1963; Zauralova, 1980a, 1980b; Lee et al., 1990). Furthermore, a low concentration of chlorophylls in hydrophytes is known to reduce the risk of cellular damage from photooxidation.

G.F. Nekrasova et al. (1998) studied The Effect of the aquatic environment on the chlorophyll and carotenoid content in numerous species of hydrophytes and emergent aquatic plants. The total chlorophyll content (a+b) (mg/dm² of leaf area) in emergent leaves of Alisma plantago-aquatica averaged 2 mg/dm² of leaf area; in floating leaves of Potamogeton natans, it was 2.5 mg/dm²; whereas in submerged leaves of P. lucens, the chlorophyll content was 2.5 times lower. The carotenoid content in emergent (A. plantago-aquatica) and floating (P. natans) leaves was 0.9 and 1.3 mg/dm² of leaf area, respectively, while in submerged leaves of P. lucens, it was 0.6 mg/dm² of leaf area—i.e., 2.5 times lower. The chlorophyll (a/b) ratio in the respective leaves was 3.5 (A. plantago-aquatica), 1.2 (P. natans), and 1.2 (P. lucens) (Nekrasova et al., 1998).

It has been established that pigment content per unit of dry mass and leaf area depends on the mesophyll structure type, which is driven by shifts in leaf mesostructure parameters that reflect the density of photosynthetic elements. Across THE SPECTRUM OF leaf types—emergent ^ floating ^ submerged—There is a reduction in the amounts of pigments and carotenoids, as well as a decrease in photosynthetic intensity due to a lower number of chloroplasts per unit of leaf area. It has been proven that the adaptation of submerged leaves to the aquatic environment under low light conditions and slow CO₂ diffusion induces alterations in the Functional Properties of chloroplasts. Specifically, an increased pigment content within the chloroplasts of submerged leaves (up to 7 x 10^-9 mg of chlorophyll, 2 x 10^-9 mg of carotenoids) compared to emergent and floating leaves was accompanied by an elevated rate of CO₂ uptake in individual chloroplasts and enhanced photosynthetic activity of chlorophyll. This activity reached 1.6 mg CO₂/mg chlorophyll·h in emergent leaves and 3.9 mg CO₂/mg chlorophyll·h in floating leaves, respectively (Ronzhina et al., 2004). It is likely that altering pigment levels enables the submerged organs of hydrophytes to regulate light-capture efficiency, optimizing their assimilatory performance under specific lighting conditions at varying growth depths and water turbidities.

A similar reduction in pigment content in submersed leaves compared to emergent leaves of the same plants has been observed in other heterophyllous species. For instance, aerial leaves of Ranunculus flabellaris contain a higher amount of chlorophyll and chloroplasts in their Cells than submersed

leaves (Yang et al., 1987; Yang et al., 1990, 1995); the synthesis of chlorophylls and carotenoids is also inhibited in submersed leaves of Hippuris vulgaris and Marsilea quadrifolia (Lin & Yang, 1999); in isolated photosynthetic cells from floating leaves of Potamogeton nodosus L., the protein and chlorophyll content accounted for about 25%, whereas in isolated photosynthetic cells from submersed leaves of P. nodosus, the protein and chlorophyll levels were 8 times lower than those in floating leaves, amounting to only 3% (Ryen, 1985).

Investigations of chlorophyll content per unit of leaf blade area in emergent and submersed leaves of Veronica anagallis-aquatica growing in different habitats (emergent under slow water currents, submersed under both high and low water velocities) demonstrated that chlorophyll content was lower in submersed leaves; moreover, it was higher under high-velocity currents than under slow-velocity currents (Table 1.9.2.1) (Boeger & Poulson, 2003). Higher chlorophyll levels in emergent leaves have also been reported in other aquatic plants (Nielsen & Sand-Jensen, 1989; Nielsen, 1993). Additionally, emergent leaves exhibit higher levels and greater activity of Rubisco (Beer et al., 1991), alongside increased CO₂ fixation (Madsen & Breinholt, 1995; Sand-Jensen & Frost-Christensen, 1999).

TABLE 1.9.2.1. Chlorophyll content in leaves of Veronica anagallis-aquatica grown under different conditions (Boeger & Poulson, 2003)

Although light regimes and the rates of photosynthetic CO₂ saturation differ between emergent and submersed leaves, fast water currents can inhibit Photosynthesis (Madsen & Sondergaard, 1983; Madsen et al., 1993; Carr et al., 1997) by acting as a mechanical stressor that causes leaf tension during wave action (MacFarlane & Raven, 1985). Photosynthetic inhibition under high flow velocities may result from leaf shading and the loss of organic metabolites (Fox, 1996).

Thus, the common Features of the pigment complex in hydrophytes—distinguishing them from terrestrial plants of temperate zones—include a low chlorophyll content per unit of fresh mass and a low ratio of total chlorophylls (a+b) to carotenoids. In emergent and floating leaves of hydrophytes growing in high-insolation environments, the adaptation of the assimilatory apparatus occurs through alterations in phototrophic tissue structure, the number of chloroplasts per unit area, and Cell size, which is also accompanied by systematic shifts in pigment distribution per unit of leaf mass and area (Ronzhina et al., 2004). Conversely, the adaptation of submersed leaves under low light conditions and slow CO₂ diffusion leads to The formation of an undifferentiated leaf blade with a low specific leaf area and a small number of chloroplasts per unit area. These structural modifications result in a high pigment content per unit mass and a low pigment content per unit area. An increase in pigment content within the chloroplasts of submersed leaves compared to emergent and floating leaves was accompanied by a decrease in photosynthetic activity, reflecting A change in the functional properties of chloroplasts (Ronzhina et al., 2004).

Later, Mommer et al. (2005), studying functional acclimatization to flooding and shading (high light = 500 µmol m^-2s^-1, low light = 13, 17, or 40 µmol m^-2s^-1) in two terrestrial plant species (Rumex palustris and R. thyrsiflorus), discovered that under flooding (at high light intensity), chlorophyll content tripled from 5.3 to 15.3 mg g^-1 of dry mass. Meanwhile, the chlorophyll (a/b) ratio remained almost unchanged under high light in R. palustris. In R. thyrsiflorus, this ratio under high light and flooding decreased from 4.0 to 2.75. Under low light, it was 2.69 (in air) and 2.85 under flooded conditions (Mommer et al., 2005).

In emergent leaves of five species of the genus Potamogeton (Potamogetonaceae), a higher chlorophyll content per unit of leaf area but a lower content per unit of volume was found compared to submersed leaves. No differences were detected in total chlorophyll content based on fresh mass (Frost-Christensen & Sand-Jensen, 1995). Submersed leaves of Ranunculus flabellaris also exhibited less chlorophyll than emergent leaves (Wells & Pigliucci, 2000). Consequently, it is evident that photosynthesis is intimately linked to the action and physical CHARACTERISTICS OF THE surrounding environment, which is consistent with the adaptive phenotypic plasticity hypothesis.