PLANT HETEROPHYLLY - O.M. NEDUKHA - 2011

CHAPTER TWO. MECHANISMS OF HETEROPHYLLY EXPRESSION IN PLANTS

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

2.1.1. Light Intensity

It has been established that changes in light intensity can induce heterophilly in plants (Ryerson, Dengler, 1994). These authors studied the structural parameters of leaves in Impatiens parviflora D.C. and Pothos aureus Linden ex André. Other researchers found that blue light also induces heterophilly in Marsilea quadrifolia (Lin, Yang, 1999). Phytochrome may be involved in the induction of heterophilly in several species (Goliber, Feldman, 1990). Since far-red light can penetrate to a sufficient depth in Water bodies, phytochrome is of considerable importance for submerged plants.

Both the quantity and quality of light underwater differ from those above the water surface. This occurs because a portion of the light is reflected from the water surface, while the rest is absorbed by the water and suspended particles (Sand-Jensen, Mebus, 1996). For instance, in the Rhine River, which contains high amounts of various suspended debris, light transmittance is less than 1% (Vervuren et al., 2003). Seasonality also affects underwater illumination. Despite this, hydrophytes are capable of surviving under very low light conditions. For example, in an experiment with Rumex crispus, plants were able to survive anywhere from 4 months (at a light intensity of 0.4 µmol quanta m^-2 s^-1) up to two years (at 17 µmol quanta m^-2 s^-1, with a 16-hour light / 8-hour dark photoperiod) (Mommer, Visser, 2005).

D. Bodkin et al. (1980), studying the effects of light and Temperature on the expression of heterophilly in Hippuris vulgaris growing naturally in two British lakes at a depth of 1.5 m and under laboratory conditions, demonstrated that The formation of aerial leaves on submerged stems in the lab occurred at a high photosynthetic photon flux density (PPFD) of sunlight, amounting to 1000 µmol quanta m^-2 s^-1. This was also observed when the red-to-far-red light ratio (660 nm to 730 nm) was low, since far-red light is more characteristic of deep waters, whereas red light predominates in the air Column. In natural lake conditions during the summer, under similarly low red-to-far-red ratios and low temperatures (below +10 ^oC), the growth of aerial leaves is inhibited, and only submersed-type leaves are formed. A decrease in photosynthetic intensity may also be the cause of this phenomenon (Bodkin et al., 1980). Another aquatic plant that responds to environmental changes is Ranunculus aquatilis, in which different leaf types develop from a submersed meristem (Cook, 1969) under a low red-to-far-red light ratio (Bodkin et al., 1980) and a high blue light photon flux density (Lin, Yang, 1999).

It has been established that photosynthetic photon flux density affects the efficiency of Photosystem II (PSII) in floating and submerged leaves of Chamaegigas intrepidus (Woitke et al., 2004). The researchers showed that under controlled lighting conditions, electron transport in PSII was 3 to 4 times higher in floating leaves compared to submerged ones. These two leaf types exhibited specific adaptations to environmental conditions, particularly to photon flux density: floating leaves adapted to high PPFD levels, whereas submerged leaves adapted to reduced PPFD levels.