Anthocyanins, Photosynthesis and Photoprotection in Succulent Plants
Anthocyanins, photosynthesis and photoprotection in succulent plants are important elements of understanding the interaction of succulent species with their abiotic environment. In general, plants are exposed to various levels of environmental stresses under natural conditions especially in arid and semi arid region. The regions are characterized by sparse and highly variable precipitation, extreme variation of diurnal and annual temperatures, high wind regime and high evaporation, powerful winds, dust storm during summer and low humidity of the atmosphere (Earth's atmosphere). Succulent plants grown in such arid and semi-arid regions often encounter various stress conditions such as high temperature, drought, high wind regime, high light intensity, etc. One of the early responses of plants to unfavorable environments is a decrease in the rate of photosynthesis.
Research on plant response to various environmental stresses is becoming increasingly important, as most climatic changes suggest an increase in aridity in many parts of the globe. All these abiotic stress factors produce deleterious effects on photosynthetic apparatus especially the photosystem II (PS II) and consequently damage the plant growth. Rubisco activase, the protein that facilitates the release of sugar phosphates from Rubisco ceases to function properly at elevated temperatures, inactivating Rubisco. Heat stress also affects processes related to membrane integrity, ion conductivity, and phosphorylation activity. The PS II complex is very susceptible to heat among the various components of the photosynthetic apparatus. Heat induces the dissociation of the manganese-stabilizing 33-kDa protein from the photosystem II (PSII) reaction center complex followed by release of the manganese atoms (Yamane et al., 1998). Heat-inactivation of PSII may be followed by dissociation of the light-harvesting complexes from the PS II. According to Cajanek et al. (1998) most of the effects of heat stress are caused by the primary inhibition of PS II photochemistry at the donor side and also a block in the electron flow at the acceptor side.
Water stress
Water stress usually inhibits photosynthetic activity in the leaf and this inhibition is accompanied by stomatal closure. As a consequence water stress indirectly decrease the amount of photosynthate exported from leaves. One of the reasons for decreased photosynthesis at low water potential may be the increased stomatal resistance. During the onset of drought, stomatal conductance normally declines before photosynthesis, suggesting that the inhibition of photosynthesis under mild stress can be mostly explained by a restriction of CO2 diffusion (Cornic, 2000). High light (beyond what is needed for maximum photosynthesis) is also a major stress for succulent plants. Extreme high light conditions lead to depression in photosynthesis efficiency (photoinhibition), mainly due to oxidative damage to the photosystem II. Recent studies have been suggested that several strategies are involved in the protection against the photoinhibition of PS II. Photorespiration (Park et al., 1996), heat dissipation via xanthophylls cycle (Gilmore, 1997), and consumption of reducing power via water -water cycle (Asada, 1999) are believed to contribute to reduction and dissipation of excess energy. In the xanthophyll cycle, zeaxanthin is formed from deepoxidation of violaxanthin via the intermediate antheraxanthin in a reaction catalyzed by a deepoxidase, and an epoxidase catalyzes the reconversion of zeaxanthin to antheraxanthin and violaxanthin. There is strong evidence that zeaxanthin functions in the dissipation of excess excitation energy, which protects the photosynthetic apparatus from the damaging effects of photoinhibition by preventing the accumulation of toxic reactive oxygen species. Demmig-Adams (1990) showed that there is a close correlation between zeaxanthin content and the capacity for quenching of chlorophyll a fluorescence and photoprotective energy dissipation (nonphotochemical quenching) in many different species and under a wide range of conditions. However, the mechanism of energy dissipation involving zeaxanthin remains largely unknown.
Ultraviolet stress
Succulent plants are naturally exposed to high solar radiation and therefore they are subjected to relatively high ultraviolet sunlight doses. In spite of the relatively low irradiance, UV-B radiation could induce severe damage to plants via direct and indirect effects on photosynthetic apparatus, nucleic acids, proteins and cell membranes (Schmitz- Eiberger and Noga, 2001). The photosynthetic apparatus of higher plants is particularly sensitive to damage by UV-B radiation. The primary targets of UV-B radiation in photosystem II are the D1 proteins of the reaction centre and the water-oxidizing system (Janssen et al., 1998). To cope with environmental UV-irradiation conditions, higher plants have evolved a number of protective mechanisms. One of the strategies involves the photoinduction of anthocyanins, which have been shown in many plant species to reduce both the frequency and severity of photoinhibition, as well as to expedite photosynthetic recovery. By intercepting the high-energy quanta, anthocyanic cell vacuoles prevent important photo-labile molecules from degradation by green light. An elegant example of this was described recently for the silver beachweed (Ambrosia chamissonis), a composite that grows at exposed sunny locations along the California coastal zone.
Photoprotective role of anthocyanins has been observed in many plant species. Dramatic induction of synthesis and accumulation of anthocyanins was observed in response to high (sun) light in apple fruits (Merzlyak and Solovchenko, 2002) and Zea mays (Singh et al., 1999). Recently Soni (2008) examined the kinetics of anthocyanin accumulation in vegetative tissues of Commiphora wightii, a succulent plant, in order to determine their protective role against the UV-B to reduce both the frequency and severity of photoinhibition and damage of photosynthetic apparatus. C. wightii transferred to direct sunlight exhibited increased accumulation of anthocyanin in young aerial branches, as compared to the plants exposed to sunlight filtered through window glasses (i.e. sunlight devoid of UV-B as the window glasses are known to absorb UV-B rays).
No accumulation of anthocyanin was observed when the plants were deprived of direct sunlight. The results clearly suggest that anthocyanin accumulated in the epidermal and hypodermal region of the young stems of C. wightii plants during acclimation to strong sunlight serve as an efficient UV-B screen and play an important role in the protection of photosynthetic apparatus from the UV-B component of solar radiation. Thus, succulent plants are quite resistant to extreme environmental conditions and are well developed morphologically as well as physiologically to survive through such conditions.
References
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