Morphogenic Responses

Studies carried out in greenhouses or in growth chambers using ultraviolet lamps and filters to simulate different solar UV-B enhancements have been conducted on a variety of terrestrial plants, including economically important crops (Santos et al. 2004) and wild plant species (Zu et al. 2010). Overall these studies showed that the UV-B enhancement besides physiological effects induces a range of morphological changes including: (a) increase/decrease of the leaf area and leaf thickness (González et al. 2002; Hilal et al. 2004);

(b) reduction of the plant height (Santos et al. 2004) and increase/decrease of the shoot/root ratio (Furness and Upadhyaya 2002); (c) axillary branching (Kakani et al. 2003); (d) increase of the leaf glandular and uniseriate trichome density (Liakopoulos et al. 2006) ; (e) deposition of the waxy surface structures (Fukuda et al. 2008); (f) opening of the cotyledon curling (Boccalandro et al. 2001; Barnes et al. 2005); (g) inhibition of the hypocotyl and stem elongation (Shinkle et al. 2004; Gerhardt et al. 2005); (h) premature leaf senescence (Pradhan et al. 2006). The effects of UV-B also include changes (increase/decrease) in the number and size of flowers as well as in the size of seeds (Kakani et al. 2003; Qaderi and Reid 2005). While some of the UV-B responses constitute a stimulation of the growth (e.g., axillary branching, leaf thickening), others reflect a growth inhibition (e.g., reduced hypocotyl elongation). However, in these experimental setups, frequently unrealistic balances between UV-B/ UV-A/PAR are obtained, and in some cases the plants have been exposed to relatively high short-term doses of UV-B, which lack the ecological relevance (Newsham and Robinson 2009) . Additionally, the levels of UV-A or PAR as well as other experimental conditions also affect the morphogenic responses, making it difficult to compare the results from different indoor studies. In addition, it is clear that not all the plant species respond in the same way to UV-B exposure (Pliura et al. 2008). In general, the monocots are more morphologically responsive to UV-B than the dicots (Pal et al. 1997). Closely related species or ecotypes, especially when occupy different habitats, also differ with respect to their morphogenic responses (Hofmann et al. 2003). Plant species also differ in the use of PAR and UV-B radiation; while some species use the PAR to trigger responses others use the UV-B radiation. Then the plants responding mainly to PAR radiation will probably be more sensitive to UV-B radiation than the UV-responding ones (Rozema et al. 2005). A critical factor in the UV-B studies is the visible light irradiance, which in growth chambers and greenhouses can be quite different to the natural sunlight (Flint et al. 2009). Indeed, it has been shown that as a result of the insufficient visible wavelengths and, therefore, of unre-alistically high UV-B/PAR ratios in indoor studies, the morphogenic effects of the UV-B radiation are magnified (Musil et al. 2002a, b). In fact, even if realistic levels of the UV-B radiation in simulating ozone reductions are used the indoor responses of plants to UV-B radiation may be quite variable and exaggerated in relation to the field. Microclimatic conditions and the interactions of different abiotic and biotic environmental factors additionally contribute to inconsistency between the results obtained in growth chambers or greenhouses with those obtained under the field conditions (Flint et al. 2003; Caldwell et al. 2007). Furthermore, the plant responses to above ambient UV-B radiation (e.g., from stratospheric ozone depletion) have rarely been assessed in the broader context of the possible effects emerging from variations in the UV-B radiation within the ambient range. Also there is a significant knowledge gap between field and laboratory studies, which has two major components: (a) the occurrence of certain effects of the UV-B radiation under laboratory conditions has not yet been demonstrated in the field studies; (b) although some indoor responses are known to occur in the field, their functional implications are still unclear. Therefore, the obvious corollary from greenhouses or growth chambers studies is: study methodologies are as varied as results. In fact, from the field grown plants, the consensus that effects of artificially changed spectral UV-B irradiances are less pronounced (Searles et al. 2001). While under UV-B enhancement among other changes, leaf thickness, reduced leaf area, decreased plant height, changes in plant architecture, and biomass/yield reduction have been observed (Searles et al. 2001; Flint et al. 2003; Barnes et al. 2005). Nevertheless, the more recent studies have suggested that in the field, primary effects of the most realistic solar UV-B enhancements are subtle morphological and chemical changes with altered carbon partitioning and allocation, but doubt reveals such changes show significant effects on both plant growth and biomass accumulation (Gilbert et al. 2009; González et al. 2009; Morales et al. 2010; Ren et al. 2010; Zu et al. 2010). The morphogenic effects of the realistic UV-B enhancements are not usually considered as primary ecological factors influencing both species abundance and species distribution in relation to other abiotic environmental factors (e.g., drought, temperature, salinity). There are, however, situations where the UV-B-induced morphogenic effects can be ecologically important, giving changes in the competitive ability with a significant impact on the composition of the plant community (Flint et al. 2003) . The UV-B enhancement alters the leaf angle and differential transmission, and absorbance of the UV-B radiation through stands of erectophilous or planophilous plant species may have an important consequence on terrestrial plant responses to the UV-B radiation (Rozema 2000) . In a model study, it was predicted that a more planophilous leaf angle in erectophilous species would reduce the UV-B/PAR ratio and therefore the UV-B damage. Of course, this effect may affect the competitive relations among species and also the ecosystem composition (Deckmyn 1996 in Rozema et al. 1997). The morphogenic effects often can be pronounced on different organisms at other trophic levels (Bassman 2004). The UV-B radiation also affects the decomposition of plant materials into ecosystems. Plants grown under the enhanced solar UV-B showed a reduced rate of the litter decomposition when compared to control plants grown under the ambient solar UV-B. The accumulation of UV-B-induced lignin and/or tannin accounts for the reduced litter decomposition rate (Cybulski et al. 2000). Nevertheless, the reduced rate of the litter decomposition can be produced as consequence of detrimental effects of the enhanced UV-B radiation on decomposing fungi and other decomposer organisms (Pancotto et al. 2003) . In opposite trend, the plant litter material exposed to the enhanced solar UV-B can be decomposed by photodegradation more rapidly than under the ambient solar UV-B (Gallo et al. 2006). Moreover, it has also been demonstrated that the species growing for several generations under enhanced UV-B radiation show accumulation and exacerbation of the UV-B effects and likelihood they might be heritable (Mpoloka et al. 2007).

Much of the UV-B research on terrestrial plants has concentrated on vegetative plant parts, but fitness of the organisms depend mainly on their successful reproduction. Of particular concern is the detrimental effect of UV-B on the pollen quality observed for some species (Koti et al. 2004; He et al. 2007). This finding suggests that pollination may be an ecologically critical developmental stage vulnerable to the UV-B

damage, even in the UV-B-tolerant species. The pollen surface of some species may transmit up to 20% of the incident UV-B radiation (Stadler and Uber 1942 in He et al. 2007), despite the presence of a variety of UV-B-absorbing pigments (Rozema et al. 2001) . Thus, the mature pollen grains are potentially susceptible to the UV-B damage during a short period between the dehis-cence of anthers and the penetration of the pollen tube into the stigmatic tissue (Koti et al. 2004). This fact may lead to both reduced pollen quality and altered patterns of competition among species affecting the composition of the ecosystem. Furthermore, the UV-B enhancement can alter the production and/or the temporal availability of flowers so as to make the plant a less attractive host for the pollinators and impinge upon competition of plants for the pollinator service, as well as on the reproductive success of the plant/pollinator system (Sampson and Cane 1999).

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