Responses of plant functional traits to atmospheric particulate pollution
Leaf thickness is often considered to be a very valuable characteristic, which may be related to resource acquisition, water conservation and assimilation (Witkowski &Lamont, 1991; Hanba & Terashima, 1999; Cooper et al., 2004). In the past, the research on leaf thickness and mesophyll thickness was usually limited to the differences between different plants (Sabrina et al., 2005). Leaf tissue thickness was positively correlated with leaf water use efficiency and also closely related to leaf water storage capacity (Vergutz et al., 2012). Previous studies have found that small and thick leaves were the characteristics of plants adapting to a relatively water-deficient environment. The leaves tend to be thicker when the surrounding environment lacks water (Reich et al., 2003; Vergutz et al., 2012). Studies show that the increase in leaf thickness or density was beneficial to increase the distance or resistance of water diffusion from the inside of the leaf to the leaf surface, and reduce the internal water loss of the plant (Durkovic et al., 2012). In this study, we found that the leaf thickness showed a significant increase trend with the increase of atmospheric particulate matter pollution. This indicates that in an environment polluted by atmospheric particles, plants can achieve water conservation by increasing leaf thickness.
Specific leaf area is closely related to the growth and survival strategies of plant, which can characterize plant adaptability to the environment and resource acquisition capabilities (Reich et al., 2003; Wilson et al., 2010). Studies have shown that plants with a lower leaf area are more adaptable to resource-poor and arid environments, while plants with a higher leaf area have a stronger ability to retain nutrients in the body (Reich et al., 2003; Denis et al., 2005; Wilson et al., 2010). In this study, we found that the specific leaf area of plants generally decreases with the increase of atmospheric particulate matter in cities. Under the influence of atmospheric particulate pollution, urban plants have already adjusted their specific leaf area. Reducing the specific leaf area of plants is very beneficial to the capture of light energy by leaves, at the same time, it reduces the harm caused by atmospheric particulate matter pollution and enhances the adaptability to polluted environment. Therefore, the relatively low specific leaf area of urban plants is the result of their long-term adaptation to urban atmospheric particulate pollution, and is also their long-term survival strategy in polluted environment.
The dry matter content of leaves is a predictive index for plants to obtain resources. It can reflect the adaptability of leaves to arid climates, and it is the most stable variable on the axis of resource acquisition (Willby et al., 2003; Denis et al., 2005). Leaf dry matter content is the preferred index in plant ecology research, it can be a good indicator of the plant's ability to preserve nutrients (Tao et al., 2019). In this study, we found that there were significant differences in dry matter content of leaves under urban atmospheric particulate pollution, and increases with the increase of pollution. With the increase of leaf dry matter content, the distance or resistance of water diffusion from leaves to leaf surfaces was increased, and the water loss in plant was reduced. Therefore, under the influence of atmospheric particles, the dry matter content in leaves of plants was higher, which may increase their environmental adaptability.
Leaf tissue density reflects the mechanical protection capability of the leaf (Yin et al., 2018). Studies have shown that the increase in leaf tissue density is beneficial to reducing transpiration, thus reducing water loss of plants (Houter & Pons 2012; Yin et al., 2018). Meanwhile, the increase in leaf tissue density can slow down the growth of plants and store more carbon for the construction of defense organizations (Reich et al., 2003). In this study, the density of leaf tissue showed an increasing trend with the aggravation of atmospheric particulate matter pollution. This shows that plants strengthen their defense structure to reduce the damage of atmospheric particles to leaves. This was similar to the response strategy of plants to high temperature and drought.
Chlorophyll content index reflects the photosynthesis ability of plants to a certain extent (Gitelson et al., 1996; Steele et al., 2008). In this study, the relative content of chlorophyll was significantly reduced by atmospheric particles. We suspect that, on the one hand, this may be related to the dust falling on the leaf. A large amount of dust was attached to the surface of the blade, which covers the contact area of the blade and the outside, especially the leaf area used for photosynthesis. On the other hand, the decrease of chlorophyll content may be the result of plant resource allocation. Under the condition of limited resources, plants use more resources to build defensive structures, thus weakening the resources for photosynthesis.
Stomatal is an important window for gas and water exchange between plants and the outside world (Allen et al., 2001; Field et al., 2010). This study shows that atmospheric particulate matter has a direct impact on stomatal density of plants, and the aggravation of pollution promotes the increase of stomatal density. As atmospheric particles were deposited on the blade surface, some air holes may be blocked, which may lead to an imbalance of the blade's ability to exchange gas with the outside world. Therefore, increasing the stomatal density of plants may be a regulation strategy for dealing with atmospheric particulate matter. At the same time, we also found that atmospheric particles have a significant impact on stomatal distribution pattern. Under the influence of atmospheric particles, urban plants can improve their gas exchange function by adjusting stomatal structure and optimizing the spatial distribution pattern of stomata, which is beneficial to their normal physiological metabolism and growth.
The trade-off strategy of plant functional traits on atmospheric particulate pollution and analysis of leaf economics spectrum
Generally, the total resources available to plants are limited. If plants invest more resources in a certain functional trait, they will inevitably reduce their investment in other traits, that is, at the expense of the construction and functional maintenance of other traits (Poorter & Bongers 2006; Tomáš et al., 2012; Wright et al., 2004b). In other words, in a limited resource environment, plants will optimize resource allocation among functional traits (Tomáš et al., 2012). In this study, due to the pressure of atmospheric particulate environment, Euonymus japonicus showed a trade-off strategy at the leaf level, which constituted a complex and orderly trade-off relationship of economics spectrum characteristics. Euonymus japonicus will adjust, transform or compensate its own functions according to its own resource conditions in the urban environment, so as to achieve and balance the three purposes of "survival, growth and reproduction", which is finally manifested in the functional characteristics of plant leaves. For example, due to the influence of dust deposition, the specific leaf area is significantly reduced, which indicates that plants may use a large part of materials to build protective structures, so as to reduce the damage of atmospheric particles to leaves. The decrease of specific leaf area indicates that the greater the leaf area per unit mass of the plant, the thicker the leaf are. At this time, the more carbon the leaves were used to build the protective structure. It is manifested by the increase of leaf dry matter content and leaf tissue density at the trait level.
It was pointed out that the leaf economics spectrum was a series of interrelated and coordinated combination of functional characteristics, and it also quantitatively represents a series of regular and changing strategies of plant resource balance (Wright et al., 2004a; Wright et al., 2004b; Freschet et al., 2012; Reich et al., 2012; Wright et al., 2012; Read et al., 2014). At one end of this economic pedigree, it shows the ability of "quick investment-return", while at the other end it shows the ability of "slow investment-return" (Wright et al., 2004; Reich et al., 2012). On the whole, under the influence of urban atmospheric particulate matter, Euonymus japonicus shows a "slow investment-profit" type in the global leaf economics spectrum, which has low specific leaf area, low chlorophyll content, large leaf thickness, high leaf dry matter content, leaf tissue density and stomatal density (See Fig. 7).