Climate change variables such as temperature (warming), precipitation (drought and flooding) and atmospheric CO2 concentrations (CO2 fertilization) are expected to impact in the short- and long-term on agricultural productivity patterns and ecosystem structure and function. As the main plant organ involved in water and nutrient uptake and major sink for the photoassimilates, the root system is a key player in determining plant and ecosystem responses to climate change. Further, the root system, by its respiration, turnover, exudation processes and interactions with the soil biota, plays a critical role in controlling the soil C storage and cycling and, ultimately, in the feedbacks of terrestrial C cycling to climate change. Hence, the root system could be considered as sink and source of carbon dioxide, the main driver of the climate change, and a better understand of its responses could provide useful information in the plant adaptation to the future atmospheric composition.
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In the present chapter, we will mainly focus on how direct (CO2, temperature, drought, flooding) and indirect (salinity) components of climate changes impact on the root form and function in higher plants.
For clarity, we consider the root form as a "photographical description" of the root system at the tri- bi- and one-dimensional levels, as determined by biometric parameters. The root form includes the root morphology, architecture, distribution and dynamics. We will refer to the root morphology as the "superficial features of the whole and single root axis" (Lynch and Nielsen, 1996) such as the length, mass, surface area, volume and diameter. Further morphological parameters derived from the formers and having a functional significance (Ryser 1998) are the root length ratio (root length per unit of the plant's dry mass, RLR), root mass ratio (root mass per unit of the plant's dry mass, RMR), specific root length (root length per unit of root dry weight, SRL), root fineness (root length per unit root volume, RF) and tissue density (root dry mass per unit root volume, RTD). The root architecture is defined as the spatial configuration of the root system (Lynch and Nielsen, 1996) and is generally estimated in terms of topology (Robinson et al., 2003). Root topology, which refers to the distribution of the branches within the system, can lie within two extreme types: the "herringbone" type , in which branching is confined to the main axis; and the "dichotomous" type, exhibiting a more random branching (Fitter and Stickland, 1991). The root distribution, which refers to the deployment of the root axis in terms of biomass or length along the soil profile, is described by root mass (root mass per unit soil volume, RMD) and root length density (root length per unit soil volume, RLD) (xxxx). Finally, the root dynamics includes the root production, mortality and turnover (ratio of root number present at a time point to the number of roots produced up to that time) and life span.
The root system, as defined by Robinson (1991) "â€¦is the result of an evolution strategy to solve the problems of soil resources acquisitionâ€¦". Hence, the main function of the roots is the capture of belowground resources, such as water and nutrients, from that "â€¦heterogeneous and porous system â€¦" (Robinson, 1991) which is the soil environment. The climate change variables impacting on the plant C allocation and respiration will also affect the water and nutrients captures which are expensive physiological processes in terms of C , i.e. root structures and energy (root growth, resource uptake transport systems, exudation processes). Finally, we will review here the impact of climate changes on the root "secondary functions" such as storage of carbon and nutrients and supply of energy to belowground food web and microorganisms.
Roots and elevated CO2, ozone and UV-B (not contributed by Maurizio Badiani)
Roots and high temperature
The Fourth Assessment Report (AR4) of the Inter governmental Panel on Climate Change (IPCC) of United Nations predicted an approx. 1.8-4Â° C increase on global mean air temperature during this century (IPCC, 2007). Soil temperatures are also expected to increase reflecting the future atmospheric temperature trend (Pollack et al., 1998). As a "resident/inhabitant" of the soil environment, the plant root system will be potentially affected by warmer soil temperatures which will have a significant impact on its form and function and therefore on plant development and productivity.. In contrast to the abundance of studies on the effects of temperature on the root system (review in Cooper, 1973; Voorhes et al., 1981, Kaspar and Bland, 1992; McMichael and Burke, 2002), relatively few information have focused on integrated root development, growth, metabolic responses to soil warming.and how roots preserve its form and function under warming soil conditions is not completely clarified
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Considerable evidence indicates that the root growth increased in response to increased soil temperature up to an optimum threshold, typical for each species and depending partly on their native temperature regime (Mc Michel & Burke, 1998), beyond which root growth decreased. Faster elongation rates of whole root system were observed in the temperature range of 5Â°-23Â°C for Eucalyptus species (Misra, 1999), 10Â°-30Â°C for sunflower (; (Seiler, 1999), 10Â°-15Â°C for winter wheat (;Gavito et al., 2001) and bog and fen plant communities (Weltzin et al., 2000). Supraoptimal soil temperatures, on the other hand, reduced the root growth in many species such as Agrostis stolonifera (>35Â°C; Huang et al., 1998), Lactuca sativa (>35Â°C; Qin et al., 2007; He et al., 2009) and wheat (>38Â°C, Tahir et al., 2008). McMichael and Burke (2002) grouped the different taxa in relation to optimum of temperature for the root growth , pointing out the influence (incidence) of diverse genetic background on temperature-dependent root growth pattern among the plant species due to their different acclimation strategies . For example, a diverse response and adaptation, in terms of both root length and mass, was evident among genetically diverse sunflower (Seiler, 1998) wheat genotypes (Tahir et al., 2008) and in two Agrostis grass species ; where the root system of A. scabra was more thermotolerant growing up to 45Â°C (Terceck et al., 2003) than that of A. stolonifera which grew until 23Â° C (Pote et al., 2006). These observations suggested that genetic diversity in root growth contributes to the survival of plant species and to improve their productivity under high soil temperature conditions and deserves further studies.
To better understand the temperature-induced root responses between and within plant species is needed to consider that the root systems comprise different root types which are distint genetically, developmentally and functionally and differently respond to soil environmental stresses (Waisel and Eshel, 2002). Several examples can be mentioned in this regard: the first root axes of pearl millet showed a higher elongation rate in response to the increase of temperature (from 20Â° to 32Â° C) respect to the second one (Gregory, 1986); the primary root of sunflower was less inhibited at temperature above 35Â°C respect than lateral roots (Seiler, 1998); the tree fine roots were more sensitive to soil warming (Pregitzer et al., 2000); the specific root length (root length / root mass) and specific root area (root area / root mass) increased in warmer soil in the root finest fraction (<0.5 mm) only (Bjork et al., 2007). However, more studies are needed for gaining a better knowledge how the different root types/orders respond to high soil temperatures.
The morphological responses and the acclimation of the root to higher temperatures involve the integration of many metabolic and physiological pathways. It is well known that the root growth depends on the supply of carbohydrates which are sharply consumed by higher root maintenance respiration in warmer soil conditions. Therefore, the maintenance of lower respiration rate may represent an important basis for the thermotolerance of the root systems and, ultimately, for the plant adaptation to the higher soil temperatures. Indeed, the lower energy required for root maintenance permitted to Agrostis scabra, to grow up to 45Â°C while the root growth of Agrostis stolonifera, heat-sensitive species, was inhibited above 27 Â°C ). Other plant species such as Citrus volkameriana (Bouma et al., 1997), Bellis perennis and Poa annua (Gunn and Farrar, 1999), adapted to warmer soil, exhibited a lower maintenance respiration rate of their root systems.
However, several authors pointed out that the temperature-induced root growth patterns also depend on radiation flux which influencing the photosynthesis determines a variation of the carbohydrate supply to root system. Indeed, the tap and lateral root elongation rate of sunflower were weakly correlated with soil temperature and sharply dependent on the amount of radiation intercepted (Aguirrezabal et al., 1994). Further, the root biomass and length of temperate northern grassland species dominated by Holcus lanatus were strongly affected by incident radiation and not by soil temperature (Edwards et al., 2004). Consequently, in order to understand root responses to warming soil, it is necessary to separate the effects of photosynthetically active radiation by soil temperature.
Root growth is not only associated with the carbohydrate metabolism but is also correlated with other cellular processes, such as cell expansion and elongation. Analyzing the spatial distribution of expansion growth along the primary root axis of Zea mays, Walter et al. (2002) observed a greater extension accompanied by a maximal expansion activity of the growing zone with the rising temperature (from 21Â°C to 26Â°C). Furthermore, several lines of evidence suggested that the root morphological changes to high temperatures might be mediated by hormones. Qin et al. (2007) demonstrated that the application of the ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC), in lettuce seedlings, mimicked the high temperature-induced root morphological changes (inhibition and increase of the root length and diameter, respectively) and, the addition of ethylene biosynthesis inhibitors such as aminooxyacetic acid (AOA) or aminoisobutyric acid (AIB) relieved these effects.
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The soil temperatures fluctuated over a wide range of temporal (diurnal and seasonal) and spatial (depth) scales (Kaspar and Bland, 1992) determining soil-zone temperatures. This soil gradient affects the orientation of the root axis, mainly nodal and seminal roots which observed (followed) a plagiotropic growth (growth at angles from the vertical). For example, the seminal roots of Zea mays which at 17 Â°C grew horizontally, above and below this temperature showed a more vertical growth (Onderdonk and Ketcheson, 1973). The result of the diverse root orientation was a different root distribution in term of length and mass within the soil profile. Root mass of a community constituted by Cardamine hirsuta, Poa annua, Senecio vulgaris and Spergula arvensis was reduced at the surface soil layers by elevated temperatures (Kandeler et al., 1998). Similar results were pointed out by Soussana et al. (1996) in root of perennial ryegrass (Lolium perenne).
Different studies have been focused on the influence of the soil temperature on the root dynamics although no consistent patterns have been observed. The root production and mortality, especially of the fine-roots, determining the soil C inputs and soil microbial activity, play a critical role in regulating agro- and ecosystem C balance and, ultimately, in the below-ground CO2 efflux. An increase of root turn-over in response to soil warming were observed in maple tree seedlings (Wan et al., 2004), Norway spruce stand (Majdi and Ohrvik, 2004) and Eriophoretum vaginatum (Sullivan and Welker, 2005) unlike of temperate steppe perennial species (Bai et al., 2010), Douglas-fir (Johnson et al., 2006), maple (Cote et al., 1998) and oak trees (Joslin et al., 2001) whose (for which) root dynamics were not affected by high soil temperatures.
Warming is one of the main factors of climate change along with altered precipitation, elevate CO2 atmospheric and N deposition and each of these may be expected to vary independently as well as to be interdependent. For example, elevated CO2 reducing the evapotranspiration increased the soil moisture (Nelson et al., 2004); the increase of the soil temperature stimulated the soil microorganism activity causing a higher N availability for the plant (Rustad et al., 2001) and, finally, soil warming interacted with the concomitant drought stress also(Kramer and Boyer, 1995). In this respect, a combination of different climate change factors are possible and it would be interesting to know the root responses to the interactive effects of these factors. For example, Tierney et al. (2003) reported a strong relationship between root production and soil temperature in a hardwood forest which contrasted with the results of Joslin et al. (2001). Tierney et al. 2003) justified this discrepancy by difference in water availability between their site with that of Joslin et al. (2001) pointing out the water availability and soil temperature interactions in the root growth responses. Bai et al. (2010) revealed that the temperature-induced inhibition on the root production and mortality of temperate perennial steppe species was observed with increased precipitation while the root dynamics was improved under ambient precipitation. Further, the effects of the increase of temperature (from 10 to 15Â°C) and N supply on the total root length of winter wheat were highly significant and additive (Gavito et al., 2001); Majdi and Ohrvik (2004) observed that the addition of N reduces the risk of root mortality in Norway spruce contrasting the effect of soil warming. The reduction of the root biomass of a mixing plant species (Cardamine hirsuta, Poa annua, Senecio vulgaris and Spergula arvensis) at 0-10 cm of soil layers (Kandeler et al., 1998) and the greater induction of the root production and mortality in Acer spp. (Wan et al., 2004) by higher soil temperatures were observed at elevated but not at ambient CO2.
Soil warming influences on several root functions such as nutrient and water captures (St.Clair and Lynch, 2010, fundamental physiological processes for the plant development and productivity and the functioning of the terrestrial ecosystems. In general, high temperatures increased the nutrient acquisition up to a peak of a maximum activity and then decline it. Over the range of the 14-34Â° C, two cultivars of red maple increased the net nitrate uptake reaching a maximum of absorption at 24Â° C (Adam et al., 2003); roots of Eucalyptus nitens treated at 20Â°C showed a greater nitrate and ammonium uptake rates than that exposed to 10Â°C (Garnett and Smethurst, 1999); the K (Ching and Barber, 1979) and the P uptake (Mackay and Barber, 1984) were increased in response to the moderate increase of temperature until 29Â°C.. However, Gavito et al. (2001) pointed out the specific absorption rates temperature-dependent for each nutrient: varying the temperature from 10Â° to 15Â° C, during the vegetative growth, winter wheat reduced both the root and shoot N concentrations leaving unchanged the P concentration. Further, Eucalyptus nitens showed the different Q10 values of the NO3- and NH4+ uptake rates estimated between 10Â° and 20Â°C 1.88 and 1.31, respectively (Garnett and Smethurst, 1999).
Moreover, nutrient uptake appeared to be controlled by warmer temperatures through a direct and an indirect effect, i.e. via root system changes and/or via plant-soil interactions, respectively. Direct changes in root morphology could explain the temperature-induced effects on K uptake (Ching and Barber, 1979) and P uptake (McKay and Barber, 1984) while changes of the fluidity of fatty acids and thermostability of plasma membrane (Clarkson et al., 1988, Sibbey et al., 1999), the uptake kinetics (BassiriRad et al., 1993, 1996, 2000; Adam et al., 2003), the root cell energy by respiration process (Atkin et al., 2000), plant nutrient demand (Lainè et al. 1993; Gavito et al., 2001) could elucidate the temperature-direct effects on root physiology of nutrient uptake. On the other hand, the temperature-induced indirect effects on nutrient uptake were mainly correlated with the capacity of the soil warming to influence the nutrient availability through changes on the biogeochemical, nodulation and mycorhizzation processes and/or nutrient transport, at the rhizosphere level. Mineral weathering, decomposition of organic matter and exchange reactions between soil solid-solution phases, the biogeochemical processes involved in the nutrient availability, occurred at accelerate rates in warmer soils (Pregitzer and King, 2005). For example, an increase of N mineralization was observed in a litter and a sandy mineral soil from forest of Pinus sylvestris (Ross et al.,1999) and in a boreal Norway spruce stand (Stromgren and Linder, 2002). The nutrient movement towards the root system was improved at moderate supraoptimal soil temperatures which increased the ion diffusion and transpiration-driven mass flow (Weast, 1982, He et al., 2004). Therefore, the nutrient uptake was enhanced by the higher nutrient concentration around the root axis in warming soil. In this respect, Ching and Barber (1984) observed that temperature changes (from 15Â° to 29Â°C) determined a sharply improvement of the K uptake due to the increase of the diffusion flux (+160%) rather than the enlargement of the root surface area (+70%).
An important agricultural and ecological function of the root system is the biological nitrogen fixation of the legume species by bacterial infection considering that half of the 320 Tg of N input to terrestrial ecosystems annually comes from the biological fixation of N2 (Paul and Clark, 1996). Symbiotic nitrogen fixation was positively influenced by increased soil temperature which raised nodule mass and activity. However, although Rhizobium can tolerate high soil temperature (30-35Â°C), the extreme temperatures (>40Â° C) strongly affected the bacterial infection and N2 fixation, at different degree among the host plant species (Zaharan, 1999). The total N2 fixation of Trifolium repens was enhanced by an increase of the temperature in the 7-13Â°C range while it was not influenced by temperatures above 13Â°C (MacDuff and Dhanoa, 1990). Soil warmer also affected other main symbiotic relationship of the higher plants i.e. the plant host-mycorrhizal fungi interactions. Increasing the soil temperature, root length colonization (LRC) was improved (Fitter et al., 2000) and an increase of the development of arbuscular mycorrhizal hyphae which consequently determined a higher P uptake by roots of pea plants was observed (Gavito et al., 2003).. However, Olsrud et al. (2004) pointed out that the positive relationship between the mycorrhizal development and soil warmer was an indirect effect due to an increased C allocation towards the roots in response of the concomitant low soil moisture content rather than a direct temperature effect on the root system (Olsrud et al., 2004).