If the root temperature, significantly affected by the management of nutrient solution temperature, strays from the optimum range, several metabolic processes may be affected. This depends on the actual temperature, the duration of the stress, the physiological stage of the crop, the species and even cultivar (Kafkafi, 2008; Rachmilevitch et al., 2006a; Sanders and Markhart, 2000). In spite of the importance of root temperature to whole-plant responses, relatively little is known in comparison to the effect of air temperature, which has been studied extensively (Rachmilevitch et al., 2006b; Zhang et al., 2007). However, Xu and Huang (2000) suggested that root temperature appears to be more critical than air temperature in controlling plant growth.
One of the most widely observed symptoms of root temperature stress is that root growth is inhibited and number of roots and root dry weight may decrease. This has been observed in many plants with their roots subjected to supra-optimal (Kafkafi, 2008; Lyons et al., 2007; Rachmilevitch et al., 2006b; Sattelmacher et al., 1990) or infra-optimal temperatures (Ali et al., 1996; Apostol et al., 2007; Bowen, 1970; Franklin et al., 2005; Sanders and Markhart, 2002). Root viability decreases (Rachmilevitch et al., 2006a) and plants may die if the stress is very severe. The cause of the reduced root growth may be due to a reduced import of photosynthates from the shoots (see ahead), but in the case of supra-optimal root temperatures, the cause seems to be mainly related to the enhanced consumption by root respiration rather than to the reduced translocation.
Root respiration increases with root temperature (Lyons et al., 2007; Rachmilevitch et al., 2006b; Xu and Huang, 2000). Oxygen is consumed at a high rate and, in addition, oxygen solubility is reduced as temperature increases (Jones, 1997). Accordingly, high root temperature is generally associated to hypoxia stress in soilless systems (Incrocci et al., 2000). Respiration is a major avenue of carbohydrate consumption and may lead to shortage of assimilates when temperatures are too high. Actually, this fact has been proposed to be a primary factor responsible for root growth inhibition and dysfunction at high root temperatures (Kafkafi, 2008; Rach-milevitch et al., 2006b). The down-regulation of plant respiratory rates and the increase of respiratory efficiency by lowering maintenance and ion uptake costs are key factors for plant acclimation to high root temperatures (Lyons et al., 2007; Rachmilevitch et al., 2006b, b).
In addition to the effect of root temperature on root growth, it also affects root morphology. Under low root temperatures, roots might be more succulent (Calatayud et al., 2008; Dielman et al., 1998; Kanda et al., 1994), whiter (Calatayud et al., 2008; Dielman et al., 1998), with lower development of lateral roots (Bowen, 1970; Dielman et al., 1998; Sanders and Markhart, 2002) and higher content of unsaturated fatty acids in phospholipids (Kanda et al., 1994). The latter has been associated with tolerance to low root temperatures (Lee et al., 2005a). In contrast, under high root temperatures, roots may be shorter and highly branched (Stout et al., 1997). These differences in root morphology may lead to changes in hydraulic properties and in roots capacity for ion and water uptake.
The majority of the studies about the effect of root temperature on water uptake have been carried out under low temperatures, although water uptake may be affected by heat stress as well (Geater et al., 1997; McMichael and Burke, 1999). Many studies have reported a decrease in water uptake as root temperatures drop (Abdel-Mawgoud et al., 2005; Calatayud et al., 2008; Cornillon, 1988; Economakis, 1997; Murai-Hatano et al., 2008; Pavel and Fereres, 1998; Sanders and Markhart, 2002). The decrease in water uptake seems to be immediate (Sanders and Markhart, 2002) and has been attributed to higher water viscosity (Abdel-Mawgoud et al., 2005; Affan et al., 2005) and higher root hydraulic resistance (Pavel and Fereres, 1998). A decrease in the permeability of the root cell membranes (Yoshida and Eguchi, 1990) caused by a reduction in the activity of the plasma membrane H+-ATPases and linked to changes in the activity (open/closed) of aquapor-ins (Kafkafi, 2008; Lee et al., 2005b; Murai-Hatano et al., 2008
Radin, 1990; Sanders and Markhart, 2002; Yoshida and Eguchi, 1990) have suggested the causes for the increase in root hydraulic resistance.
In addition to water uptake, nutrient uptake is very sensitive to nutrient solution temperature (Xu and Huang, 2006). A restriction of nutrient uptake has been observed under supra-optimal (Rachmilevitch et al., 2006b) or infra-optimal (Ali et al., 1996; Dong et al., 2001; Macduff et al., 1987) temperatures. Actually, crops may suffer from nutrient deficiencies during long cold periods (Sanders and Markhart, 2002). However, in some studies neither any significant effect has been measured (Osmond et al., 1982) nor an increase of nutrient uptake has been determined under low temperatures (Calatayud et al., 2008). This might be dependent on the tolerance of the species and the specific temperature used in the study. Nutrient uptake may be limited by uptake per unit of root or by reduced root growth. The latter may become more significant over the long term (Sanders and Markhart, 2002). Regarding supra-optimal temperatures, the reduction of nutrient uptake per unit of root may be due to the shortage of root assimilates consumed by the enhanced respiration. With regard to the decrease of nutrient uptake per unit of root under low root temperatures, it has been associated with the change in the structure of membrane lipids in roots and with the decrease in the activities of enzymes responsible for nutrient uptake such as H+-ATPase (Dong et al., 2001). The uptake of different nutrients may have different sensitivities to root temperature. For example, NO3~ absorption appears more sensitive than NH4+ absorption at low root temperatures (Clarkson and Warner, 1979; Kafkafi, 2008; Macduff et al., 1987). This maybe due to the lower energy demand for NH4+ assimilation (Kafkafi, 2008).
Another root function that is influenced by root temperature is the synthesis and translocation of hormones like cytokinins, gibberellins and ABA (Ali et al., 1996; McMichael and Burke, 1999; Rachmilevitch et al., 2006b; Singh et al., 2007). A high level of cytokinin in the roots (Kanda et al., 1994)
has been associated with tolerance to infra-optimal temperatures. Moreover, there is evidence that ABA is involved in cold-temperature signalling (Franklin et al., 2005), and that it may be a means of long-distance root-to-shoot signalling in plants with cooled root systems (Franklin et al., 2005).
The reduced water uptake at low root temperatures might decrease leaf water potential and leaf turgor (Radin, 1990; Sanders and Markhart, 2002). Nevertheless, plants can respond to their decreased water status by increasing ABA concentrations in the shoot (Udomprasert et al., 1995; Zhang et al., 2008), which triggers stomatal closure (Apostol et al., 2007; Zhang et al., 2008). The decrease in transpiration caused by stomatal closure has been indirectly determined by measuring leaf temperature (Ahn et al., 1999; Malcolm et al., 2008), which has been suggested as a very sensitive parameter in identifying stress caused by low root temperature (Ahn et al., 1999). In sensitive species, stomata may be slow to respond and water stress may occur, which can result in transient or permanent wilting (Sanders and Markhart, 2002).
The closure of stomata results in a decrease in CO2 assimilation rate (Zhang et al., 2008). A decline in photosynthetic rate has been measured under high (Lyons et al., 2007; Rachmilevitch et al., 2006a, b; Xu and Huang, 2000) and low (Apostol et al., 2007; Malcolm et al., 2008) root temperatures. Accordingly, a decrease in the maximum and the effective quantum yield of photochemical efficiency of PSII and the fraction of open PSII reaction centres has been observed at non-optimal temperatures (Rachmilevitch et al., 2006a; Repo et al., 2004; Zhang et al., 2007, 2008). In contrast, the effective quantum yield and the fraction of open PSII reaction centres increased in rose plants with their root exposed at 10°C (Calatayud et al., 2008). In addition to the closure of stomata, changes in the ultrastructure of cortical cells that may affect the photosynthetic apparatus have been observed under low root temperatures (Lee et al., 2002). The decline in photosynthetic activity results in the reduction of shoot growth, shoot dry weight and/or leaf area under both supra-optimal (Kafkafi, 2008) and infraoptimal root temperatures (Ali et al., 1996; Apostol et al., 2007; Field et al., 2009; Franklin et al., 2005; Malcolm et al., 2008; Sanders and Markhart, 2002; Solfjeld and Johnsen, 2006). In addition, a high root temperature may also accelerate the senescence of aerial parts (Guedira and Paulsen, 2002).
The assimilate use in plants is altered by root temperature but differently depending on whether temperatures are above or below the optimum range. Under low temperatures, the leaf content of total non-structural carbohydrates increases (Ali et al., 1996; Repo et al., 2004; Solfjeld and Johnsen, 2006). This been attributed to a lower partitioning of assimilates into structural carbohydrates (Solfjeld and Johnsen, 2006), a delayed loss of starch (Repo et al., 2004), a reduction of translocation (phloem loading/ unloading) or a decrease of root sink demand (Sanders and Markhart, 2002). In contrast, some authors (Ali et al., 1996; Calatayud et al., 2007) have measured an increase of carbohydrates in the roots, which has been associated with tolerance to low root temperatures (Kanda et al., 1994). On the other hand, at high root temperatures total non-structural carbohydrates decrease in shoots and roots (Guedira and Paulsen, 2002; Kubota et al., 1987; Xu and Huang, 2000) due to the imbalance between photosynthesis and respiration in which carbon consumption exceeds production (Xu and Huang, 2000). Also, high root temperatures lead to changes in allocation pattern favouring root growth at the expense of shoot growth (Rachmile-vitch et al., 2006a).
The reduced nutrient uptake under non-optimal root temperatures may lead to a decrease in the leaf concentration of several nutrients (Kafkafi, 2008; Malcolm et al., 2008). Besides nutrient uptake, nutrient partitioning and assimilation are also altered by root temperatures (Sanders and Markhart, 2002). For example, an increase of NR activity has been measured under low root temperatures in leaves (Calatayud et al., 2008) and roots (Sanders and Markhart, 2002), while nitrate assimilation rate seems to decrease under high root temperatures (Rachmilevitch et al., 2006a). Besides, both an increase of ammonium content in leaves (Calatayud et al., 2008) and a decrease of amino acid content (Kubota et al., 1987) have been measured under low root temperatures. These divergences may depend on the species and the specific temperature of the study.
The exposure of plant roots to non-optimal temperatures may lead to oxidative stress. Actually, membrane injury has been pointed as the cause of the inhibition of root functions (Sanders and Markhart, 2002). H2O2 (Rhee et al., 2007) and MDA (Zhang et al., 2007) have been detected in plant tissues under non-optimal root temperatures. In order to prevent the accumulation of ROS in root cells, plants may respond to unfavourable root temperatures by increasing their synthesis of ascorbate and glutathione, or the activity of SOD, CAT or APX (Zhang et al., 2007). Plants tolerant to non-optimal root temperatures should be capable of dealing with ROS (Rhee et al., 2007) and preventing injury of the membrane (Rachmilevitch et al., 2006b).
To conclude, diagnosis of stress caused by non-optimal root temperatures in plants may be assessed by different techniques: measurements of biomass production and yield, leaf area, shoot/root ratio, root morphology, root respiration, water and nutrient uptake, nutrient content in plant tissues, photosynthetic activity, CF, stomatal conductance, transpiration, root hydraulic resistance, hormone accumulation in roots and shoots, carbohydrate content and partitioning in the plant, amino acid and ammonium content in plant tissues, lipid peroxidation and ROS species, the amount of antioxidant compounds, leaf temperature and the activities of several enzymes.
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