Factors affecting root function

2.4.3.3.1 Root responses affecting nutrient uptake

Root morphological characteristics strongly influence nutrient uptake (Boot, 1989) for both mobile and immobile nutrients. In a L. perenne sward higher nitrate interception is associated with comparatively larger root systems than the average root system (Crush et al., 2005). Modelling for nitrate uptake in relation to root system size and distribution suggest that a deeper root system and increasing root length density deeper in the soil profile reduce nitrate-leaching (Thorup-Kristensen, 2001). High N supply has been associated with increase in root biomass and root:shoot ratio but specific root length, specific root area, mean root diameter and frequency of fine roots were unaffected (Boot and Mensink, 1990).

As for N, the increased interception of inorganic phosphorus which is a non-renewable and immobile nutrient element is associated with root architecture, frequency and length of root hairs and distribution of roots deeper to the soil (Lambers et al., 2006) and mycorrizhal symbiosis. L. perenne breeders aim to develop cultivars which are more efficient at acquiring inorganic phosphorus from soil and/or more efficient at using phosphorus. Some articles suggest that phosphorus-fertilization has a direct effect on biomass partitioning (De Groot et al., 2001). A low phosphorus status decreased production and export of cytokinins from roots (Kuiper et al., 1989). One theory is that change of biomass partitioning may be linked to change in cytokinin production, and possibly associated with a decreased rate of uptake and metabolism of N (Kuiper et al., 1989). Lower phosphorus status was also associated with higher total root-length production without a proportional change in root biomass (Steingrobe et al., 2001), leading to greater amounts of uptake of immobile resources, such as phosphorus. Other observations also showed that increase in specific root length is also associated with lower inorganic phosphorus-supply (Schroeder and Janos, 2005). These results suggest that

Chapter 2 Literature review

36 when phosphorus is limited plants produce more fine roots as an adaptation strategy for higher phosphorus-acquisition.

As nutrients are distributed in soil in a heterogeneous manner roots need to respond to local nutrient patches to enhance nutrient capture (Hodge, 2004). Grass roots show different types of plasticity responses when encountering nutrient-rich patches. The plastic changes can be limited to individual roots such as, elongation of individual roots (Bilbrough and Caldwell, 1995), or the structure of the whole root system might be changed such as, increase in total root length (Hodge et al., 2000), increase in root production (Gross et al., 1993), and increase in extent of lateral root branching (Farley and Fitter, 1999). Faster growing grass species produce significantly higher root numbers, root length density and root biomass in nutrient rich patches under heterogeneous nutrient supply compared to slow growing species (Fransen et al., 1998; Robinson and Van Vuuren, 1998). The increase of both root biomass (Robinson, 1994) and specific root length (Eissenstat and Caldwell, 1988) significantly increase root length density.

Reduction in specific root length is usually reflected in increased root diameter and vice versa but an increased tissue density can also reduce specific root length. Morphological and physiological plasticity responses of roots in a heterogeneous soil environment have been reviewed by Hodge (2004). Increased nutrient uptake rate of the grass root system is a physiological plasticity response which is influenced by uptake capacity or ion affinity of the roots. Local nitrate supply at a higher concentration (1.0 mM nitrate) yielded many lateral branches compared to the remainder of the root system in contact with lower nitrate (0.01 mM nitrate) (Drew and Saker, 1975). In fact, following a local nutrient-deposition event, physiological responses occur before morphological responses (Drew and Saker, 1978; Robinson, 1994).

2.4.4.2.2 Root responses affecting water uptake

Water uptake capacity of plants depends on root morphological characteristics and physiological properties and the plasticity of roots at different levels of soil water availability. Morphological plasticity of the root system allows plants to adapt to different soil nutrient status and moisture availability (Robinson, 1994; Hodge, 2004). Root length density was positively correlated with water uptake rate under adequate soil moisture availability, at various soil depths in an experiment of Ehlers et al. (1991). Maximum

Chapter 2 Literature review

37 water uptake rate from individual soil layers (normalized by evapo-transpiration rate) found linearly associated with root length (normalized by specific root length) (Ehlers et al., 1991). Ehlers et al. (1991) also reported that maximum specific uptake rate per cm root length is inversely related to the square-root of root length density and also to the square- root of specific root length. Huang and Fry (1998) reported that root morphological and physiological characters which influence water uptake may differ from genotype to genotype of a species. In F. arundinacea, under limited water supply, deeper rooted genotypes had increased specific root length, and lower electrolyte leakage compared to a shallow-rooted genotype. The shallow-rooted genotype also produced lower dry weight and suffered greater turgor loss (Huang and Fry, 1998). Generally, plants producing thinner roots have higher specific root length, which is considered more efficient in terms of biomass utilization required to achieve nutrient acquisition (Eissenstat, 1992). A deep rooting system has been identified as one of several traits that is important for water deficit (drought) resistance (Sheffer et al., 1987; Marcum et al., 1995; Carrow, 1996). Root branching at deeper rooting depth has been considered to be an important drought tolerance mechanism (Marcum et al., 1995). Jupp and Newman (1987) studied root morphological and physiological traits of L. perenne under low water potential and reported that more negative water potential promotes lateral root initiation and elongation, and increases total root length. The degree of morphological plasticity is sometimes species-specific or genotype-specific and also depends on morphological traits of the root system to some extent. Molyneux and Davies (1983) studied the capacity for deep rooting of three pasture species and observed that in water-deficit conditions, seedlings of Dactylis

glomerata L. can penetrate to a much deeper soil depth than L. perenne and P. pratense.

2.4.4 Intra-plant competition