3.5 ± 0.5

63.3 ± 4.7

Fig. 8.2 Effects of salinity (0-200 mM NaCl) on the frequency (%) of different root diameter classes of primary (a) and lateral roots (b) of Lens culinaris landrace (Ustica)

root system and is frequently used as indicator of root fineness. Then, higher SRL results from longer and thinner roots per unit construction cost (root mass) and this root apparatus is more effective in water and nutrient uptake (Fitter 2002). SRL is a complex parameter that includes variations in root diameter and RTD, which respond to environmental conditions differently (Ryser 2006) . In the Ustica variety, a salinity increase leads to an increase in SRL (Table 8.1), due to thicker lateral roots and the root diameter distri bution was shifted toward larger diameter classes (Fig. 8.2a, b). Root diameter distribution is usually expressed as the mean diameter but sometimes it does not necessarily characterize a response of root system structure adequately. In fact, fine and coarse roots show different responses, indicating that root diameter classes should be considered as functionally distinct and regarded separately to fully understand stress responses of root systems. It is known that roots with a smaller root diameter can contact a larger soil volume per unit root surface area; however, the maintenance carbon cost of producing finer roots may be higher as these will have to be replaced more frequently (Fitter 2002). In Ustica plants, coarse roots, for both principal and lateral prevailed under high salinity conditions. This result can be explained by considering that under salinity, the construction costs per root length should be minimized because of the onset of growth-limiting conditions, and the root development resulted further inhibited to counter water stress and ion toxicity due to the salt around the root. Apart from the effects on root biomass production, contrasting root morphological responses of eco-types to salt treatments might be partially responsible for dissimilar abilities to tolerate salinity. Structural and morphological differences in roots certainly play an essential role for nutrient and water uptake by plants from saline soil and the study of these parameters can help to determine different mechanisms underlying salt toxicity and the way plants can cope with saline conditions. Some modifications of root morphology should not be considered a simple growth stopping, but rather an induced reorientation of growth which is related to stress avoidance. This information could be considered an important tool in studies that involve salt tolerance improvements in plants.

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