Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 310

Figure 1. Pedigree of major seedlings of walnut cultivars and advanced selections in the breeding at the California,
Davis, [Courtesy Dandekar et al., 2004]
of variation among walnut varieties in critical traits setting the potential for breeding? (3) Can
diagnostic tools be developed for identification of critical traits that could serve as selection
tools in breeding programs? In this chapter we review and discuss the available literature and
current knowledge regarding abiotic stress in walnut. Following we explain the morphologi‐
cal, physiological and molecular aspects of abiotic stress responses in plants emphasizing on
wlanut in three separate sections and summarize related research in each case.
2. Morphological responses to abiotic stresses
2.1. Soil-root interface and water absorption
The efficiency of any tree in terms of water relations depends on its ability to absorb water at
a rate able to prevent internal water deficits during periods of high transpiration. Water supply
to trees implies two major steps: absorption and transport of water (i.e. ascent of sap), both
driven by transpiration. The efficiency of soil water absorption in trees depends on both spatial
extension and density of their root system [18]. Spatial extension: water uptake by individual
trees depends on fine root exchange surface, i.e., on their cumulated length or biomass. Both
vertical root distribution and seasonal root growth dynamics depend closely on physical soil
properties (mainly texture like clay content, bulk density, content of coarse elements etc.) and
the physiological constraints on root survival and development (water table, oxygen supply,
nutrients, aluminum or manganese toxicity, soil pH). Climate itself could also influence fine
root dynamics [19]: data from a literature survey support the view that rainfall is one of the
major environmental factors controlling fine root biomass [20].
Surprisingly, an exceptionally small fine root biomass was detected in a dry beech stand when
compared to five other stands with higher rainfall [20]; this could be due to a large mortality
of fine roots during peak drought. It is also well established under continental [21-22] or like
mediterranean climates [21] that soil water uptake displays a gradual downward shift as the