Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 309

The various parts of a walnut tree differ in their needs during the year for photosynthate for
respiration, growth of new plant parts, and developing nut crop. Depending on the cultivar,
heavy crop loads may adversely impact the following year's crop by reducing female flower
initiation and the amount of stored carbohydrates. The processes associated with nut produc‐
tion appear to be under strong genetic control; thus annual heavy nut production will require
selection of seedlings of walnut cultivars exhibiting multiple leaf layers to maximize photo‐
synthetic production, tendencies toward lateral bearing, good resistance to anthracnose, and
efficient use of photosynthesis for tree growth and nut production.
The genus Juglans consists of four sections. Three of these, Rhysocaryon (black walnuts native
to the Americas), Cardiocaryon (Japanese, Manchurian and Chinese walnuts, including
selections known as heartnuts) and Trachycaryon (the butternut of eastern North America),
exhibit thick shells and non-dehiscent hulls [234]. The fourth section, Juglans, is comprised of
a single species, Juglans regia L., distinguished by a dehiscent hull which separates from the
shell at maturity [234]. J. regia , the Persian walnut, is native to central Asia and grows as a wild
or semi-cultivated tree in a wide area from south-eastern Europe and the Caucasus to Turkey
and Iran, through southern portions of the former Soviet Union into China and the eastern
Himalayas. It has been cultivated for its nut crop for at least several thousand years and was
probably introduced into European commerce and agriculture by the ancient Greeks. It was
prized by the Romans as Jovis glans and was utilized in medieval Europe as an herbal
medicine, particularly for brain and scalp ailments. Since its introduction into North America
it has commonly been referred to as the English walnut to distinguish it from the American
black walnut while the correct name is Persian walnut [15; 217].
Species of walnut are distributed in temperate and subtropical areas of the Northern Hemi‐
sphere, mainly in mountain forests. Walnuts are distributed in three separate regions,
Mediterranean, East-Asian Himalayas, and North American. Walnut is deciduous, monoe‐
cious, and wind-pollinated. Walnut trees are sharply differentiated from other fruit trees by
their size and vigor, tree height, and crown diameter, often reaching 30 m and trunk diameters
as large as 2 m. Trees may have single trunks or be multi-stemmed [13].
The pedigree of major seedlings of walnut cultivars and advanced selections in the breeding
program in California is shown in Figure 1.
Walnut ( Juglans spp. ) is generally very sensitive to specific ion toxicities [218]. but genetic
variation in growth indices and morphological, physiological, biochemical and cellular
responses to water stress, especially in germination and early growth of walnut seedlings, have
been studied to some extent. There are clear economic incentives for identification of drought-
adapted walnut genotypes that can be used successfully in extensive arid and semi-arid
regions. Our preliminary work has identified some walnut seedlings that are very tolerant to
drought and especially to salt stresses at the germination stage [192; 212]. Mechanisms of
adaptation and tolerance in selected walnut rootstocks of walnut were also investigated.
To facilitate breeding for improved water use and drought resistance, a number of questions
should be addressed: (1) What physiological traits contribute to efficient water use and high
drought resistance and how do they interact with traits of rapid growth? (2) What is the range