Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 340

tissues [171]. Several factors appear necessary to confer desiccation tolerance. Evidence
implicates the accumulation of soluble sugars, especially sucrose and raffinose family oligo‐
saccharides [172-173]. However, such sugars have also been detected in immature desiccation-
intolerant embryos of maize and wheat [174]. Other factors, such as heat-stable late
embryogenesis abundant proteins, may be involved [175], but some of these have been
identified in recalcitrant (desiccation-intolerant) seeds [176]. Hence, examining the drought
response of desiccation tolerant and intolerant seeds fails to provide conclusive evidence of a
role in desiccation tolerance for either soluble sugars or heat-stable proteins. Soluble sugars
and heat-stable proteins were equally likely (or unlikely) to be involved in the development
of seed quality [178].
4.6. Chlorophyll pigments and photosynthetic activity under abiotic stress
It is clear from numerous similar studies of water and salt relations that turgor maintenance
alone does not assure continued leaf expansion [196]. It may be that photosynthetic capacity
is insufficient to provide carbon for both wall synthesis and ''turgor-driven cell expansion“.
Or it may be that some higher level controls operate to limit expansion in spite of the available
potential [221]. The Chl a and Chl b contents as well as the photosynthetic electron transport
rate in leaves of stressed 'Lara' and 'Serr' seedlings decreased significantly at all drought and
salt periods tested, but stressed 'Panegine20' and 'Chandler' seedlings did not differ signifi‐
cantly from the controls in regards to these traits at any time during the applied stress. The
decreases were more apparent with longer drought exposure time [180]. The Chl a/b ratios
remained constant in all cases and there were no significant differences observed within
genotypes [180].
The stability of chlorophyll content and chlorophyll a/b ratio in 'Panegine20' and 'Chandler'
seedlings suggests that the pigment apparatus is comparatively resistant to dehydration in
these tolerant walnut cultivars. Drought and salt stress can directly or indirectly reduce the
photochemical efficiency of PS2 due to either inefficient energy transfer from the light-
harvesting complex to the reaction centre, or to inability of the reaction centre to accept photons
as a result of structural alterations in the PS2 complex [201; 210]. The results obtained indicate
that abiotic stress like drought and salt affects both the light-harvesting complex and the
reaction centre of PS2. Also Rosati et al [121] revealed that Kaolin application in walnut under
water stress did not affect dark respiration rate, nor Amax2500, but significantly reduced
Amax2000/Amax2500 and apparent quantum yield, while compensation point was signifi‐
cantly increased. The modeled leaf photosynthetic response to PAR was different for the
kaolin-coated and the control leaves [121]. Assuming that only 63% of the PAR incident on the
kaolin-coated leaves actually reached the leaf surface, the modeled curves for the two treat‐
ments overlapped perfectly at any PAR [121].
Drought reduces photosynthesis in walnut ( Juglans regia L.), but it is not known whether this
is mainly due to the closure of stomata, or to possible effects on leaf biochemistry. In an attempt
to answer this question, Rosati et al [121] studied diurnal changes in the water status and gas
exchange in droughted [50% crop evapotranspiration (ETc)] and fully irrigated (100% ETc)
walnut trees, over 2 d. They resulted that stem water potential (Ψs) ranged from -0.5 MPa in