Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 325

rain events, the water potential rapidly increased and was significantly higher than in the dry
season (Figure 3). The embolism level increased with xylem δD. Similar analyses were
performed between xylem δD and leaf pre-dawn water potential, leaf transpiration and
photosynthesis. The former two parameters had significantly negative and linear correlations
with xylem δD, while photosynthesis was not significantly correlated with xylem δD (Figure
4b-d) [17].

The daily sap flow varied significantly between seasons and was mainly determined by the
daytime sap flow. Generally, in summer, even in early October, the flow was higher than in
spring (17 April). On 15 August, at the point of lowest soil moisture, the daily sap flow was
also restricted. The daily sap flow was significantly correlated to transpiration demand and
also to mean air temperature. For all individuals, the sensors showed negligible night-time sap
flow with lowest values on 15 August [17].

The leaf transpiration rate exhibited similar dynamics to the pre-dawn water potential in the
growing season [17]. There was a significant difference in transpiration between the dry and
wet seasons. During the dry season, the transpiration rates ranged as 0.9-1.6 mmol m −2 s −1 ,
significantly lower than the range of 2.1-2.5 mmol m −2 s −1 observed in the wet season. The
assimilation rate did not completely follow the dynamic pattern of transpiration (Figure 4d).
Photosynthetic rate was lowest on 15 August, when the soil moisture was lowest in the growing
season; however, the highest photosynthetic rate occurred on 17 April, when the soil moisture
was not highest [17] (Table 2).

Regression equation R value R 2 value P value

Transpiration Y = -1.770 + 0.398x 0.951 0.904 0.013

Air temperature Y = -2.141 + 0.541x 0.894 0.800 0.041

Table 2. Relationships between the daily sap flow and transpiration and air temperature in walnut [Courtesy Sun et
al., 2011].

3.5. Vulnerability to cavitation

Cavitation occurs when negative sap pressure exceeds a threshold value defined by anatomical
characteristics [62-65]. Many species have been found to operate very close to the point of
embolism. Therefore, stomata control both plant water losses and sap pressure and, thus, may
actively control the risk of xylem embolism [63-69].

Vulnerability curves (VCs) were constructed by plotting the changes in the percentage loss of
xylem conductance (PLC) versus xylem pressure were demonstrated by Cochard et al at [29].
Significant differences were found between organs. Leaf rachises were the most vulnerable,
roots the least vulnerable, and leaf veins and shoots intermediate. Turgor pressure (P leaf 0) at
full turgor averaged 0.93 ± 0.06 MPa (n = 5, ±SE) and the turgor loss point averaged -1.53 ± 0.04
MPa. When plants were continuously exposed to a constant and high light intensity for 1 week,
a higher level of water stress was obtained. Eplant and g s dropped close to zero whereas