The Effects of Simultaneous Drought and Salinity Stresses on Growth, Water Uptake and Leaf Water Potential of Almond in Semi-Arid Condition

Almond (Prunus dulcis) has an important role in the agricultural economy of north-west of Iran, especially Azerbaijan provinces. Due to arid and semi-arid conditions of our country, large areas of cultivated land are affected by salinity. Almond trees have good tolerance to water stress, and are suitable for such conditions. However, sensitivity of almond trees to salinity calls for special attention to the integrated effect of salinity and water stress on its water relations. This trial aimed to evaluate the combined effect of salinity and drought stress on water uptake, vegetative growth and leaf water potential (LWP) of almond trees.

The experiment was conducted at the Sahand Horticultural Station located in East Azerbaijan, Iran (37o 55' 43'' N, 45o 57' 29'' E) during 2014 growing season at 7 years old almond (cv Azar) trees grafted on GF677 rootstock based on randomized complete block design with three replications. Treatments comprised three irrigation salinity levels viz. 2 (T1), 4 (T2), and 5 (T3) dSm-1. The soil of the experiment site was coarse loamy mixed calcareous mesic typic xerofluvents. Undisturbed and composite disturbed soil samples were taken from three diagnostic layers. Twelve undisturbed core samples were taken from each layer. Composite disturbed soil samples were air-dried and ground to pass a 2-mm sieve.  All the appropriate soil chemical (Organic matter and Calcium carbonate content, pH and EC, Total N, Available P and K) and physical (Particle size distribution, natural bulk density) properties were measured by the routine laboratory methods. Water contents at field capacity (FC) and permanent wilting point (PWP) were determined by the pressure plate apparatus. After irrigation of all trees with 200 mm water enough for saturating of soil in rooting depth on 20th of May, the measurements began. The volumetric soil water content (SWC) was measured at three locations around each tree 30 cm apart from tree trunk at three depths (0‒20, 0‒40 and 0‒70) using a TDR probe. During the experiment (20th May till 17th October), air temperature and relative humidity were obtained from the meteorological site located in the station. The midday leaf water potential (LWP) was measured from the leaves located in north part of trees close to stem between 12 and 14 o’clock.

Results indicated that salinity has significant effect (p<0.01) on LWP, vegetative growth and remaining water content. The difference between T3 and other treatments was not significant in SWC more than 8%. Therefore, it is obvious that, at SWC less than 8%, reduction in soil water potential due to increased osmotic pressure of soil solution in T3 have caused that, trees unable to uptake more water. Therefore, at SWCs less than 8%, remaining water content in T3 was significantly more than other treatments. Seasonal averages of annual vegetative growth and increase in trunk diameter in unstressed tree was 104 cm and 53%, respectively and decreased to 62 cm and 18% in T3 respectively. Seasonal averages of LWP for treatment T1 to T3 were ˗1.78, ˗1.93 and ˗2.16 MPa respectively. Whereas unstressed trees had highest LWP (-1.53 MPa). Highest and lowest LWP for treatment T1 to T3 were -1.20, -1.32 and -1.35 and -2.38, -2.47 and -2.73 MPa respectively. LWP of unstressed trees was between -1.1 and -2.0 MPa. There was significant negative correlation between LWP and VPD. The slope of regression equation increased as stress severity increased. This means that, for a given VPD, leaf water potential was declined with increase in salinity of irrigation water. LWP is affected by two stresses namely evaporative demand of the atmosphere (atmospheric-induced stress) and unavailability of water due to the reduction of soil water content (soil-induced stress). In well-watered plants, LWP is affected only by atmospheric factors (VPD) and therefore the relationship between LWP and SWC should not be significant as it took place in our experiment for unstressed trees. But there was a significant relationship between LWP and SWC in stressed treatments (T1 to T3) because of soil-induced stress. Threshold value of LWP for initiating stress was obtained to be -1.78 MPa. Based on the threshold LWP, values of SWC for initiating stress for treatment T1 to T3 can be 10.1, 11.8 and 13.5%, respectively.

Based on our findings, the midday leaf water potential is a suitable criterion for determining water status of almond trees in the studied area and can be used as an indicator for tree water and salinity stresses. Irrigation water salinity had significant effect on LWP. Due to relationship between LWP and soil water content (SWC), this indicator can be used for determination of soil available water and non-limiting water range of almond trees. Besides LWP, salinity also had a significant effect on vegetative growth and extractable soil water content. At high water content, the effect of salinity on extractable water content was not significant. But with decreasing water content, the effect of salinity increased so that, at SWC less than about 8%, the remaining SWC in saline condition was significantly higher than non-saline condition (extractable water in the saline condition was less than non-saline condition). Salinity also reduced soil available water range of almond trees., LWP reached to its threshold value (-1.78 MPa) at SWC equal to 10 and 13.5% in non-saline and saline condition respectively.