The Effect of Different Mulch Types on Some Photosynthesis and Water Relation Parameters of Olive Cultivar ‘Manzanilla’ Under Water Stress Conditions

To study the effect of mulch and two irrigation levels on olive cultivar ‘Manzanila’, two experiments were conducted. The first experiment evaluated the effect of seven types of mulch in plant-less pots; the second one was conducted on olives as a factorial according to a complete randomized design with 4 replications. In this experiment, seven mulch types (gravel mulch, pistachio shell mulch, wheat straw mulch, de-oiled olive pomace mulch, super absorbent polymer, organic-based emulsion polymer, and control (no mulch)) in the presence of two irrigation levels (control: irrigation to reach 100% of field capacity) and water stress (irrigation to reach 50% of field capacity) were investigated. The results showed that mulching decreased surface evaporation rate and improved soil water content, keeping the underneath soil moist longer than bare soil; the best types of mulch for conserving soil water were de-oiled olive pomace mulch and pistachio shell mulch. The results of the second experiment indicated that the shoot water potential and the olive chlorophyll index were decreased significantly in the water stressed treatment; however, using pistachio shell mulch improved the latter traits under stress conditions. The stomatal size and stomatal density were also affected by irrigation conditions; on the other hand, mulch usage, especially de-oiled olive pomace mulch, could decrease drought effects in drought stress conditions, improving leaf conditions for photosynthesis, and growth. All types of mulch in the experiment increased stomatal conductance in the water stress treatment (50% of crop capacity), as compared to the control (100% of crop capacity). Overall, the results showed that the application of pistachio shell mulch and de-oiled olive pomace mulch could be a suitable criterion to reduce the damaging effects of drought stress in olive.