Influence of White Plastic and Water Replacement Rates on Pomegranate Orchard Phenology, Fruit Yield, and Quality

Currently, 98% of domestic commercial pomegranate fruit (Punica granatum L.) are produced in California on over 13,000 ha.  In 2013, a pomegranate orchard, established in 2010 with a density of 558 trees/ha, was irrigated at water replacement rates of  35, 50 and 100% based on rainfall, tree water removal, and soil surface water loss as measured by a weighing lysimeter.  A durable white plastic was installed in one half of each water replacement plot.  Orchard floor reflectance, surface and soil temperatures, soil moisture (30 cm), leaf greenness and leaf porometric measurements were made during the growing season.  White plastic improved mid-day orchard floor reflectance 3.5 fold and reduced surface temperatures in both sunlit and shaded areas.  Irrigation level did not influence SPAD readings, but trees grown under white plastic were greener (P ≤ 0.0001), which was visually observable during leaf senescence in the fall.  Stomatal conductance on Aug. 2 was higher (P=0.008) in leaves grown under plastic compared to those grown in bare ground.  Weekly soil water content (SWC) throughout the season was always higher in soil covered with plastic and the standard error between plastic vs. bare ground suggested that SWC under plastic was more uniform.  Soil temperatures at 0, 5, 10 and 30 cm depth were less variable under plastic.  A single harvest was made on Oct. 23.  Total marketable fruit was increased by 17% and total fruit yield (all grade categories) was increased by 20% with the use of plastic (P=0.025).  Total fruit yield was linearly increased with increasing water replacement rates.  Increased irrigation also increased the amount of superficially cracked and cracked (visible arils) fruit.  Neither the white plastic nor irrigation rates improved external fruit color.  Fruit raw product attributes (% SS, initial pH, aril wt.) were little changed by treatments, however, increasing water application rates resulted in lower CDM values (darker juice color) and juice color from fruits grown under plastic were also darker (P=0.06).  Trees grown under white plastic had more basal sucker production (% of trees and sucker size) at the end of the season; irrigation rate also increased sucker frequency and size.  Trunk cross-sectional area was increased linearly with increasing water replacement rates.  The use of an in-row durable white plastic appears to be beneficial in retaining soil moisture, reducing the need for chemical weed control, and improving pomegranate total fruit yield and juice color.