Evaluating deficit irrigation
Watermelon quality and lycopene content not affected
By Jenna Smith
For more information regarding this subject, contact:
Daniel Leskovar
(830) 278-9151
d-leskovar@tamu.edu
The United States produces, on average, 1.77 million tons of watermelons each year.
In Southwest Texas, watermelons constitute a large portion of the vegetable crop as well; however, strict pumping limitations of underground and surface water as well as competition for water use have caused farmers in the region to reevaluate the efficiency of their irrigation systems.
Daniel Leskovar, associate professor of horticulture and principal scientist with the Rio Grande Basin Initiative at the Texas A&M Research and Extension Center in Uvalde, is using evapotranspiration (ET) rate values to evaluate deficit irrigation (0.75 ET and lower) on watermelon crops. ET describes the amount of water removed from soils by soil evaporation and plant transpiration.
Deficit irrigations of 0.5 and 0.75 ET indicate that plants are only utilizing 50 or 75 percent of the water typically used with full irrigation. When less water is supplied to the plant, the plant must usethe available water more efficiently.
Using subsurface drip irrigation systems at three locations-Uvalde, Weslaco and Lubbock-Leskovar determined how decreased water application affected yield, fruit quality and lycopene content of diploid (seeded) and triploid (seedless) watermelons. By examining which cultivar of watermelon adapts more readily to specific environments, watermelons can be grown in their favored environment, increasing survivability and overall fruit quality.
Watermelons require 12 to 22 inches of water for maximum production. The amounts of water applied under deficit irrigation were 6.8 inches (0.5 ET) and 11.7 inches (0.75 ET) compared to 15.5 inches (1.0 ET) for fully watered melons.
"Trends, in general, across all three locations show that watermelon yields decrease sharply under deficit irrigation scenarios, 0.75 ET and below," he said.
"However, triploids have a 34 percent higher yield and fewer culls than diploids when supplied with deficit water."
Although watermelon yields decreased, deficit irrigation had inverse effects on fruit quality and lycopene content. For example, watermelon firmness increased at 0.5 ET, the lowest deficit water level. Triploid watermelons had higher soluble solids content at the lower irrigation rate as well.
Another important factor to note is that lycopene content was not affected by deficit irrigation. Watermelons contain 60 percent more lycopene-an antioxidant known to reduce the risk of heart attacks and cancer-than tomatoes.
Lycopene content increased with plant maturity, and ripe and overripe melons had a substantial increase in lycopene at the lower irrigation rates.
"The fact that lycopene content is still high in overripe melons may have practical applications for human use," he said.
"Although overripe fruits are left in the field as wastes, they can be harvested and used to provide an additional source of this beneficial antioxidant."
Understanding that lycopene content is affected more strongly by genetics and plant breeding than environmental conditions will allow for proper cultivars of watermelons to be placed in areas of Texas based solely on their reactions to drought conditions.
"If water pumping restrictions from underground aquifers are further imposed, deficit irrigation will become a reality for vegetable producers," he said. "Limited irrigation will directly reduce yield and size, but fruit quality, color and lycopene content will more likely remain constant in triploid watermelons."
