Irrigation effects on seasonal growth and rubber production of direct-seeded guayule

Understanding guayule's response to environmental factors, such as location, soil type, drought stress, and seasonal growth variation is critical for irrigation management to maximize and estimate rubber and resin accumulation throughout the growing seasons. A study was conducted at two sites with different soil types (sandy loam soil at Maricopa, AZ and a clay soil at Eloy, AZ) to compare plant growth and rubber accumulation among different irrigation treatments during a two-year growing season. The above- and below-ground biomass, biomass growth, rubber/resin content, and rubber/resin accumulation were measured every other month from establishment to final harvest in well-watered treatments, which received 100% replacement of crop evapotranspiration (ET_c) and irrigated with subsurface drip and furrow (denoted as D100 and F100, respectively), and compared to reduced irrigation treatments (D50 and F50), which received 50% replacement of ET_c. Drip irrigation with high water input (D100) decreased root mass partition, but leaf, stem, and flower partitions were not significantly affected by irrigation treatment. Biomass yield was higher in the well-watered treatments as expected, while rubber and resin content were lower, indicating rubber and resin dilution by higher biomass. For all treatments, rubber and resin yield increased linearly over the two-year growing season. However, the rates of increase were different among the irrigation treatments. The D100 treatment had a higher rubber yield increase rate compared to F100 and D50 in sandy loam soil at Maricopa, while the D100 treatment had the lowest increase rate compared to the F100, F50, and D50 treatments in clay soil at Eloy. Top branches of guayule plants in the D100 treatment at Eloy lodged in the second year and likely contributed to lower rubber content and rubber yield in the treatment. The drip irrigation treatments D50 and D100 had higher water productivity for rubber yield at Maricopa. However, the D50 and F50 treatment had the highest water productivity for guayule rubber yield, while the D100 treatment had the lowest due to lower rubber content at Eloy. Root rubber content was 31–39% lower than stem at the two locations. This study indicates that rubber biosynthesis occurred in guayule year-round and that it is possible in clay soils to reduce irrigation without a significant loss in rubber yield.