Nitrogen fertilization effects on jojoba seed production

Although jojoba (Simmondsia chinensis (Link) C. Schneider) has been cultivated in the United States for many years, information is limited on the plant's nitrogen (N) requirement. A 10-year field study, which began with 2-year-old plants, evaluated the effect of N application rates on growth, flower bud survival, seed yield, seed weight, seed wax content and leaf blade total N content of three clones. Nitrogen rates were 0, 30 and 60 kg N per ha per year for the first 4 years and then were increased to 0, 60 and 120 kg N per ha per year for the remaining 6 years of the study. Nitrogen rates had a significant effect on growth in height in only 1 year. Growth was generally less in years when high seed yields were produced. Seed yields were increased linearly by N rate in 4 of the 7 years that crops were produced. Nitrogen rate had no effect on flower bud survival. Highest bud survival followed winters with temperatures no lower than - 3°C and little or no survival occurred when temperatures reached - 6 or - 7°C. Seed weight was increased by N in 3 of the 7 years. A negative linear relationship existed between seed yield and seed weight. In 5 of the 7 years, seed wax content was reduced as the N rate was increased. Seed yields of clones varied among years, but AT-1310 had the highest seed wax content in 4 of the 7 years. Leaf blade total N content generally increased linearly as the N rate was increased after the first 3 years of the study. Based on leaf N content and seed yields obtained over a 7-year period, it appears that a leaf N content lower than 2% from July through September is associated with reduced seed yield in the following crop year.