Jojoba (
Simmondsia chinensis) is a perennial, dioecious, evergreen shrub suitable for cultivation in arid and semi-arid areas. It is native to the arid regions of the Sonora and Mojave deserts in the southwestern United States and northwest Mexico. In its natural habitat, jojoba usually grows in coarse desert soils on the rocky slopes of mountains in areas with 100–300 mm annual rainfall [
1]. The transcriptomic, proteomic, and metabolomic pathways that confer jojoba’s tolerance to the extreme heat-stress conditions in its habitat have been recently characterized [
2].
Jojoba oil composition was first analyzed in 1933, revealing its potential as a replacement for sperm whale oil [
3]. Jojoba is the only plant species known to accumulate wax esters in seeds instead of triglycerides [
4]. Its commercial potential comes from the odorless and colorless wax extracted from the seeds and used in a variety of products, including cosmetics, pharmaceuticals, lubricants, and petrochemicals [
5,
6,
7].
Nitrogen (N) is a major nutritional element in plants, required in many physiological and developmental processes, such as photosynthesis [
8], metabolite biosynthesis [
9], and flowering [
10]. Most N uptake in perennial crops occurs during the intensive vegetative growth and postharvest stages [
11], March–May and September–December for jojoba growing in the northern hemisphere [
12]. On the other hand, N pollution has been identified as one of the top emerging threats facing humanity and the planet due to its impact on climate, environment, and public health [
13,
14], and N excess originating from agricultural systems is considered to be a major source of this pollution [
15].
Despite the increasing importance of jojoba cultivation and its transformation from an extensive, rain-fed crop to an intensive, irrigated one, few studies have focused on its N requirements. Jojoba has relatively low reliance on fertilizers and has traditionally been produced in soils with marginal fertility compared to other crops [
16,
17]. Early work in Arizona and California showed a negligible response of jojoba to N fertilization in terms of seed yield [
18] and a relatively modest amount of 58 kg N ha
-1 year
-1 was suggested as sufficient for a yield of 3000 kg seeds ha
-1 [
19]. Nonetheless, more recent studies have demonstrated a positive effect of N on commercial jojoba plantations. N, phosphorus (P) and potassium (K) fertilization in jojoba (80, 8 and 80 kg ha
-1, respectively) shortened the period to fruit production and provided better vegetative growth compared to unfertilized plants [
20]. However, this experiment was conducted with old cultivars that had limited productivity and were irrigated only once a month. The response was attributed to the combined effect of fertilization and irrigation. Benzioni (1995) illustrated the jojoba plant’s response to nitrate fertilizer by measuring nitrate reductase activity and found much higher levels after application, mainly during growth and fruit filling [
21]. More recent research conducted under North Sinai conditions evaluated the effect of N foliar spray on yield and seed quality in jojoba. Treatments included two rates of a N-P-boron (B) foliar spray (1%-0.75%-0.4% and 1.5%-1.25%-0.8% of N, P, and B, respectively) applied three times (October, March, and April) and a control treatment with no fertilizer application. Higher rates of N (together with P and B) were found to increase growth, yield, and seed oil content [
22]. Research done in Arizona on 2-year-old local cultivars treated with N as a solid fertilizer showed a correlation between N dosage and seed yield. Levels were 0, 30, and 60 N kg ha
-1 year
-1 for the first 4 years, increased to 0, 60, and 120 N kg ha
-1 year
-1 for the remaining 6 years of the study. Differences between treatments in plant height were only significant in one year, whereas seed yield increased with increasing N application in 4 of the 7 years. An effect on leaf N concentration was visible after 3 years of N application (Nelson and Watson, 2001).
As mentioned earlier, Optimizing N application is essential for maximizing productivity on the one hand, and reducing environmental contamination on the other. Hence, the objective of the current study was to determine N-fertilization requirements of modern intensively cultivated jojoba by evaluating the effect of N availability on vegetative and reproductive development. In contrast to previous studies, in which irrigation was by furrow and N was applied once or twice annually [
23], or where N treatments were combined with differential P, B, and K levels [
20,
22,
24,
25]—making it difficult to isolate the effects of N, this study focused on applying N throughout six seasons via drip irrigation (fertigation).