Impact of Daily and Seasonal Variation on the Phytochemical Profile of Larrea cuneifolia in Northwestern Argentina

<i>Larrea cuneifolia</i> Cav. (common name: jarilla macho) is an endemic Argentinian medicinal shrub that has traditionally been used by the Diaguita-Calchaquí communities in the Monte Desert region in northwestern Argentina. The aim of the present study was to analyze the phytochemical profile and biological activity of the aerial parts of jarilla collected in different places throughout the year, in different seasons and times of day, to determine the optimal harvesting conditions for promoting its medicinal use. The aerial parts were collected three times a day over the course of four seasons in eight <i>L</i>. <i>cuneifolia</i> populations. The total phenolic compounds (TPCs), total flavonoid (TF) content, total lignans (TL), sugars (S) and soluble protein (SP) content were quantified by using spectrophotometric methods and HPLC-DAD. Antioxidant activity was determined by using ABTS scavenging. Significant seasonal, diurnal and spatial variations in the accumulation of TPC (52.61 to 113.52 mg GAE/g), TF (3.71 to 17.92 mg QE/g), TL (283 to 582 μg NDHGAE/g); S (5.73 to 15.17 mg GE/g) and SP (36.75 to 103.10 mg BSAE/g) in aerial parts of <i>L. cuneifolia</i> were revealed. The highest concentrations of TPC and TF were recorded in spring mornings. Maximum accumulation of nordihydroguaiaretic acid (291.8 ± 2.8 μg NDHGAE/mg dry weight) and other lignans were also observed in spring. Heat map analyses pinpoint Ampimpa (Site 1) as a site for jarilla sustainable harvesting, balancing high metabolite content with population abundance, especially in spring, when the highest antioxidant activity (SC₅₀ = 1.560 ± 0.021 μg GAE/mL) coincides with increased phenol levels. These studies highlight the importance of integrating ecological and phytochemical data to define harvesting strategies; collecting during spring mornings optimizes the yield of bioactive compounds, simultaneously minimizing ecological pressure. This study demonstrates how seasonal bioprospecting can inform pharmacological research and local development while safeguarding the endemic plant population.