Humulus lupulus L. Strobilus Photosynthetic Capacity and Carbon Assimilation

Abstract

The economic value of Humulus lupulus L. (hop) is recognized, but the primary metabolism of the hop strobilus has not been quantified in response to elevated CO₂. The photosynthetic contribution of hop strobili to reproductive effort may be important for growth and crop yield. This component could be useful in hop breeding for enhanced performance in response to environmental signals. The objective of this study was to assess strobilus gas exchange, specifically the response to CO₂ and light. Hop strobili were measured under controlled environment conditions to assess the organ's contribution to carbon assimilation and lupulin gland filling during the maturation phase. Leaf defoliation and bract photosynthetic inhibition were deployed to investigate the glandular trichome lupulin carbon source. Strobilus-level physiological response parameters were extrapolated to estimate strobilus-specific carbon budgets under current and future atmospheric CO₂ conditions. Under ambient atmospheric CO₂, the strobilus carbon balance was 92% autonomous. Estimated strobilus carbon uptake increased by 21% from 415 to 600 µmol mol^-1 CO₂, 14% from 600 to 900 µmol mol^-1, and another 8%, 4%, and 3% from 900 to 1200, 1500, and 1800 µmol mol^-1, respectively. We show that photosynthetically active bracts are a major source of carbon assimilation and that leaf defoliation had no effect on lupulin production or strobilus photosynthesis, whereas individual bract photosynthesis was linked to lupulin production. In conclusion, hop strobili can self-generate enough carbon assimilation under elevated CO₂ conditions to function autonomously, and strobilus bracts are the primary carbon source for lupulin biosynthesis.

Keywords: CO2 enrichment; bracts; carbon autonomy; flowering crops; lupulin; respiration.

Figure 1

The net photosynthesis of hop strobili as a function of estimated intercellular CO₂ concentration (C_i). Cuvette O₂ was atmospheric ambient (~21%). Strobilus temperature was controlled at 25 °C and photosynthetically active radiation at 400 m⁻² s⁻¹. Samples were pooled across four measurement intervals (means ± SE).

Figure 2

The net photosynthesis of hop strobili as a function of photosynthetically active radiation (PAR). Cuvette O₂ was atmospheric ambient (~21%). Strobilus temperature was controlled at 25 °C and CO₂ at approximately atmospheric ambient (415 μmol mol⁻¹). Samples were pooled across four measurement intervals (means ± SE).

Figure 3

Hop strobili daily carbon gain estimates per m² in Yakima, WA, (representative days of year 215–235, 2022) at ambient (415) and 600, 900, 1200, 1500, and 1800 μmol mol⁻¹ CO₂.

Figure 4

The hop strobili bracts without (top row) and with (bottom row) contact herbicide treatment. Brown tissue illustrates the contact herbicide treatment and green tissue represents the control. Treated and control bracts on the strobili above the bracts illustrate the random individual treatment of bracts within a strobilus.

Figure 5

Isolation of hop bracts with (left) and without (right) contact herbicide treatment. Brown tissue illustrates the contact herbicide treatment and green tissue represents the control. Glandular trichomes are visible at the lower inner surface of each bract. Unlike control bracts, bracts treated with contact herbicide did not fill their glands with yellow lupulin.