The plant hormone jasmonic acid (JA) fine tunes the growth–defense dilemma by inhibiting plant growth and stimulating the accumulation of secondary compounds. We investigated the interactions between JA and phytochrome B signaling on growth and the accumulation of selected secondary metabolites in Hypericum perforatum L., a medically important plant, by spraying plants with methyl jasmonate (MeJA) and by adding far-red (FR) lighting. MeJA inhibited plant growth, decreased fructose concentration, and enhanced the accumulation of most secondary metabolites. FR enhanced plant growth and starch accumulation and did not decrease the accumulation of most secondary metabolites. MeJA and FR acted mostly independently with no observable interactions on plant growth or secondary metabolite levels. The accumulation of different compounds (e.g., hypericin, flavonols, flavan-3-ols, and phenolic acid) in shoots, roots, and root exudates showed different responses to the two treatments. These findings indicate that the relationship between growth and secondary compound accumulation is specific and depends on the classes of compounds and/or their organ location. The combined application of MeJA and FR enhanced the accumulation of most secondary compounds without compromising plant growth. Thus, the negative correlations between biomass and the content of secondary compounds predicted by the growth-defense dilemma were overcome.

Introduction: Greenhouse tomato growers face the challenge of balancing fruit size and chemical quality traits. This study focused on elucidating the interplay between plant branching and light management on these traits, while maintaining consistent shoot density.Methods: We evaluated one- and two-shoot plants under varying top light intensities using high-pressure sodium lamps and light-emitting diode (LED) inter-lighting.Results: The reduced yield in the two-shoot plants was mainly due to smaller fruit size, but not due to source strength limitations, as evaluated through leaf weight ratio (LWR), chlorophyll index, specific leaf area (SLA), leaf dry matter percentage, and stem soluble carbohydrate accumulation. Enhanced lighting improved fruit weight and various fruit traits, such as dry matter content, total soluble carbohydrate content, and phenolic content, for both one- and two-shoot plant types. Despite lower mean fruit weight, two-shoot plants exhibited higher values for chemical fruit quality traits, indicating that the fruit growth of two-shoot plants is not limited by the available carbohydrates (source strength), but by the fruit sink strength. Diurnal analysis of fruit growth showed that two-shoot plants had reduced expansion during light transitions. This drop in fruit expansion was not related to changes in root pressure (measured as xylem sap exudation from decapitated plants), but might be related to diminished xylem area in the stem joint of the two-shoot plants. The concentration of several hormones, including cytokinins, was lower in two-shoot plants, suggesting a reduced fruit sink capacity.Discussion: The predominant impact of branching to two-shoot plants on sink capacity suggests that the fruit growth is not limited by available carbohydrates (source strength). Alongside the observation that light supplementation and branching exert independent additive effects on fruit size and chemical traits, this illuminates the potential to independently regulate these aspects in greenhouse tomato production.

Continuous light (CL) or a predominant nitrogen supply as ammonium (NH₄⁺) can induce leaf chlorosis and inhibit plant growth. The similarity in injuries caused by CL and NH₄⁺ suggests involvement of overlapping mechanisms in plant responses to these conditions; however, these mechanisms are poorly understood. We addressed this topic by conducting full factorial experiments with tomato plants to investigate the effects of NO₃⁻ or NH₄⁺ supply under diurnal light (DL) or CL. We used plants at ages of 26 and 15 days after sowing to initiate the treatments, and we modulated the intensity of the stress induced by CL and an exclusive NH₄⁺ supply from mild to strong. Under DL, we also studied the effect of nitrogen (N) deficiency and mixed application of NO₃⁻ and NH₄⁺. Under strong stress, CL and exclusive NH₄⁺ supply synergistically inhibited plant growth and reduced chlorophyll content. Under mild stress, when no synergetic effect between CL and NH₄⁺ was apparent on plant growth and chlorophyll content, we found a synergetic effect of CL and NH₄⁺ on the accumulation of several plant stress hormones, with an especially strong effect for jasmonic acid (JA) and 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, in xylem sap. This modulation of the hormonal composition suggests a potential role for these plant hormones in plant growth responses to the combined application of CL and NH₄⁺. No synergetic effect was observed between CL and NH₄⁺ for the accumulation of soluble carbohydrates or of mineral ions, indicating that these plant traits are less sensitive than the modulation of hormonal composition in xylem sap to the combined CL and NH₄⁺ application. Under diurnal light, NH₄⁺ did not affect the hormonal composition of xylem sap; however, N deficiency strongly increased the concentrations of phaseic acid (PA), JA, and salicylic acid (SA), indicating that decreased N concentration rather than the presence of NO₃⁻ or NH₄⁺ in the nutrient solution drives the hormone composition of the xylem sap. In conclusion, N deficiency or a combined application of CL and NH₄⁺ induced the accumulation of JA in xylem sap. This accumulation, in combination with other plant hormones, defines the specific plant response to stress conditions.

Efficient production of artemisinin, a valuable secondary metabolite from Artemisia annua, remains a challenge for pharmaceutical applications. This study investigated the use of ex vitro composite plants—generated by inoculation with Agrobacterium rhizogenes strains 2659 and 1523—under hydroponic and aeroponic conditions to enhance artemisinin and phenolic compound accumulation. In leaves, artemisinin content increased in a cultivation-specific, strain-dependent manner: strain 2659 was effective under aeroponics (+36%), while strain 1523 enhanced accumulation under hydroponics (+32%). In roots, strain 2659 led to higher artemisinin accumulation than strain 1523 under both systems, with increases of up to 145% in hydroponics and 75% in aeroponics. Strain 1523 strongly promoted artemisinin exudation, especially in hydroponics, suggesting active regulation of artemisinin export. Aeroponic cultivation increased total phenolic content (TPC) in roots, while strain 1523 reduced TPC in leaves. Although total biomass was unaffected, A. rhizogenes altered assimilate partitioning, decreasing the shoot-to-root ratio and enhancing root metabolism. These findings demonstrate that ex vitro composite plants, combined with optimized soilless cultivation, represent a flexible tool to boost accumulation and secretion of high-value compounds in A. annua. The strain and environment-specific responses emphasize the importance of selecting appropriate bacterial strain–cultivation combinations for scalable production systems.