This study evaluated the physiological responses, hormonal signaling, osmotic and nutrient levels, as well as the performance of essential oils, antioxidant enzymes, and secondary metabolites in Lavender plants subjected to chromium and fluoride toxicity and biochar application. The findings indicated that the administration of raw and especially multiple-chemical engineered biochars decreased fluoride (about 16-40%) and chromium (39-60%) levels in Lavender leaves, whereas raised CEC and soil pH, nitrogen (10-37%), potassium (20-47%), phosphorus (10-60%), magnesium (30-49%), calcium (20-50%), zinc (39-240%), iron (40-120%), plant biomass, and photosynthetic pigments of Lavender plant leaves under toxic fluoride and chromium conditions. The treatments with multiple-chemical engineered biochars decreased the osmotic stress and osmolyte concentration (carbohydrates, soluble proteins, and proline) in the leaves of Lavender plants. Both raw and multiple-chemical engineered biochars significantly enhanced the water content of plant leaves, reaching up to 10% under toxic circumstances. Moreover, these treatments decreased the synthesis of stress hormones such as jasmonic acid (4-17%), salicylic acid (29-49%), and abscisic acid (30-66%), while increasing the production of Indole-3-acetic acid (IAA) (15-29%) in Lavender plants subjected to chromium and fluoride stress. The use of multiple-chemical engineered biochars showed notable efficacy in enhancing antioxidant enzyme's activity against oxidative damage induced by metal toxicity and decreasing proline accumulation. Maximum concentrations of linalyl acetate, borneol, camphor, and linalool were achieved under fluoride and chromium stress conditions by metaphosphoric acid-engineered biochar. Multiple-chemical engineered biochars application can be inferred as valuable approach to enhance both the quality and quantity of lavender essential oil under conditions of fluoride and chromium-induced stress.

Salinity stress is a major threat to agricultural productivity and sustainability, often causing irreversible damage to photosynthesis. Lavender, a valuable aromatic plant, experiences growth impacts under salt stress. However, the regulatory mechanisms of photosynthesis related to its adaptation to salt stress remain unclear. In this study, lavender was exposed to 0, 100, 200, and 300 mM NaCl for 28 days. It was observed that lavender effectively maintained chlorophyll stability when salt concentrations were below 200 mM and stress duration was under 21 days. The most effective model for lavender under salt stress was identified as a right-angled hyperbolic modified model. Under moderate salt stress (100 mM, 200 mM), genes such as LaPSB28, LaPSBS, and LaPSBR contributed to PSII core stability, enhanced photosynthetic pigment levels, and sustained high electron transfer rates to improve salt-tolerance. Additionally, LaLHCB4-1 and LaPSAK-1 regulated stomatal size, thereby facilitating gas exchange and supporting the photosynthetic process. Conversely, under high salt stress (300 mM), LaPSBW-1, -2, and LaPSAB were found to reduce photosynthetic pigment levels and inhibit photosynthetic activity. However, genes such as LaCHLG-2, LaGLG-3, LaBAM1-1 and -3, and LaCHLP-3 aided in starch synthesis by increasing pigment content, thus promoting energy balance and enhancing salt tolerance. This regulation involved photosynthesis-antenna proteins and pathways related to starch, sucrose, and chlorophyll metabolism. These findings may support the cultivation of salt-tolerant lavender varieties and maximize saline soil usage.