1. Introduction: Today, agriculture faces challenges such as an increasing global population, climate change, and heavy dependence on synthetic inputs that threaten sustainability. Eco-friendly biostimulants are gaining attention as alternative strategies to enhance crop productivity and resilience. Smoke-water (SW), derived from burning plant residues, is rich in bioactive compounds such as karrikins, which stimulate seed germination and influence physiological and biochemical processes. Previous studies have reported improvements in photosynthesis and yield traits in several species. Seed priming is also an efficient and low-cost technique to increase germination, seedlings' vigor, and establishment, as well as the productive capability of crops. Although various studies have evaluated the effect of SW on plant developmental characteristics over the last two decades, the physiological mechanisms and their relationship with increased crop yield, especially under different SW application methods, have not yet been investigated simultaneously. Therefore, the present study is a comparative study between two SW application methods, seed priming and foliar spraying, on some morphological, physiological, and yield traits of pepper (Capsicum annuum L.), a crop valued both as a spice and medicinal plant under field conditions.
2. Materials and Methods: The study was conducted in 2024 at the Research Farm of the Faculty of Agriculture, Razi University, using a two-factor factorial design within a randomized complete block design with three replications. The factors included five levels of seed priming (control, hydropriming, and priming with SW at 0.01%, 0.1%, and 1% concentrations) and four levels of foliar application (control and SW sprays at 0.01%, 0.1%, and 1% concentrations). SW was prepared by transferring smoke from burning wheat straw through distilled water, filtering, and diluting to the desired concentrations. For seed priming, chili pepper seeds were soaked in distilled water (for hydropriming) and different concentrations of SW (for SW-priming) for 18 hours at 20°C in a germinator. Before radicle emergence, priming was stopped, and the seeds were rapidly dried using a blower. Untreated dry seeds were used as a control. Foliar applications with different concentrations of SW were applied three times during the growth period. Gas exchange parameters, including photosynthetic and transpiration rate, stomatal and mesophyll conductance, intercellular CO₂ concentration, and photosynthetic WUE, were measured using a portable photosynthesis system (LCi, BioScientific Ltd., UK). Measurements were performed three days after the second foliar application, under a photosynthetically active radiation (PAR) intensity of 1200–1400 µmol photons m^-2s^-1. Fruits were harvested from each experimental unit throughout the growth period. After the final harvest, the total number of fruits and their fresh and dry weights per m^-2 were recorded and used to calculate the final fruit number and yield. At the end of the experiment, plant height, the number of primary and secondary branches, and total biomass yield were recorded from ten representative plants, and their mean values were included in the statistical analyses. Data analysis was conducted using SAS (ver. 9.1) software, and mean comparisons were performed using the LSD method.
3. Results and Discussion: Both seed priming and foliar application with SW significantly enhanced physiological and yield traits compared with the control. Seed priming with SW at a concentration of 1% increased photosynthetic rate (15.82%), transpiration (13.69%), stomatal conductance (51.54%), mesophyll conductance (15.62%), plant height (19.73%), and fresh and dry fruit yield (32.90% and 30.58%, respectively). Foliar application with SW at a concentration of 1% led to improvement photosynthesis rate (19.62%), transpiration rate (20%), stomatal conductance (48.97%), plant height (16.07%), number of branches (34.24% primary, 46.12% secondary), number of fruits (39.31%), fresh and dry fruit yield (28.47% and 33.76%) and dry/fresh weight ratio (4%). The treatment of SW at 1% priming + foliar spray resulted in an increase of 164% in biomass yield over the control. Harvest index was reduced by 7-10% under 1% SW treatments. The overall increase in the gas exchange parameters, photosynthesis efficiency, and accumulation of biomass validates the great potential of SW as a biostimulant. These results confirm earlier reports of karrikin-mediated improvements in photosynthetic activity and growth, while providing new evidence that foliar application may be more effective than seed priming in directing assimilate partitioning toward fruit dry matter.
4. Conclusion: Utilization of SW, particularly at 1% concentration, significantly improved gas exchange traits, morphological attributes, and yield of chili pepper. Foliar spraying was more effective than seed priming in increasing fruit biomass accumulation, reflecting greater assimilate allocation to fruits. Although a decrease in harvest index was observed, the overall improvement in yield quantity and quality suggests SW as a promising, eco-friendly, and cost-effective biostimulant for the sustainable growth of chili pepper.