Abstract
Controlled abiotic stress, or "eustress," can be leveraged to improve the quality of horticultural crops. This study investigates the impact of intermittent salinity applications on the sugar content of 'Albion' strawberries grown in two hydroponic systems: a passive Kratky method and a substrate-based system. Results indicate that alternating freshwater with 77 mM saltwater every 72 hours in a substrate-based system significantly increases Total Soluble Solids (TSS) to 13.28 °Brix—a 24% improvement over control—without compromising yield or fruit morphology.
Introduction
In high-value strawberry production, sweetness—quantified as Total Soluble Solids (TSS) or °Brix—is a primary determinant of market quality. While salinity is traditionally viewed as a detrimental stressor that reduces plant growth and yield, controlled exposure to salt can trigger physiological defense mechanisms. These mechanisms often involve the accumulation of sugars and antioxidants to maintain osmotic balance. This study explores "intermittent eustress" as a strategy to boost strawberry quality in hydroponic environments while avoiding the toxic effects of continuous high salinity.
Methodology
The research compared two distinct hydroponic techniques using the 'Albion' cultivar:
- Kratky Method: A passive, non-circulating system where roots are suspended directly in a nutrient solution.
- Substrate-Based System: Plants grown in a 70:30 mixture of coco coir and perlite, irrigated via pots.
Researchers applied salinity treatments using "Instant Ocean" salt at two concentrations: 43 mM (Electrical Conductivity ~5.9 mS/cm) and 77 mM (EC ~9.0 mS/cm). These were applied in intermittent cycles of 24 hours or 72 hours, alternating with freshwater cycles to allow for plant recovery.
Environmental parameters were monitored using a custom Arduino-based logger. Key performance metrics included TSS (°Brix), individual fruit weight, total yield per plant, and the percentage of deformed fruits (marketability).
Results and Discussion
Enhancing Sweetness through Intermittent Stress
The application of intermittent salt stress successfully increased TSS across several treatments. The most effective protocol was the 77 mM salt application on a 72-hour cycle within the substrate-based system, which achieved a TSS of 13.28 °Brix. This represents a 24% increase compared to the 10.72 °Brix observed in the control group.
System Performance: Substrate vs. Kratky
A critical finding was the superior performance of the substrate-based system. The coco coir and perlite mixture provided a physical and chemical buffer that likely moderated the osmotic shock to the root system. In contrast, plants in the Kratky system were more sensitive to salinity; while they showed increased TSS, they also suffered from significantly higher rates of fruit deformity. Specifically, the 77 mM/72h treatment in the Kratky system resulted in over 10% deformed fruit, whereas the substrate-based system maintained deformity rates (5.08–7.16%) comparable to the non-stressed control.
Yield Stability
Importantly, the intermittent nature of the stress allowed plants to maintain productivity. No significant reduction in total yield or individual fruit weight was observed in either system under the tested salt concentrations. This suggests that the "off-periods" (freshwater cycles) provide sufficient recovery time for the plants to maintain biomass production while shifting their metabolism toward sugar accumulation during the "on-periods."
Conclusion
Intermittent salinity application is a viable technique for enhancing the flavor profile of hydroponic strawberries without sacrificing yield. For commercial growers, substrate-based systems are recommended over passive liquid systems like Kratky when applying salt stress, as the substrate protects fruit quality and plant health. Future research should investigate the long-term effects of salt accumulation in substrates and the specific sugar profiles (glucose, fructose, and sucrose) influenced by these treatments.