Combined effects of biostimulants, N level and drought stress on yield, quality and physiology of greenhouse-grown basil

Precise nitrogen (N) supply is an agronomic practice of crucial importance to achieve optimal crop performance without compromising product quality. However, excessive use of synthetic N fertilizers may have deleterious effects on both agroecosystem and human health. Thus, the development and use of strategies aiming to ameliorate the losses caused by water constraints and N deficiency are essential for fostering resilient and sustainable agroecosystems. In this regard, the impact of three drought stress levels (DS) [100%, 80% and 60% of the field capacity (FC)] in combination with four N supply rates (0, 50, 100 and 150 kg ha(-1)) on sweet basil cultivated in a protected environment was investigated. The interactive biostimulatory action of Kelpstar (R) seaweed extract (SWE) and Tyson (R) protein hydrolysate (PH) was also explored. The study focused on the effects of these treatments on yield, physiological attributes, functional traits, and volatile compounds profile. Drought stress led to a reduction in yield by 12.5% and 21.1% under irrigation at 80% and 60% FC, respectively, compared to well-watered plots (100% FC). Furthermore, drought stress levels linearly decreased total leaf area (-15.4% and-26.2% for DS80 and DS60, respectively), stomatal conductance (-14.2% and 34.1% for DS80 and DS60, respectively), nitrogen use efficiency (NUE) (4.0% and 10.0% for DS80 and DS60, respectively), and volatile compounds, such as trans-2-hexanal, 1-octen-3-ol and alpha-bergamotene. Conversely, an increase in N application rate positively influenced yield (8.6% and 12.2% for N100 and N150, respectively), total leaf area (22.2% and 16.5% for N100 and N150, respectively), specific leaf area (SLA), total chlorophyll (7.7% for N150), nitrate content, and the presence of specific volatile compounds, such as 1-octen-3-ol and alpha-bergamotene, when compared to no N application. Seaweed extract application caused an upsurge in yield (+17.5%), stomatal conductance (+25.8%), WP (+13.5%), total chlorophyll (+2.3%), nitrate (+3.4%), phenolics (+14.2), ascorbic acid (+28.2), as well as, 1-octen-3-ol, beta-cis-ocimene, linalool and eugenol, compared to the control. Similarly, plant protein hydrolysate increased yield (+16.1), stomatal conductance (+10.4), WP (+13.7), total chlorophyll (+4.3), phenolics (+10.7%), ascorbic acid (+9.7%), beta-cis-ocimene and eugenol, compared to the control. Notably, the increased yield, improved quality, and enchanced physiological traits observed after biostimulant application, especially under drought stress or N deficiency conditions, underscore the potential role of bio-stimulants in increasing resilience of basil plants. Thus, the foliar application of SWE and PH offer a valuable strategy for enhancing plant yield and quality under sub-optimal conditions, while simultaneously enhancing water and N use efficiency.