Interactive Effects of Ascophyllum nodosum Seaweed Extract and Silicon on Growth, Fruit Yield and Quality, and Water Productivity of Tomato under Water Stress

Agricultural crops including tomato (Solanum lycopersicum L.) are severely affected by drought, which is a critical abiotic stress. Biostimulants, such as Ascophyllum nodosum seaweed extract (ASE), and silicon (Si) are independently used in alleviating drought stress and in enhancing growth and productivity of agronomic and horticultural crops. The present study was conducted to assess the combined effects of ASE and Si on growth, fruit yield, fruit quality, and water productivity of tomato under water stress. Five doses of ASE (0, 1.25, 2.5, 3.75, and 5 mL L⁻¹) were applied in combination with 60 kg ha⁻¹ soluble Si in the form of monosilicic acid (as soil incorporation regardless of ASE doses) along with a control (where no ASE or Si was applied) under three soil moisture regimes of 50%, 75%, and 100% field capacity (FC). Data on growth, fruit yield, water productivity, fruit quality, and physio-biochemical parameters of tomato were collected. The results revealed that severe water stress of 50% FC negatively affected growth, physiological traits, and fruit yield of tomato (43–80% lower yield across ASE doses) compared with those at 100% FC, whereas fruit quality parameters (total soluble solids, fruit firmness, color index, and fruit pH) increased with reduced soil moisture regime. Application of ASE at 3.75 and 5 mL L⁻¹ in combination with soluble Si at 60 kg ha⁻¹ resulted in statistically similar fruit yields under a sufficient soil moisture level of 100% FC and a moderate soil moisture level of 75% FC, respectively. A consistent trend of higher fruit yield and water productivity was observed for plants supplemented with 5 mL L⁻¹ ASE and 60 kg ha⁻¹ soluble Si regardless of soil moisture regimes. Similarly, individual Si supplementation at 60 kg ha⁻¹ was also effective and caused 207% increase in fruit yield even at severe water stress of 50% FC compared with the control. However, a combined application of ASE and Si had more promising results than the sole application of Si. Exogenous soil application of ASE at 5 mL L⁻¹ along with soluble Si at 60 kg ha⁻¹ holds promise for tomato production under moderate to sufficient soil moisture availability.