Using silicon-based liquid fertiliser to improve growth, yield and quality of beetroot (Beta vulgaris L.) grown under combined phosphorus and water stress in Mpumalanga

Abstract

Beetroot (Beta vulgaris L.) is a vital vegetable crop with significant health benefits due to its rich nutritional profile and bioactive compounds. However, its production, like any other crop, is threatened by the abiotic multi-stress conditions involving soil nutrient deficiencies, water deficit and acidity. In recent years, the silicon-based liquid fertilizer (SB) has emerged as a promising tool to enhance crop resilience to single abiotic stress; however, there is a dearth of information on the effects of SB application on crop performance under combined abiotic stresses. Hence, this study aimed to assess the (i) beetroot growth, yield and quality, and (ii) soil quality parameters in response to SB application and combined phosphorus (P) and water (W) deficit levels at crop harvest. To achieve the above-mentioned objectives, a 90-day experiment was conducted in the net house at the Nooitgedacht Research Station in Mpumalanga, South Africa, during the 2023/2024 and 2024/2025 cropping seasons. The 4 × 6 factorial design experiment was arranged in a completely randomised block design with three replications. The treatments consisted of four SBs with different concentrations [0 - distilled water as a control (SB0), 0.2 (SB0.2), 0.4 (SB0.4), and 0.6 (SB0.6) L/ha], a factorial combination of two P deficit levels [20 kg/ha (P20) and 40 kg/ha (P40)]), and three W deficit levels [25% (W25), 50% (W50), and 100% (W100) of evapotranspiration]: P20W25, P40W25, P20W50, P40W50, P20W100, and P40W100. The (W) stress and foliar SB application began at 14 and 28 days after transplanting (DAT), respectively. Plant height, number of leaves per plant, the leaf area, and leaf chlorophyll content were determined at 63 DAT, while stomatal conductance was measured at 70 DAT. The beetroot was harvested at 90 DAT to determine fresh leaf mass, dry leaf mass, fresh root mass, dry root mass, total fresh biomass, total dry biomass, root diameter, and beetroot quality parameters. The Shapiro-Wilks test and homogeneity of variance were performed on standardised residuals to test for any deviations from normality and homoscedasticity, respectively, to measure the equality of variances between seasons. Heteroscedasticity between the two growing periods was indicated by the results, which showed a lack of variance homogeneity (P<0.05). The P20W25 significantly decreased the chlorophyll content by up to 38%, compared to P40W100. The SB0.2 application significantly increased the stomatal conductance by 21%, 30%, and 26% compared to SB0, SB0.4, and SB0.6, respectively, at 70 DAT. Compared to SB0×P20W25, SB0.2×P40W100 increased stomatal conductance by 153% in season 1 and 60% in season 2. Plant height, number of leaves per plant, leaf area, and root quality parameters were not affected by the SB application and SB×PW. Leaf P content was significantly reduced by 120% for the P20W25 compared to P20W100. The application of SB0.2, SB0.4 and SB0.6 significantly reduced leaf Cu content by 32%, 39%, and 78%, respectively, relative to SB0. Fresh root mass, dry root mass, total fresh biomass, and total dry biomass varied significantly with combined P and W deficit levels, season and the interaction of SB, combined P and W deficit levels and season. The SB0.4×P20W100 led to a substantial increase in fresh root mass (97%) and dry root mass (86%) compared to SB0×P20W25 in season 2. Fresh root mass, total fresh biomass, and total dry biomass recorded from P20W25 were significantly reduced by 46%, 28%, and 26%, respectively, compared to P40W100 in season 2. All yield parameters in season 2 were quantitatively higher than in season 1, with SB0.4×P20W100 recording the highest values across all yield parameters. Compared to P20W100, P20W25 significantly reduced root P content by 50%, indicating that the extreme stress limits P uptake. The application of SB0.2, SB0.4, and SB0.6 significantly reduced root Ca content relative to the control. The SB0.4×P20W25 significantly reduced the root Na content compared to SB0×P40W25. This may be associated with osmotic adjustment and ionic balance regulation. Compared to control, SB0.4 and SB0.6 reduced Co content by up to 41% in season 1. The P40W100 treatment significantly reduced B content by 82% compared to the P40W25 treatment in season 1, while no significant effect was observed in season 2. The principal component analysis results showed that, in conclusion, the beetroot Ca, Mg, K, Na, P, Fe, Zn, and Mn content were strongly and positively associated with SB0.4×P20W50 in season 1. Season 2 produced higher crop yield, whereas season 1 exhibited higher root P, K, Ca, Mg, Na, B, Co, Cu, Fe, Mn, and Zn content, suggesting a biomass dilution effect, in which increased yield in season 2 led to lower nutrient content due to the distribution of nutrients across greater biomass. The effect of the SB on soil Ca, Na, K, Mg, and pH (KCl) was not significant.