Effect of a natural bio-stimulant on the photosynthetic capacity and yield of spinach under drought stress
Liatile, Pule Clement
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Spinach (Spinacia oleracea L.) is a green leafy vegetable that is cultivated worldwide due to its high nutritional value, containing many vitamins and minerals. Spinach is susceptible to drought stress and its cultivation and production is limited by low water availability in arid and semi-arid areas. The potential of a natural bio-stimulant (Xcell Boost) to improve the physiological and biochemical responses as well as vegetative growth in spinach grown under different water stress regimes was evaluated under controlled conditions in the greenhouse. The trial design was a split-plot, where the main plot was the water treatments and subplot were Xcell Boost treatments. Three water levels were maintained in the soil: 100% (full irrigation), 50% (mild drought stress) and 30% (severe drought stress) water holding capacity (WHC). Three levels of Xcell Boost treatments were used: namely the control (no bio-stimulant, BX0), single (BX1) and double (BX2) concentration of bio-stimulant. Non-invasive and invasive techniques were used to assess the photosynthetic, biochemical, and vegetative growth parameters. Drought stress showed insignificant effect on the normalised difference vegetation index (NDVI) under optimal water treatments. Under drought stress, NDVI values were maintained above 0.68 units, indicating that plants were green and healthy. Even so, the application of BX1 further increased the NDVI values under water deficiency. Because NDVI is a measure of vegetation “greenness”, it can be associated with potentially high chlorophyll content. This was supported by an increase in Chlorophyll a content under drought conditions. Application of BX2 increased the levels of Chlorophyll a, b, and carotenoids under drought stress. Drought stress slightly reduced the stomatal conductance under severe water stress. Application of Xcell Boost improved the stomatal conductance across all water regimes with BX1 showing the highest increase in stomatal conductance, particularly under severe water deficiency stress, which could have resulted in a high carbon dioxide fixation and increased photosynthetic rates. The application of BX1 enhanced the accumulation of osmoprotectants (proline and total soluble sugars (TSS) and reduced electrolyte leakage under mild stress (50% WHC) but failed to boost the levels of TSS under severe drought stress (30% WHC). Positive correlations between proline and performance index absorbance (PIABS) shows that high proline level is linked to better photosynthetic capacity of spinach. Xcell Boost (BX2) induced substantial increases in the activities of the reactive oxygen species (ROS) scavenging enzymes, ascorbate peroxidase (APX) and guaiacol peroxidase (GPX), under severe drought stress. BX1 was only effective at inducing these enzymes under mild drought stress. Xcell Boost applications at both concentrations reversed the negative effects of drought stress by inducing remarkable increases in plant height, leaf area, stem dry weight, root length and root moisture. This indicates that Xcell Boost application enhanced spinach resilience under drought stress. Stem dry weight also had a positive correlation with APX and stomatal conductance. Also, there was a significant positive correlation between total performance index (PITotal) and root moisture content, this relationship could ensure that enough water is available for transport from the roots to the leaves to participate in the photosynthesis process. Based on the results, applying Xcell Boost enhances several physiological, biochemical, and vegetative parameters in spinach during drought stress. This makes Xcell Boost a highly beneficial bio-stimulant for improving yield in spinach, and most probably for other crops, cultivated in semi-arid and arid regions.