Effects of Plant Biostimulation Time Span and Soil Electrical Conductivity on Greenhouse Tomato 'Miniplum' Yield and Quality in Diverse Crop Seasons

Abstract

Biostimulants help plants cope with environmental stresses and improve vegetable yield and quality. This study was conducted to determine the protein hydrolysate (PH) effect of three different durations (weekly applications: three, six, or nine times plus an untreated control) in factorial combination with four soil electrical conductivities (EC: 1.5, 3.0, 4.5, or 6.0 mS·cm^-1) on yield, fruit quality, and elemental composition of tomato 'miniplum' grown in a greenhouse. Fruit yield was best affected, during the summer, by six and nine biostimulant applications at EC 3.0 mS·cm^-1, and in the same season, the six treatments led to the highest fruit number with no difference compared to nine applications; during the winter, the three and six treatments improved the mentioned variables at each EC level. Fruits' dry residue and Brix^o were positively affected by biostimulation both in summer and winter. In summer, the 6.0 mS·cm^-1 EC led to the highest dry residue and Brix^o values, though the latter did not show any significant difference compared to 4.5 mS·cm^-1; in winter, the best results corresponded to 4.5 and 6.0 mS·cm^-1. A higher beneficial effect of PH on fruit antioxidant status, i.e., lycopene, polyphenols, ascorbic acid levels, and lipophilic (LAA) and hydrophilic (HAA) activity, was recorded in winter compared with summer. Positive correlations between polyphenols and LAA, as well as ascorbic acid content and HAA were found for all EC and PH treatments. Most of the mineral elements tested demonstrated concentration stability, whereas the highest EC decreased P, Mg, Cu, and Se accumulation. The opposite effect was shown by PH application on Se and Mn levels, with P tending to increase. The concentrations of Fe, Zn, and Cu were the lowest under the longest duration of PH supply. These results further confirm the essential role of plant biostimulation in enhancing tomato yield and quality, with a particular focus on the treatment duration.

Keywords: Solanum lycopersicum L.; antioxidant activity; ascorbic acid; firmness; greenhouse; lycopene; mineral composition; phenolics; protein hydrolysate.

Figure 1

Yield (a) and number of fruits per m² (b) of tomato (Proxy F₁) affected by the interaction between soil EC and the number of biostimulant applications during the spring–summer season. Within each parameter, the values followed by different letters are statistically different according to Duncan’s multiple range test at p ≤ 0.05.

Figure 2

Protein hydrolysate (PH)’s effect on changes of the ascorbic acid, polyphenols, lycopene content, and LAA and HAA activities in summer and winter tomato fruits. For each parameter, values with different letters differ statistically according to Duncan test at p < 0.05.

Figure 3

Correlation between (a) HAA and ascorbic acid content (r = 0.870, p < 0.001) and (b) LAA and polyhenols (r = 0.800, p < 0.001) in tomato fruits.