Drought is one of the most relevant abiotic stresses that currently affect horticultural species, and it is mostly due to climate change. The negative effects of water shortage are mostly significant for high-water-demanding crops, such as tomato (Solanum lycopersicum L.), and they can occur as morphological, physiological, and biochemical alterations. Nowadays, biostimulants are considered a emerging key strategy for enhancing plant productivity and resilience to abiotic stresses, including drought.
The main goal of this work is to evaluate the effects of new foliar biostimulants on the physiological and biochemical properties of tomatoes (leaves and fruits) grown under water stress conditions in an open field.
Tomato plants (cv. ‘Heinz 1301’) were arranged on three experimental rows in open field conditions, each of which was subjected to three different irrigation (I) regimes: 100% (I-100), 60% (I-60), and 30% (I-30) of IRRIFRAME recommendations. The biostimulants were distributed by spraying directly onto the leaves: non-fermented kiwi residual biomasses (T2), kiwi residual biomasses fermented with Lactiplantibacillus plantarum 4193 (T3), and kiwi residual biomasses fermented with Companilactobacillus farciminis 4841 (T4). Control plants (T1) were sprayed with water only. Plant health was assessed after fruit colour change (R1, five weeks from the start of stress) and fruit ripening (R2, eight weeks from the start of stress), measuring leaf colour and SPAD index. At both R1 and R2, leaf samples were collected and analysed for their chlorophylls, polyphenols, total antioxidant content (DPPH assay), and proline content. Fruits were harvested when fully ripe, characterized by size and analysed biochemically (polyphenol, lycopene, and soluble sugar content; titratable acidity; and antioxidant activity).
Severe water stress (I-30) caused an increase in leaf SPAD index (R1), a reduction in a* (redness) colour coordinates (R1 and R2), and a decrease in the total chlorophyll content (R1 and R2). This latter negative effect was reduced by all biostimulants. Leaves counteract drought stress by increasing their total polyphenols (R2) and proline content (R1) (only I-30), and no positive effect was detected for any treatments.
The reduction in fruit weight and size was observed under severe drought conditions. In I-60, an increase in the lycopene content was detected, along with a decrease in the percentage of titratable acidity and the fructose content. The use of the T3 biostimulant had a positive effect on the fruit size both in I-100 and I-60, also showing the highest content of polyphenols in I-30, in comparison with the other treatments.
