Drought Stress Affects the Response of Italian Local Tomato (Solanum lycopersicum L.) Varieties in a Genotype-Dependent Manner

Abstract

Drought stress is one of the most severe conditions for plants, especially in the face of the emerging problem of global warming. This issue is important when considering economically relevant crops, including the tomato. For these plants, a promising solution is the valorization of local agrobiodiversity as a source of genetic variability. In this paper we investigated how six Italian tomato varieties react to a prolonged period of water depletion. We used a multidisciplinary approach, from genetics to plant physiology and cytology, to provide a detailed overview of the response of plants to stress. The varieties analyzed, each characterized by a specific genetic profile, showed a genotype-specific response with the variety 'Fragola' being the most resistant and the variety 'Pisanello' the most susceptible. For all the parameters evaluated, 'Fragola' performed in a manner comparable to that of control plants. On the contrary, 'Pisanello' appeared to be more affected and showed an increase in the number of stomata and a drastic increase in antioxidants, a symptom of acute oxidative stress. Our work suggests the existence of a valuable reservoir of genetic biodiversity with more drought-tolerant tomato genotypes opening the way to further exploitation and use of local germplasm in breeding programs.

Keywords: Italian varieties; drought stress; oxidative stress; physiological response; tomato.

Figure 1

Hierarchical clustering (UPGMA algorithm) of six local varieties and one commercial cultivar (‘Supersteak’).

Figure 2

The maximum quantum yield of photosystem II (F_v/F_m) in six local varieties (A–F) and one commercial cultivar ‘Supersteak’ (G) of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05).

Figure 3

Performance index (PI_ABS) in six local varieties (A–F) and one commercial cultivar ‘Supersteak’ (G) of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05).

Figure 4

Growth Index (GI) in six local varieties (A–F) and one commercial cultivar ‘Supersteak’ (G) of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05). GI (_1,0) = (h₁ − h₀)/2; GI (_2,0) = (h₂ − h₀)/2.

Figure 5

Stem diameter in six local varieties (A–F) and one commercial cultivar ‘Supersteak’ (G) of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05).

Figure 6

Calcofluor White (CFW) staining in leaves of two local varieties (A,B—‘Fragola’; C,D—‘Pisanello’) and one commercial cultivar (E,F—‘Supersteak’). A,C,E are the controls (regularly well-watered plants) and B,D,F are the stressed (no water for 16 days). PP, palisade parenchyma; SP, spongy parenchyma. Scale bar: 100 µm.

Figure 7

Leaf Area in two local varieties (‘Fragola’ and ‘Pisanello’) and one commercial cultivar ‘Supersteak’ of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05). The p-values for ‘Pisanello’ and ‘Supersteak’ are near to significance, respectively p = 0.07 and p = 0.08. t₀ = before the start of treatment, t₁ = half duration of the stress, t₂ = the end of the stress.

Figure 8

(A) LaL (Lamina Length) and (B) LaW (Lamina Width) in two local varieties (‘Fragola’ and ‘Pisanello’) and one commercial cultivar ‘Supersteak’ of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two- way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05). t₀ = before the start of treatment, t₁ = half duration of the stress, t₂ = the end of the stress.

Figure 9

Stomatal density in two local varieties (‘Fragola’ and ‘Pisanello’) and one commercial cultivar ‘Supersteak’ of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05).

Figure 10

Nitro Blue Tetrazolium (NBT) staining in leaves of two local varieties (A—’Fragola’; B—’Pisanello’) and one commercial cultivar (C—’Supersteak’). On the left are the controls (regularly well-watered plants) and on the right are the stressed samples (no water for 16 days). The side of the square in the background is 0.5 mm.

Figure 11

(A) total antioxidant content, (B) polyphenols content and (C) flavonoids content of two local varieties (‘Fragola’ and ‘Pisanello’) and one commercial cultivar (‘Supersteak’) of tomato (Solanum lycopersicum L.) subjected to drought stress (no water for 16 days) relative to the controls (regularly well-watered plants). Two-way factorial ANOVA followed by Tukey post-hoc test was used to determine the statistical significance of differences. Points with the same lower-case letters do not differ significantly (p > 0.05).