Foliar brassinosteroid analogue (DI-31) sprays increase drought tolerance by improving plant growth and photosynthetic efficiency in lulo plants

Abstract

The use of agronomic alternatives such as plant hormone sprays has been considered a tool to mitigate drought stress. This research aimed to evaluate the use of foliar brassinosteroid analogue DI-31 (BRs) sprays on plant growth, leaf exchange and chlorophyll a fluorescence parameters, and biochemical variables in lulo (Solanum quitoense L. cv. septentrionale) seedlings grown under drought stress conditions. Seedlings were grown in plastic pots (3 L) using a mix between peat and sand (1:1 v/v) as substrate. Lulo plants were subjected to drought stress by suppressing 100% of the water needs at 30-37 and 73-80 days after transplanting (DAT). Foliar BRs analogue (DI-31) sprays were carried out at four different rates (0, 1, 2, 4, or 8 mL of analogue per liter) at different times (30, 33, 44, 60, 73, and 76 DAT). Drought stress caused a reduction in the F_v/F_m ratio, leaf gas exchange properties, total biomass, and relative water content. Foliar DI-31 sprays enhanced leaf photosynthesis in well-watered (WW) (∼10.7 μmol m^-2 s^-1) or water-stressed plants (WS) (∼6.1 μmol m^-2 s^-1) when lulo plants were treated at a dose of 4 and 8 mL·L^-1 compared to their respective controls (0 mL·L^-1 for WW: 8.83 μmol m^-2 s^-1 and WS: 2.01 μmol m^-2 s^-1). Also, DI-31 sprays enhanced the photochemical efficiency of PSII, and plant growth. They also increased the concentration of photosynthetic pigments (TChl and Cx + c) and reduced lipid peroxidation of membranes (MDA) under drought conditions. The results allow us to suggest that the use of DI-31 at a dose of 4 or 8 mL·L^-1 can be a tool for managing water stress conditions caused by low water availability in the soil in lulo-producing areas to face situations of moderate water deficit at different times of the year.

Keywords: Andean fruit species; Drought stress; Foliar spray; Leaf photosynthesis; Malondialdehyde; Plant hormones.

Figure 1

Relative water content (RWC) in well-irrigated (light grey bars) and drought-stressed (dark gray bars) lulo (Solanum quitoense Lam.) plants sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Each column represents the mean of five data ± standard error (n = 5). Bars followed by different letters indicate statistically significant differences according to the Tukey test (p ≤ 0.05).

Figure 2

Plant height (A), total dry weight (TDW) (B) and dry matter (DM) partitioning (C) in well-irrigated (light grey bars) and drought-stressed (dark gray bars) lulo (Solanum quitoense Lam.) plants sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Each bar chart summarizes the mean of five data ± standard error (n = 5). Bars followed by different letters indicate statistically significant differences according to the Tukey test (p ≤ 0.05).

Figure 3

Net photosynthetic rate (P_N) (A), stomatal conductance (g_s) (B), transpiration rate (E) (C) and intrinsic water-use efficiency (WUE_i) (D) in well-irrigated (light grey bars) and drought-stressed (dark gray bars) lulo (Solanum quitoense Lam.) plants sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Each bar chart summarizes the mean of five data ± standard error (n = 5). Bars followed by different letters indicate statistically significant differences according to the Tukey test (p ≤ 0.05).

Figure 4

Maximum quantum efficiency of PSII (F_v/F_m) (A), photochemical quenching (qP) (B) and non-photochemical quenching (NPQ) (C) in well-irrigated (light grey bars) and drought-stressed (dark gray bars) lulo (Solanum quitoense Lam.) plants sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Each bar chart summarizes the mean of five data ± standard error (n = 5). Bars followed by different letters indicate statistically significant differences according to the Tukey test (p ≤ 0.05).

Figure 5

Total chlorophyll content (TChl) (A), carotenoids (Cx + c) (B) and malondialdehyde production (MDA) (C) in well-irrigated (light grey bars) and drought-stressed (dark gray bars) lulo (Solanum quitoense Lam.) plants sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Each bar chart summarizes the mean of five data ± standard error (n = 5). Bars followed by different letters indicate statistically significant differences according to the Tukey test (p ≤ 0.05).

Figure 6

Correlation between the total dry weight (TDW) (A) or malondialdehyde production (MDA) (B) and the relative tolerance index (RTI) (C), and the decrease in the maximum quantum efficiency of PSII (DQE) (D) in lulo (Solanum quitoense Lam.) plants exposed to drought and sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Bars and points represent the mean of five data ± standard error (n = 5). Bars followed by different letters indicate statistically significant differences according to the Tukey test (p ≤ 0.05).

Figure 7

Three-dimensional plot (total dry weight (TDW), decrease in the maximum efficiency of PSII (DQE) and relative tolerance index (RTI)) for lulo (Solanum quitoense Lam.) plants exposed to drought and sprayed with 1, 2, 4 and 8 mL·L⁻¹ of brassinosteroid analogue (DI-31) at 80 days after transplanting (DAT). Data represents the mean of five data ± standard error (n = 5).

Figure 8

Concept model of the impact of the drought stress and foliar brassinosteroid analogue (DI-31) sprays in lulo plants. Red arrows and blue arrows indicate the negative or positive effect of interaction between drought stress and foliar DI-31 applications on the physiological and biochemical responses, respectively. g_s: stomatal conductance; P_N: net photosynthesis rate; E: transpiration; F_v/F_m: maximum quantum efficiency of PSII; qP: photochemical quenching; NPQ: non-photochemical quenching.