Water-Deficit Tolerance in Sweet Potato [ Ipomoea batatas (L.) Lam.] by Foliar Application of Paclobutrazol: Role of Soluble Sugar and Free Proline

Abstract

The objective of this study was to elevate water deficit tolerance by improving soluble sugar and free proline accumulation, photosynthetic pigment stabilization, photosynthetic abilities, growth performance and storage root yield in sweet potato cv. 'Tainung 57' using a foliar application of paclobutrazol (PBZ). The experiment followed a Completely Randomized Block Design with four concentrations of PBZ: 0 (control), 17, 34, and 51 μM before exposure to 47.5% (well irrigation), 32.3% (mild water deficit) or 17.5% (severe water deficit) soil water content. A sweet potato cultivar, 'Japanese Yellow', with water deficit tolerance attributes was the positive check in this study. Total soluble sugar content (sucrose, glucose, and fructose) increased by 3.96-folds in 'Tainung 57' plants treated with 34 μM PBZ grown under 32.3% soil water content (SWC) compared to the untreated plants, adjusting osmotic potential in the leaves and controlling stomatal closure (represented by stomatal conductance and transpiration rate). In addition, under the same treatment, free proline content (2.15 μmol g^-1 FW) increased by 3.84-folds when exposed to 17.5% SWC. PBZ had an improved effect on leaf size, vine length, photosynthetic pigment stability, chlorophyll fluorescence, and net photosynthetic rate; hence, delaying wilting symptoms and maintaining storage root yield (26.93 g plant^-1) at the harvesting stage. A positive relationship between photon yield of PSII (Φ_PSII) and net photosynthetic rate was demonstrated (r² = 0.73). The study concludes that soluble sugar and free proline enrichment in PBZ-pretreated plants may play a critical role as major osmoprotectant to control leaf osmotic potential and stomatal closure when plants were subjected to low soil water content, therefore, maintaining the physiological and morphological characters as well as storage root yield.

Keywords: paclobutrazol; pigment; proline; sweet potato; total soluble sugar; water deficit.

FIGURE 1

Storage root characteristics of sweet potato pretreated with paclobutrazol (PBZ) grown under well watering [WW; 47.5% soil water content (SWC)] or severe water deficit (SWD) (17.5% SWC; 14 days water withholding) subsequently recovery prior to storage root harvesting process (150 days after planting).

FIGURE 2

Free proline and total soluble sugar in leaf tissues of sweet potato pretreated with PBZ subsequently subjected to mild water deficit (32.3% SWC; 7 days water withholding) (A,C) or SWD (17.5% SWC; 14 days water withholding) (B,D). Different letters in each bar show significant difference at p ≤ 0.01 by Tukey’s HSD. Data presented as mean ± SE.

FIGURE 3

Leaf osmotic potential (Ψ_s) and stomatal conductance (g_s) of sweet potato pretreated with PBZ subsequently subjected to mild water deficit (32.3% SWC; 7 days water withholding) (A,C) or SWD (17.5% SWC; 14 days water withholding) (B,D). Different letters in each bar show significant difference at p ≤ 0.01 by Tukey’s HSD. Data presented as mean ± SE.

FIGURE 4

Maximum quantum yield of PSII (F_v/F_m) and proton yield of PSII (Φ_PSII) of sweet potato pretreated with PBZ subsequently subjected to mild water deficit (32.3% SWC; 7 days water withholding) (A,C) or SWD (17.5% SWC; 14 days water withholding) (B,D). Different letters in each bar show significant difference at p ≤ 0.01 by Tukey’s HSD. Data presented as mean ± SE.

FIGURE 5

Net photosynthetic rate (P_n) and transpiration rate (E) of sweet potato pretreated with PBZ subsequently subjected to mild water deficit (32.3% SWC; 7 days water withholding) (A,C) or SWD (17.5% SWC; 14 days water withholding) (B,D). Different letters in each bar show significant difference at p ≤ 0.01 by Tukey’s HSD. Data presented as mean ± SE.

FIGURE 6

Relationship between chlorophyll a content and F_v/F_m (A), total chlorophyll content and Φ_PSII (B), Φ_PSII and net photosynthetic rate (P_n) (C), and P_n reduction and plant dry weight reduction of sweet potato pretreated with PBZ subsequently subjected to water deficit conditions (D). Data presented as mean ± SE.