Use of Non-Destructive Measurements to Identify Cucurbit Species (Cucurbita maxima and Cucurbita moschata) Tolerant to Waterlogged Conditions

Abstract

Limited information is available regarding the physiology of squash plants grown under waterlogging stress. The objectives of this study were to investigate the growth and physiological performances of three cucurbit species, Cucurbita maxima cultivar (cv.) OK-101 (OK) and Cucurbita moschata cv. Early Price (EP) and Strong Man (SM), in response to waterlogging conditions, and to develop a precise, integrated, and quantitative non-destructive measurement of squash genotypes under stress. All tested plants were grown in a growth chamber under optimal irrigation and growth conditions for a month, and the pot plants were then subjected to non-waterlogging (control) and waterlogging treatments for periods of 1, 3, 7, and 13 days (d), followed by a 3-d post-waterlogging recovery period after water drainage. Plants with phenotypes, such as fresh weight (FW), dry weight (DW), and dry matter (DM) of shoots and roots, and various physiological systems, including relative water content (RWC), soil and plant analysis development (SPAD) chlorophyll meter, ratio of variable/maximal fluorescence (Fv/Fm), quantum photosynthetic yield (YII), normalized difference vegetation index (NDVI), and photochemical reflectance index (PRI) values, responded differently to waterlogging stress in accordance with the duration of the stress period and subsequent recovery period. When plants were treated with stress for 13 d, all plants exhibited harmful effects to their leaves compared with the control, but EP squash grew better than SM and OK squashes and exhibited stronger tolerance to waterlogging and showed less injury. Changes in the fresh weight, dry weight, and dry matter of shoots and roots indicated that OK plants suffered more severely than EP plants at the 3-d drainage period. The values of RWC, SPAD, Fv/Fm, YII, NDVI, and PRI in both SM and OK plants remarkably decreased at waterlogging at the 13-d time point compared with controls under identical time periods. However, the increased levels of SPAD, Fv/Fm, YII, NDVI, and PRI observed on 7 d or 13 d of waterlogging afforded the EP plant leaf with improved waterlogged tolerance. Significant and positive correlations were observed among NDVI and PRI with SPAD, Fv/Fm, and YII, indicating that these photosynthetic indices can be useful for developing non-destructive estimations of chlorophyll content in squashes when screening for waterlogging-tolerant plants, for establishing development practices for their cultivation in fields, and for enhanced cultivation during waterlogging in frequently flooded areas.

Keywords: chlorophyll fluorescence; cucurbit; spectral reflectance; squash; waterlogging tolerance.

Figure 1

Appearance of EP, SM, and OK squash plants grown under optimally irrigated (control, C) or waterlogging (W) conditions for 0 (non-waterlogging), 3, 7, and 13 d periods. Bar indicates 5 cm.

Figure 2

Effects of waterlogging on (A) relative water content (RWC, %) and (B) electrolyte leakage (EL, %) in EP, SM, and OK plants. Plants were grown under optimally irrigated (control, C, black bars) or waterlogging (W, white bars) conditions for 13 days. Means followed by different letters are significantly different at p ≤ 0.05 by least significant difference (LSD). Vertical bars indicate the standard error (n = 4).

Figure 3

The regression correlations between (A) SPAD and NDVI, (B) Fv/Fm and PRI, and (C) YII and PRI in plants under control and waterlogging treatments. Each symbol represents the average of ten plants which were randomly selected from each treatment. Each NDVI or PRI index was calculated using leaf SPAD, Fv/Fm, and YII data (n = 30) from the model validation datasets. The R² coefficients of NDVI and PRI with leaf SPAD, Fv/Fm, and YII were calculated. The white circle represents data from control treatments, whereas the black circle represents data from waterlogging treatments. ***p < 0.001.