Morpho-physiological traits and yield quality for cassava genotypes planted under drought during canopy establishment

Experimental detail

This experiment was conducted under field conditions from August 2021 to August 2022 (2021/2022) and from August 2022 to August 2023 (2022/2023) at the Field Crop Research Station of Khon Kaen University, Khon Kaen, Thailand (16°28′ N, 102°48′ E, 200 m a.s.l.). The soil type for the experimental field was Yasothon Series (Yt: Oxic Paleustults). The experiment was a 2 × 6 split-plot design with four replications (main plot factor = water regime, subplot factor = genotype). Two water regimes, including drought conditions in the dry season and full irrigation, were assigned as main plots. Six cassava genotypes, Kasetsart 50, Rayong 9, Rayong 72, CMR38-125-77, CMR 35-91-63, and CM523-7, were assigned as subplots. The cassava genotypes were selected for high environmental adaptability (Kasetsart 50), high yield and high starch content (Rayong 9 and CMR38-125-77), high yield and drought tolerance (Rayong 72), high yield (CMR35-91-63), and low yield and drought tolerance (CM523-7).

Land preparation and tillage were conducted, and soil ridges were created with a 1 m distance between the ridges. The plot size was 7 × 10 m. Cassava stem cuttings of 20 cm from healthy 12-month-old plants were planted at 1 × 1 m spacing after soaking for 15 min in thiamethoxam (3-(2-chloro-thiazol-5-ylmethyl)-5-methyl-(1,3,5)-oxadia-zinan-4-ylidene-N-nitroamine, 25% water-dispersible granules) to prevent pest infestation. The stakes were inserted vertically to a depth of 14 cm into the soil ridges. Manual weed control was conducted between 30 to 90 days after planting (DAP). At 30 DAP, chemical fertilizer was applied according to the nutrient requirements for cassava as suggested by Howeler (2002) and the soil characteristics that were identified before planting. Chemical fertilizer (N-P-K) formula of 15-7-18 was applied at a rate of 312.5 kg ha⁻¹ at 60 DAP (Department of Agriculture, 2010). Before planting, soil at 0–30 cm and 30–60 cm depths were sampled to assess physical and chemical properties (Table 1). The soil texture at the Field Crop Research Station of Khon Kaen University was a sandy loam; the values for soil pH ranged from 6.3 to 6.8, total nitrogen varied from 0.2 to 0.3 g kg⁻¹, available phosphorus was between 8.0 to 36.8 mg kg⁻¹, and exchangeable potassium varied from 13.6 to 54.7 mg kg⁻¹. The soil chemical analysis indicated low total nitrogen and exchangeable potassium. From 30 to 90 days after planting (DAP), full irrigation based on a mini-sprinkler system was applied to all experimental plots under both irrigation and drought conditions. In the dry season (90 to 150 DAP), drought treatment was imposed by withholding irrigation, and supplementary irrigation was applied back for the recovery period from 151 to 360 DAP. For the plots that received full irrigation, the plants were irrigated throughout the crop duration. Irrigation was conducted based on the amount of crop water requirement (ETcrop) that was calculated as described by Doorenbos & Pruitt (1992): (1)${\displaystyle \text{ET crop}=\text{ETo}\times \text{Kc}}$ where ET crop is the crop requirement (mm day⁻¹), ETo is the evapotranspiration of a reference plant under specified conditions calculated by the pan evaporation method, and Kc is the crop water requirement coefficient that varies as a function of the growth stage. The Kc value for cassava was provided by the FAO, but it is inappropriate for the cassava growing conditions. The Kc value for FAO was calculated using a crop duration of 210 days. However, the crop duration of the cassava was 360 days. Therefore, we decided to use the Kc of sugarcane, which has a crop duration that covers 360 days (Doorenbos et al., 1986). In addition, the period for yield formation for cassava is also similar to sugarcane. The amount of water for irrigation was then calculated.

Data collection

Soil moisture content was monitored by sampling at 90, 120, 150, 180, and 360 DAP for depths of 0–30 cm and 30–60 cm. The soil samples were oven-dried at 105 °C for 72 h or until weights were constant, and the moisture percentage was calculated. Soil moisture was determined by the gravimetric method described by Shukla et al. (2014) as shown below Eq. (2): (2)${\displaystyle \text{Soil moisture content}\left(\text{\%}\right)=\frac{\text{Soil wet weight}\left(\mathrm{g}\right)-\text{Soil dry weight}\left(\mathrm{g}\right)}{\text{Soil dry weight}\left(\mathrm{g}\right)}\times 100.}$

The relative water content (RWC) values were recorded at 180, 210, 240, 270, and 360 DAP. Measurements were taken from the second or third fully expanded leaves on the top of the main stem of two plants from each plot. The sampling occurred between 9:00 a.m. and 12:00 p.m. Leaf discs, each measuring one cm², were collected from 10 leaves per plot. Leaf fresh weight was determined in the laboratory and then soaked in distilled water for 24 h, after which it was weighed to obtain the turgid weight. Sampled leaves were oven-dried at 80 °C until constant weight, then weighed. RWC was calculated by following Kramar (1980): (3)${\displaystyle \mathrm{RWC}\left(\text{\%}\right)=\frac{\text{Fresh weight}\left(\mathrm{g}\right)-\text{Dry weight}\left(\mathrm{g}\right)}{\text{Turgid weight}\left(\mathrm{g}\right)-\text{Dry weight}\left(\mathrm{g}\right)}\times 100.}$

Crop data was collected from two plants of each plot at 90, 120, 150, 180, and 360 DAP. The plants were separated into leaves, stems, storage roots, and fibrous roots. All plant parts were subsampled (about 10% of the total fresh weight of each organ). A subsample of fresh leaves was then used to measure leaf area by using a leaf area meter (LI-3100, LI-COR, Inc., Lincoln, NE, USA). Subsamples were oven-dried at 80 °C to achieve a constant dry weight. The harvest index (HI) was calculated as the ratio of the dry weight of storage roots to the total dry weight of the crop. The starch content of the storage root was assessed using the specific gravity method, and the starch yield was calculated by multiplying the starch content with the dry weight of the storage root. Calculations of CGR during 90 to 120 DAP, 120 to 150 DAP, 150 to 180 DAP, and 180 to 360 DAP were performed based on the function below (Sawatraksa et al., 2019) Eq. (3): (4)${\displaystyle \mathrm{CGR}\left(\mathrm{g}{\mathrm{m}}^{-2}{\mathrm{d}}^{-1}\right)=\left(\frac{1}{\mathrm{G}}\right)\times \left(\frac{{\mathrm{DW}}_2-{\mathrm{DW}}_1}{{\mathrm{T}}_2-{\mathrm{T}}_1}\right)}$

where G is sample area (m²) and DW₁ and DW₂ are crop dry weight (g) at the times T₁ and T₂(d). The equation for CGR was applied to calculate LGR, SGR, and SRGR.

Relative growth rate (RGR) was calculated using the following equation (Sawatraksa et al., 2019) Eq. (4): (5)${\displaystyle \mathrm{RGR}\left(\mathrm{g}{\mathrm{g}}^{-1}{\mathrm{d}}^{-1}\right)=\frac{\mathrm{In}\left({\mathrm{DW}}_2\right)-\mathrm{In}\left({\mathrm{DW}}_1\right)}{{\mathrm{T}}_2-{\mathrm{T}}_1}}$

where DW₁ and DW₂ are crop dry weight (g) at the times T₁ and T₂(d).

Statistical analysis

Analysis of variance (ANOVA) was performed for all crop traits by following a model for split-plot design (Gomez & Gomez, 1984) and by using the statistix10 program (Statistix10, 2013). Mean comparisons were conducted for the least significant difference test (LSD) at p≤ 0.05.