Physiological activities and yield of yacon potato are affected by soil water availability

2.1 Characterization of the experimental area

The study was conducted at Fazenda Garganta, in the district of Celina, municipality of Alegre, located in southern Espírito Santo, at an altitude of 680 m, latitude 20°47′01″S, and longitude 41°36′56″W. According to the Köppen classification, the climate in southern Espírito Santo is classified as “Aw,” characterized by two well-defined seasons: a hot and rainy season from October to March and a cold and dry season from April to September, with an average annual temperature of 23°C and annual precipitation of approximately 1,200 mm.

To monitor the meteorological conditions outside the greenhouse, an automatic weather station (model E5000, Irriplus^®) was installed, and to periodically monitor humidity and temperature both inside and outside the greenhouse, a digital thermo-hygrometer (CEM DT-625^®, Akso) was used.

During the experimental period, relative humidity remained between 70 and 80%, both inside and outside the greenhouse, regardless of the year of cultivation. The average temperature recorded was 19.40°C for the first year of experimentation and 20.21°C for the second year. The external environment recorded temperatures 2.7°C (first experiment) and 2.5°C (second experiment) lower than those inside the greenhouse at the time of evaluation. Thus, temperatures were within the range considered optimal for yacon development (between 18 and 25°C), according to Seminario et al. [2].

2.2 Experimental setup

Two experiments were conducted, the first to evaluate the behavior of yacon plants subjected to a wide range of soil water tensions, aiming to determine a survival range for the plants. In the second year, a narrower range of soil tensions was tested based on the results from the previous year, aiming to refine the range for optimal development. The experimental periods ran for 210 days, from April to October in 2020 and 2021.

Both experiments were conducted in a protected environment, using a greenhouse with a “rain-shelter” design (to prevent rainfall from influencing the treatments), covered with 150 micron agrofilm, with open sides at the canopy level to facilitate air circulation. 25 L pots were used (Measurements: 35 cm high × 34 cm mouth wide), filled up to 90% of their capacity (22.5 L) with soil collected from a depth of 0.0–30.0 cm, which was crumbled, sieved through a 4 mm mesh, and homogenized.

The soil used in both experiments was sourced from the same location and classified as a medium-textured Red-Yellow Latosol [7]. A chemical and physical analysis of the soil revealed the following characteristics: pH 5.68 in water, 6.36 mg dm⁻³ of P, 79.00 mg dm⁻³ of K, 2.41 cmol_c dm⁻³ of Ca, 0.60 cmol_c dm⁻³ of Mg, 0.00 cmol_c dm⁻³ of Al, 3.23 cmol_c dm⁻³ of exchangeable sum of bases, 6.78 cmol_c dm⁻³ of effective cation exchange capacity, and a base saturation index of 47.65%. The soil contained 48 g kg⁻¹ of coarse sand, 554 g kg⁻¹ of fine sand, 210 g kg⁻¹ of silt, and 188 g kg⁻¹ of clay. Based on this analysis, 0.78 kg of dolomitic limestone with a relative total neutralizing power of 95% was applied per 1,000 L of soil to raise the base saturation to 70%.

Additionally, based on the soil analysis, the planting fertilization was carried out with 22.35 g of single superphosphate per pot, and topdressing with 20.47 g of urea and 17.85 g of potassium chloride per pot, split into two applications at 60 and 100 days after transplanting (DAT), following the nutritional requirements of yacon determined by Mendes [8].

Soil samples were also collected to determine the soil water retention curve using a Richards extractor membrane. The data were then fitted to the model proposed by Van Genuchten [9], which describes soil moisture behavior as a function of tension (matric potential).

where θ is the volume-based soil moisture (m³/m³); Ψ _m is the soil water matric potential (kPa); θ is the residual volumetric water content (m³/m³); θ _s is the saturation volumetric water content (m³/m³); and m, n, and α are the model fitting parameters.

Based on the observed data, a soil water retention equation was generated for the soil layer collected (Table 1).

Using the adjusted Van Genuchten equation, volumetric moisture content was determined at specific soil water tensions for the sampled points. Thus, the corresponding soil moisture levels for each water availability level were defined. These moisture levels were correlated with readings from the HIDROFARM^® soil moisture sensor (Falker) in the first experiment, and the Field Scout TM TDR300^® sensor (Spectrum Technologies) in the second experiment (Figure 1). The devices were calibrated to account for the specific soil type used, a crucial step in minimizing errors and ensuring accurate moisture readings.

Figure 1

Calibration curves of the devices: correlation between soil moisture determined by the standard gravimetric method as a function of the HIDROFARM device in the first experiment, and the TDR device in the second experiment.

Readings from the HIDROFARM and TDR were taken daily at 11:00 h and 17:00 h from four plants per treatment. Irrigation was triggered whenever the average soil moisture in these four experimental units reached the tension level required by the treatment, restoring soil moisture to the level corresponding to a tension of 10 kPa.

Drip irrigation was used, with a flow rate of 4.0 L h⁻¹, delivered by GA-4 pressure-compensating emitters with screw base, installed in each pot. Polyethylene hoses with a nominal diameter of 16 mm were used, with each tested tension irrigated by a separate hose line, totaling five plots. Different water volumes were applied based on emitter flow rates and operating time for each plot. The hoses were connected to derivation lines (DN 32), each equipped with a manual valve for irrigation control. A 1,000 L water tank, positioned approximately 10 m above the system, was used to provide gravity-fed irrigation.

System testing was conducted to determine average emitter flow rates and the uniformity coefficient of water distribution (UCW) for the irrigation system. Water collection was performed over a 2 min period from five emitters per line, after which the average depths were used to calculate UCW. The system demonstrated 95% uniformity, classifying it as excellent [10].

Yacon plants were propagated from rhizophores with 4–5 buds, weighing approximately 10 g, which were pre-disinfected in a 1% chlorinated solution and planted at a depth of 10 cm.

2.3 Physiological evaluations

Photosynthetic variables were measured using an infrared gas analyzer (LI-6800XT^® Portable Photosynthesis System, IRGA, Licor) at 160 DAT, when the plants were actively accumulating photoassimilates in their roots. The following parameters were evaluated: net CO₂ assimilation rate (A, µmol CO₂ m⁻² s⁻¹), stomatal conductance (gs, mol H₂O m⁻² s⁻¹), transpiration rate (E, mmol H₂O m⁻² s⁻¹), and intercellular CO₂ concentration (Ci, µmol CO₂ mol⁻¹). Photosynthetically active radiation was standardized using artificial saturating light at 1,000 μmol photons m⁻² s⁻¹, and CO₂ concentration was maintained at 420 ppm in the chamber. Measurements were taken in the morning, around 09:00 h, on cloudless days, targeting the youngest fully expanded leaves free from visible anomalies. Two leaves per plant were evaluated each time soil moisture reached the tension level required by the experimental treatment.

Additionally, chlorophyll index, nitrogen balance, anthocyanin, and flavonoid levels were measured using a Dualex^® optical sensor (Force-A, Orsay, France). These variables were 60, 110, 160, and 210 DAT.

2.4 Experimental design

Both experiments were arranged in a completely randomized design with four replications and five treatments, corresponding to different levels of water availability based on soil water tension. In the first year, tensions of 30, 60, 100, 200, and 300 kPa were tested to assess the plant’s ability to withstand water deficit over a wide range, and in the second year, tensions of 20, 30, 45, 60, and 75 kPa were evaluated to obtain a more detailed understanding of the plant’s response to decreasing soil water availability.

For the measurements of chlorophyll index, nitrogen balance, anthocyanin, and flavonoid levels, which were taken at four plant ages, a split-plot design (5 × 4) was adopted, with four replications for the analysis of variance (ANOVA).

The data were subjected to Cochran’s test to verify homogeneity of variance. Additionally, the Shapiro–Wilk test was performed, demonstrating that the data followed a normal distribution. Subsequently, analysis of variance (p ≤ 0.05) was conducted, and when the F-test was significant, regression analyses were applied using the R software, version 4.3.0 [11].

3.1 Net CO₂ assimilation rate

The rates of net CO₂ assimilation, stomatal conductance, intercellular CO₂ concentration, and transpiration decreased with increasing soil water tension, with coefficients adjusted to the linear model (Figure 2). The results demonstrate how yacon’s photosynthetic activity is affected by soil water availability, as all rates showed higher values when the plant was subjected to lower tensions, that is, with a greater volume of water available in the soil. In the first year, the maximum rate (27.332 µmol CO₂ m⁻² s⁻¹) was observed at the lowest applied tension (30 kPa), indicating that the plant still had the potential for a photosynthetic response under conditions of greater water availability (lower tension).

Figure 2

Net CO₂ assimilation rate (a1) and (a2), stomatal conductance (b1) and (b2), intercellular CO₂ concentration (c1) and (c2), and transpiration rate (d1) and (d2) in yacon plants subjected to different soil water tensions, over 2 years of cultivation. *The numbers next to the letters identify the year of the experiment, with (1) referring to the first experiment (2020) and (2) to the second experiment (2021).

Another interesting result was the demonstration of yacon’s survival capacity under water deficit, which extended up to a tension of 200 kPa, where the estimated soil water content (0.2058 m³/m³) was 68.8% of the field capacity estimated at 10 kPa (0.2991 m³/m³) (Table 1). At 200 kPa, the plant reached the lowest assimilation rate (11.607 µmol CO₂ m⁻² s⁻¹), representing a 57.53% reduction in the plant’s photosynthetic capacity before death was caused by the 300 kPa tension (Figure 2(a1)). This production of photoassimilates was used solely for the plant’s survival, as there was no accumulation of biomass in the tuberous roots, which are the storage organs. Even in the second year, the maximum assimilation rate (29.276 µmol CO₂ m⁻² s⁻¹) was observed at the lowest applied tension (20 kPa) (Figure 2(a2)), close to the soil’s field capacity (10 kPa), demonstrating how responsive yacon is to water availability.

3.2 Stomatal conductance and intercellular CO₂ concentration

The observed gains in net assimilation rate are closely related to stomatal conductance, which increased with greater water availability in the soil (lower tensions) during both years of the study (Figure 2(b1) and (b2)). Stomatal conductance values ranged from 0.07 mol H₂O m⁻² s⁻¹ at a tension of 200 kPa in the first experiment to 0.30 mol H₂O m⁻² s⁻¹ at a tension of 20 kPa in the second experiment. This result indicates an increase of more than 400% in stomatal conductance under conditions of greater water availability. With greater stomatal opening, there will be higher intercellular CO₂ concentration, also observed (Figure 2(c1) and (c2)), consequently increasing the flow of CO₂ toward the carboxylation site, facilitating the physiological process.

3.3 Transpiration rate

Transpiration rate is also related to stomatal conductance, exhibiting similar behavior, increasing as water became more available in the soil (lower tensions). In the first year, the highest transpiration rates were observed at a tension of 30 kPa (5.95 mmol H₂O m⁻² s⁻¹), and in the second year at 20 kPa (6.86 mmol H₂O m⁻² s⁻¹) (Figure 2(d1) and (d2)). Similar behavior was noted in Chinese cabbage, which recorded the highest transpiration rate (5.8 mmol H₂O m⁻² s⁻¹) at the lowest tension range tested (13–17 kPa) [12].

3.4 Chlorophyll and nitrogen balance

There was significant interaction between soil water tensions and evaluation periods for variables such as chlorophyll index and nitrogen balances. For the chlorophyll index in the first year of study (2020), when higher tension amplitudes (60–200 kPa) were applied, the best fit was a decreasing linear model over the cycle, except for plants growing under 30 kPa tension, where the best fit was a quadratic model. In this case, there was a decrease in this index until around 180 days, followed by an upward trend at the end of the cycle (210 days) (Figure 3(a1), Table 2).

Figure 3

Chlorophyll index (a1) and (a2), nitrogen balance index (b1) and (b2), and flavonoid index (c1) and (c2) in yacon plants, throughout the cycle, subjected to different soil water tensions over 2 years of cultivation. *The numbers next to the letters identify the year of the experiment, with (1) referring to the first experiment (2020) and (2) to the second experiment (2021). The coefficients of the adjusted models are presented in Table 2.

This behavior likely reflects better conditions, with greater water availability favoring the maintenance of some investment in photosynthetic apparatus. In the second year, the best fit was a decreasing linear model for all tensions throughout the cycle (Figure 3(a2), Table 2). Generally, the highest chlorophyll index values were observed under the lowest applied tensions (30 kPa in the first year and 20 kPa in the second year), reinforcing the result that yacon’s photosynthetic activity benefits from greater water availability.

For nitrogen balance, a similar behavior to chlorophyll was observed, which is expected given the direct relationship between these variables, as nitrogen is necessary for chlorophyll synthesis [15]. In the first year, under lower tensions (30 and 60 kPa), the best fit was a quadratic model, with a decrease in this index until around 180 days, followed by an upward trend at the end of the cycle (210 days). For the 100 kPa tension, the best fit was a decreasing linear model throughout the cycle. Under 200 kPa tension, the best fit was a quadratic model, showing a slight increase in this index, with a maximum around 90 DAT, followed by a decrease until the end of the cycle (Figure 3(b1), Table 2). In the second year, the best fit was a decreasing linear model throughout the cycle for all applied tensions (Figure 3(b2), Table 2).

3.5 Anthocyanin and flavonoid index

There was significant interaction between soil water tensions and evaluation periods for variables such as flavonoids in both years. Therefore, the data were broken down to assess the physiological behavior of yacon throughout the cycle, according to the applied tension.

The flavonoid index, under the condition of higher water restriction (tension of 200 kPa), a quadratic model was fitted, describing a decrease until about 135 days into the cycle, followed by an increase in values until the end of the cycle (210 days). Under conditions of greater water availability (tensions of 30 and 60 kPa), a quadratic model was also fitted, but with an inverse pattern, showing an increase in values up to around 135 days of the cycle, followed by a decrease until the end of the cycle (210 days). In contrast, with the application of 100 kPa tension, no significant differences were observed for the flavonoid index throughout the cycle (Figure 3(c1), Table 2). In the second year, no significant differences were observed for the flavonoid index between the applied tensions (20–75 kPa) over the cycle (Figure 3(c2), Table 2).

The anthocyanin index did not show significant effects in either experiment. The absence of a significant effect may be related to the function of anthocyanins in plants, which are secondary metabolites that do not play a fundamental role in basic life cycle processes but have important secondary functions. For instance, they act as a solar filter with antioxidant properties, neutralizing free radicals and protecting leaves against damage caused by UV light [13]. Typically, stress conditions induced by solar radiation lead to an increase in anthocyanin content in the leaf epidermis [14]. However, since the stress in this study was induced by water deficit, this may not have activated the defense mechanism involving anthocyanin synthesis, leading to the absence of significant differences between treatments.

3.6 Root production

Analyzing the influence of soil water availability (based on the applied tensions) on the total production of fresh tuberous roots (an important parameter for the crop’s commercial performance), it was observed that, in the first experiment, significant tuberous root production occurred only at 30 kPa, reaching approximately 1,100 g plant⁻¹. In the other treatments, the production of fresh tuberous roots was around 100 g plant⁻¹, but without meeting consumption standards (roots were small and very thin).

In the second experiment, it was noted that yacon expresses its highest productive capacity near the water availability at 10 kPa tension, which is considered the soil’s maximum water retention capacity. Production showed a linear decrease as soil water availability decreased. Overall, production ranged from 1156.3 to 135.2 g plant⁻¹, between the highest and lowest levels of water availability applied (tensions of 20 and 75 kPa, respectively) (Figure 4).

Figure 4

Total production of fresh tuberous roots in yacon plants subjected to different soil water tensions in the second year. Alegre-ES, 2021.