Potato ranks the first highest produced non-cereal food crops and the fourth highest produced crop after wheat, corn, and rice worldwide (FAOSTAT, 2019). It is cultivated in over 100 countries, and the global production of potatoes was estimated to be 370 million tons in 2019, feeding over a billion people worldwide. Potato is considered to be a healthy source of carbohydrates, dietary fiber, protein, vitamins, antioxidants, and minerals (Beals, 2019). Hence, enhancing the productivity of potato crops can contribute to fulfilling the nutritional requirements of the rising population (Birch et al., 2012). Potato is mainly cultivated for its tuber, which is mainly composed of carbohydrates generated through photosynthesis in the source leaves. The photosynthetic end product, i.e., sucrose, is transported from source leaves to the stolon where it is converted to starch, leading to tuber initiation and growth (Aliche et al., 2020). The effective coordination among these processes is crucial for tuber growth and productivity. The shallow root system of potatoes makes this crop one of the most drought-sensitive species (Zarzynska et al., 2017). Drought strongly inhibits key physiological and biochemical processes, leading to poor plant performance and tuber yield loss. The magnitude of this loss, however, mostly depends on the duration and severity of drought episodes as well as plant growth stage and cultivar (Evers et al., 2010; Stark et al., 2013; Aliche et al., 2018; Plich et al., 2020; Hill et al., 2021). Drought during the early growth stage is considered to be the most harmful as it substantially reduces total leaf area, reduces photosynthetic rates, and assimilates partitioning to tubers leading to poor tuber initiation, bulking, and tuber yield (Evers et al., 2010; Obidiegwu et al., 2015). Drought during tuberization leads to fewer stolon per stem, reflected by lower tuber number and yield (Eiasu et al., 2007). If potato plants experience drought during the tuber bulking stage, they will produce fewer and smaller-sized tubers. Nevertheless, it has been suggested that the initiation of stolon and the formation of tuber are the most critical stages of drought stress (Aliche et al., 2020). The reduction in tuber yield under drought is suggested to be mainly associated with the inhibition of photosynthesis (Plich et al., 2020). The drought-induced stomatal closure, which is aimed at reducing the transpiration water loss and conserving plant water status, also restricts CO₂ diffusion in the leaf making the Calvin cycle CO₂ substrate-limited (Pinheiro and Chaves, 2011; Dahal et al., 2014, Aliche et al., 2020). This may result in the accumulation of ATP and NADPH since their rates of generation by the photosynthetic electron transport chain exceeds their utilization by the Calvin cycle. Consequently, there is an energy imbalance in the chloroplast level that favors the generation of reactive oxygen species leading to oxidative stress and damage of cell components. Thus, plants experience both stomatal and biochemical limitations of photosynthesis in response to drought stress (Lawlor and Tezara, 2009; Pinheiro and Chaves, 2011; Dahal et al., 2015; Dahal and Vanlerberghe, 2017).

Potato plants employ various strategies at the molecular, biochemical, physiological, and whole plant levels to cope with drought stress (Boguszewska-Mańkowska et al., 2018, Dahal et al., 2019). At molecular and genomic levels, drought tolerance has been conferred by the expression of various stress-related genes that encode proteins including transcription factors and enzymes involved in drought stress tolerance (Shinozaki and Yamaguchi-Shinozaki, 2007). The products of drought-related genes play a key role in stimulating initial stress response and in inducing stress tolerance at the cellular level. Drought stress initiates the synthesis and deprotonation of abscisic acid (ABA), a well-known phytohormone (Yao et al., 2018). ABA serves as signaling molecules that induce the expression of several stress-related genes including those involved in closing stomata (Cutler et al., 2010). Drought-related genes are believed to be governed through both ABA-dependent and ABA-independent mechanisms (Takahashi et al., 2018). Although the application of exogenous ABA has confirmed the ABA-induced expression of stress-related genes, several drought-induced genes are insensitive to exogenous ABA application. The ability of the cultivars to tolerate drought stress is considered to be governed by upregulation of the expression of chloroplast-localized antioxidants and molecular chaperones (Vasquez-Robinet et al., 2008). It has been reported that the drought-tolerant capacity of potato cultivars is conferred with the induction of the expression of the dehydration-responsive element-binding protein (DREB1A) regulons (Kasuga et al., 1999; Kudo et al., 2017). For instance, the transgenic potato genotypes overexpressing AtDREB1a exhibited an improved drought tolerance in comparison with wild type (Watanabe et al., 2011; Movahedi et al., 2012). Pino et al. (2013) compared ScCBFI transgenic potato with non-transgenic lines. Their study suggested an improved drought tolerance in ScCBFI transgenic lines as indicated by improved overall plant performance and extensive root development following drought stress (Pino et al., 2013).

At the biochemical level, potato plants display an increased accumulation of compatible solutes in response to drought stress (Chen and Murata, 2008; Evers et al., 2010; Sprenger et al., 2016). These solutes have been believed to decrease the leaf water potential without affecting turgor pressure. As a result, leaf cells are capable of taking up more water from the soil to maintain leaf water status and survive drought. For instance, the elevated accumulation of sugar alcohol (Vasquez-Robinet et al., 2008) and proline levels (Sprenger et al., 2016) has been observed in potato leaves following drought stress. In another study, the increased accumulation of glycine betaine has been reported in higher plants in response to drought, salinity, and low-temperature stress (Rontein et al., 2002). Using transgenic potato genotypes overexpressing betaine aldehyde dehydrogenase—an enzyme required in the biosynthesis of glycine betaine—Zhang et al. (2011) reported improved drought stress tolerance in potatoes.

At the whole plant and physiological levels, potato plants improve instantaneous WUE by minimizing transpiration water loss and concomitantly conserving leaf water status through the decrease in stomatal conductance, leaf number, and leaf area (Liu et al., 2005; Coleman, 2008; Albiski et al., 2012; Ierna and Mauromicale, 2020; Kassaye et al., 2020). However, the associated cost of improved WUE is a reduction in photosynthetic leaf surface area, resulting in a negative impact on carbohydrate synthesis. The leaf develops hair and turns to a narrower size to lessen the light absorbance and prevent photooxidative damage. A few studies have revealed that potato cultivars exhibit an increase in root/shoot ratio due to the extensive and large root architecture in response to drought stress (Zarzynska et al., 2017; Boguszewska-Mańkowska et al., 2020). The drought tolerance has also been conferred to enhanced water and nutrient uptake efficiency as a result of higher root/plant biomass ratio following drought stress (Wishart et al., 2013; Villordon et al., 2014; Zarzynska et al., 2017; Boguszewska-Mańkowska et al., 2020). Studying five potato cultivars subjected to drought stress, Zarzynska et al. (2017) revealed a correlation between tuber yield to root length and area. Their study suggested that potato cultivars tend to improve drought tolerance with deeper root length and larger root systems. In another study, Banik et al. (2016) reported that severe drought treatment following the drought acclimation cycles reduced leaf wilting, induced thicker cuticular layers, and increased open stomata compared with plants without acclimation treatment. Consequently, potato plants acclimated to mild drought stress exhibited reduced yield losses as compared with non-acclimated controls (Banik et al., 2016).

Research on intensive potato breeding is primarily centered on selecting the drought-resistant cultivars by considering indicators at the whole plant and leaf levels such as yield, plant phenotype, leaf morphology, and leaf water content, with less effort at the physiological and biochemical levels. Although the regulation of the physiological and biochemical traits is critical for drought survival, only a few studies have attempted to integrate changes observed at the leaf and whole plant levels with those at the physiological and biochemical levels during drought. Thus, the main objectives of this study were to develop a method to (1) identify and characterize the drought-tolerant potato genotypes and (2) identify the physiological and biochemical traits governing drought tolerance in potatoes.

Plants experience drought stress when they receive insufficient water than their actual demand. The effects of drought stress may range from disruption in the molecular and biochemical functions at the cellular level to the physiological and morphological functions at the leaf and whole plant levels (Shinozaki and Yamaguchi-Shinozaki, 2007). Potato plants use several strategies at the physiological, biochemical, and molecular levels to combat drought stress (Boguszewska-Mańkowska et al., 2018; Dahal et al., 2019). These strategies enable plants either to maintain water potential by escaping the drought or to develop the adaptation mechanisms to tolerate lower water potential.

In this study, we used 56 potato genotypes including commercial cultivars to assess their tolerance to drought stress. Based on their capacity to maintain tuber yield under drought, potato genotypes differed in their ability to tolerate drought stress (Table 1). To further examine the drought tolerance mechanisms, we selected six genotypes, namely, Bannock Russet, Nipigon, Onaway, Denali, Fundy, and Russet Norkotah, with diverse response to drought tolerance based on tuber yield (Table 1, Figure 1A). We measured leaf RWC to determine whether the ability of plants to maintain tuber yield under drought is governed either by drought avoidance or by adaptation mechanism to tolerate lower water potential. The comparable leaf RWC across genotypes under drought stress (Figure 2) suggests that the ability of the potato genotypes Bannock Russet and Nipigon to tolerate drought was not associated with drought avoidance but rather associated with the physiological and biochemical tolerance mechanisms to lower water potential.

One of the important physiological strategies used by plants to survive drought stress is the improvement in WUE (Liu et al., 2005; Coleman, 2008; Albiski et al., 2012; Ierna and Mauromicale, 2020; Kassaye et al., 2020). The tolerant genotypes, Bannock Russet and Nipigon, exhibited a substantial increase in instantaneous WUE than did moderately tolerant genotypes, Onaway and Denali, and the sensitive genotypes, Fundy and Russet Norkotah, under drought (Figure 3A). Consistent with previous findings, our study revealed that the capacity of potato genotypes to enhance WUE under drought stress is reflected in their ability to tolerate drought stress. The differential stimulation of instantaneous WUE of tolerant and moderately tolerant genotypes vs. sensitive genotypes was accounted for by the differential suppression of stomatal conductance and concomitantly transpiration rates across genotypes (Figures 3B,C). Another strategy that plants employ following drought stress is a considerable increase in the drought-related proteins (Shinozaki and Yamaguchi-Shinozaki, 2007). Drought induces the expression of numerous stress-related genes that encode proteins including transcription factors and enzymes involved in drought stress tolerance (Shinozaki and Yamaguchi-Shinozaki, 2007). For instance, the ability of potato plants to tolerate drought stress is believed to be governed by upregulation of DREB1A regulons (Movahedi et al., 2012, Pino et al., 2013). In this study, the tolerant genotypes, Bannock Russet and Nipigon, exhibited a considerable increase in leaf proteins relative to moderately tolerant and susceptible genotypes following drought stress (Figure 4). The differential stimulation of leaf protein content under drought stress will further magnify when corrected on a chlorophyll basis in tolerant and moderately tolerant genotypes. Future study needs to be focused on the identification of proteins that are upregulated following drought treatment to advance our understanding of molecular mechanisms governing drought tolerance.

Photosynthesis converts CO₂ to carbohydrates in the presence of light energy and with the help of photosynthetic pigments, mainly chlorophylls. The carbohydrate is then translocated to the stolons for tuber initiation and growth. Hence, an enhancement in tuber yield can be expected through the stimulation of photosynthetic carbon fixation and their translocation to stolon. In this study, the drought-induced reduction in tuber yield was mainly related to the strong perturbation of photosynthesis (Table 2). However, the reduction in the rates of photosynthesis was similar in all genotypes, regardless of their differential tolerance ability to drought. Therefore, unlike previous findings (Plich et al., 2020), our current study suggests that the ability of the potato genotypes, Bannock Russet and Nipigon, to tolerate drought stress is not associated with the maintenance of photosynthesis under drought as compared with sensitive genotypes. Future research needs to confirm whether the differential ability of potato genotypes to maintain tuber yield under drought is due to the differences in the carbohydrate translocation to the stolon. The reduced F_v/F_m following drought stress indicates that all plants were experiencing stress under drought, which was also mirrored by decreased photosynthesis (Table 2). Consequently, all potato genotypes dissipated energy, excess of that used by photosynthesis, through enhanced NPQ under drought (Table 2). The precise mechanism that activates NPQ under drought is the area of further research.

The production of potatoes is constrained by frequent and severe drought episodes (Obidiegwu et al., 2015). Improving the productivity of potatoes under such suboptimal growth conditions is important to achieve the nutritional demand of an increasing population. Understanding the stress-related physiological, biochemical, and molecular processes is crucial to develop the screening procedures for selecting potato cultivars that can better adapt to drought. The elucidation of such processes may offer new insights into the identification of specific characteristics that may be useful in breeding new cultivars aimed at maintaining or even enhancing potato yield under the changing climate. Our current study has revealed that leaf protein content, instantaneous WUE, stomatal conductance, and transpiration rates can be used as the screening criteria for selecting the drought-tolerant potato genotypes. We suggest that future research needs to be concentrated on the identification and characterization of signaling molecules and target genes governing drought tolerance and tuber yield potential.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

KD planned the research and wrote the manuscript. KD and TG planned and designed the experiments. TG and AC conducted the experiments and performed measurements and laboratory analyses. KD and TG analyzed the data. TG, X-QL, BB, and DD revised the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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