Introduction

In natural environments, the availability of resources, such as water, CO2, O2, light, and nutrient elements, is very important for plant survival and growth. Although the drought, cold and nutrient deficiency are types of environmental stress, the mechanisms of these stresses are completely different. Drought is considered a major environmental factor that arrests plant growth and results in lower crop yields. In order to survive under conditions of drought stress, it is necessary for plants to reduce leaf water potential, stomatal conductance, gas exchange, and normal growth (Neumann 2008; Reddy et al. 2004). Cold stress, or the low temperature exposure, can decrease plant metabolism activity and even impose severe yield penalty on crop plant (Stitt and Hurry 2002; Nord and Lynch 2009). Nutrient deficiency, in particularly the nitrogen and phosphorus deficit, is always regarded as another important limited factor for plant normal growth and metabolism (Tewari et al. 2004).

All these stresses can cause oxidative stress and lead to the formation of reactive oxygen species (ROS). These ROS can be detrimental to lipids, proteins, carbohydrates, and nucleic acids, which can accelerate plant senescence after long-term exposure (Prochazkova et al. 2001; Mittler 2002; Mittler et al. 2004). Plants have developed both enzymatic and non-enzymatic defense systems for the removal and detoxification of ROS (Mittler 2002; Mittler et al. 2004). The enzymatic system consists mainly of superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2). Non-enzymatic antioxidant metabolites or low molecular weight compounds, such as ascorbates, carotenoids, tocopherols, and flavonoids, also play an essential role in ROS scavenging. The balance between ROS production and ROS removal capacity determines the rate of plant metabolism and growth (Tiwari et al. 2002; Baxter et al. 2007; Godon et al. 1998).

Polyploidization is a major driving force in plant evolution, and it is estimated that most angiosperms exist as polyploids (Comai 2005). It has long been held that polyploid species can colonize a wider range of habitats and survive better in high altitude and latitude zones that are characterized by high UV irradiation, low temperatures, nutrient-poor soils, and drought (Ehrendorfer 1980; Levin 1983). Improved stress tolerance has been reported in polyploids compared with the corresponding diploids under drought (Chandra and Dubey 2010), cold (Sugiyama 1998), UV (Niwa and Sasaki 2003), and salt (Saleh et al. 2008) stress challenges. Although these data were collected from several different plants, we have demonstrated that increases in the tolerance to more than one type of stress can be acquired to different extents by plant polyploidization (data not shown). In addition, some of these increases in stress resistance capacity are considered to be due to the enhanced SOD enzyme activity of the polyploid compared with the corresponding diploid (Chandra and Dubey 2010; Niwa and Sasaki 2003), although the conflicting results were also reported (Zhang and Kirkham 1994).

It is well known that few plants possess tolerance to more than one type of stress due to limitations in morphological and physiological trade-offs (Niinemets and Valladares 2006; Mittler 2006). Accordingly, this raises a further question: how does polyploidization confer increases in plant multi-stress tolerance to different extents compared with the corresponding diploid? The gene dosage theory can partly explain this increased tolerance (Chandra and Dubey 2010; Niwa and Sasaki 2003), and the epigenetic theory is an excellent approach for resolving this problem, although the precise mechanism has yet to be characterized (Comai 2005; Chen 2010). Hence, it is necessary to look for a simple and easily comprehensible theory that explains this polytolerance of polyploids compared with diploids at the physiological level for most stress factors. In this study, the natural allotetraploid tobacco, which is a fast-growing, moderately low nutrient/drought-tolerant, and cold-sensitive plant, and an artificially produced octaploid tobacco were compared under conditions of drought, cold and nutrient stress.

Materials and methods

Material preparation and identification

Seeds of Nicotiana benthamiana (allotetraploid tobacco) came from our own laboratory and were germinated in a cultivation vessel for 24 h. When the bud reached a length of 2 mm, half were transferred to another cultivation vessel and treated with 0.3% colchicine plus 2% dimethyl sulfoxide for 72 h (Nakamura et al. 1993; Zhu et al. 2006). The resulting octaploids and tetraploids were initially identified based on their special phenotypes (Paucã-Comãnescu et al. 1999; Romero-Aranda et al. 1997), and their identities were subsequently confirmed by the stomatal cell chloroplast number, a rapid and reliable method for large sample analysis (Zhu et al. 2006; Ho et al. 1990; DeMaggio and Stetler 1971).

Growth conditions and treatment

When they had reached the 7- to 8-leaf stage, plants of both ploidy levels were selected for uniformity, transplanted to plastic pots filled with vermicullite and watered with full strength Hoagland solution twice per week. After 10 days acclimative culture, the plants were divided into two groups for the following treatments.

For the cold treatment, the plants were grown under three levels of cold conditions: 25 (control), 10 and 4°C for 7 days. For the drought treatment, 0 (control), 5 and 10% PEG6000 solution were applied to each plant and their subsequent growth state was monitored. For the nutrient stress experiment, we used the 1/10 strength Hoagland solution as the control, the 1/30 and 1/100 strength as the moderate and severe deficiency treatment, respectively. For abscisic acid (ABA) treatment, two treatments of 2 and 20 mg L−1 ABA (S-cis) were applied to the two groups of tobacco plant leaves with different ploidy levels for 7 days. Plants were preserved under well-watered conditions, and water-treated plants without ABA were used as the controls.

In addition, a comparison was made of the survival time of the two ploidy levels of tobacco plants under conditions of drought (5% PEG), cold (4°C), nutrient deficiency (1/100 strength of HS), shade (20 μmol s−1 m−2 irradiance) and waterlogging stress (water above the soil) exposures.

All plants were grown under controlled conditions on a 16 h light/8 h dark cycle (25/21°C) and with irradiation of 200 μmol s−1 m−2. Seven days later, the functional leaves of the plants (mid position of the stem) were sampled for analysis. In addition, the upper-, mid- and lower-positioned leaves under favorable conditions were also collected, respectively, for carbohydrate content analysis. The data obtained for the 0 day plants (stress-free) were considered as control data.

H2O2 and oxidative damage determinations

H2O2 content was measured using the method of Gay et al. (1999). Lipid peroxidation was determined by the method of Heath and Packer (1968) with reference to the malondialdehyde content by the thiobarbituric acid reaction. Protein damage was assayed with the method of Levine (2002).

Antioxidant enzyme extraction and assay

Tobacco leaves (0.5 g) were homogenized with liquid nitrogen and then extracted with 5 mL of 50 mmol L−1 phosphate buffer (pH 7.0), 10 mmol L−1 reduced ascorbate and 4.0% (w/v) polyvinylpyrrolidone. The homogenate was centrifuged at 10,000×g for 30 min at 4°C, and the supernatant was collected for the enzyme assays. The SOD activity was measured with the method of Dhindsa et al. (1980); the reaction medium for APX activity was determined by the method of Nakano and Asada (1981); CAT activity was determined by following the method of Aebi (1984); the GR activity was assayed with the method of Smith et al. (1988).

Total antioxidant capacity measurement

The total antioxidant capacity reflects the non-enzymatic antioxidant capacity and was evaluated by employing two widely used methods: the Fe3+ reducing power (FRAP method) and α,α-diphenyl-β-picrylhydrazyl (DPPH) free radical scavenging capacity (DPPH method). The antioxidant power of foliar extracts was determined using the method of Benzie and Strain (1996). The DPPH free radical scavenging capacity of foliar extracts was determined using the method of Brand-williams et al. (1995).

Ascorbate and glutathione content determination

The ascorbate pool (ASC + DHA) and ASC/DHA were assayed according to the protocol of Fryer et al. (1998). The total glutathione pool (GSH + GSSG) and GSH/GSSG were assayed according to the protocol of Nagalakshmi and Prasad (2001).

Survival time and nonstructural carbohydrate determination

Survival time was recorded as the day on which half the plant was considered to be dead. The soluble sugar and starch content were measured with the method of Chow and Landhäusser (2004).

Analysis of gas exchange

Gas exchange was determined using a portable photosynthetic apparatus (LI-6400XT, LI-COR, USA). Photon flux density was measured at 800 μmol m−2 s−1 (outside room) and the temperature, relative humidity, and CO2 concentration were noted to be dependent on ambient conditions. Pn, Gs, and Ci were also monitored and recorded simultaneously.

Statistical analysis

All data were analyzed using SPS 13.0 software. Means followed by the same letter are not significantly different (Duncan’s multiple range test P < 0.05). For each stress treatment, three replicates were analyzed.

Results

Ploidy level identification and phenotype description

Nicotiana benthinana is a typical allotetraploid tobacco plant. The artificial octaploid was obtained following treatment with colchicine (Nakamura et al. 1993). The octaploid exhibited typical phenotypes of autopolyploids, such as delayed ontogenesis, slow growth, thickened and rippled leaves, a shortened stem and enhanced leaf color (Fig. 1a, b). Additionally, enhanced epidermal and stomatal cell size and decreased stomatal density were observed in contrast to their tetraploid counterparts (Fig. 1c, d). Chloroplast number was also found to be twice as large in the octaploid stomatal guard cell when compared to the tetraploid (Fig. 1c, d).

Fig. 1
figure 1

Comparison of the tetraploid and octaploid tobacco plants. a Morphological differences between the octaploid and tetraploid; b biomass accumulation during the same time period; c, d contrasting sizes and densities of the stomata cells in the octaploid and tetraploid tobacco leaves, respectively. Scale bar = 30 μm; sc stomatal cell, chl chloroplast

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H2O2 accumulation and oxidative damage

When compared to the octaploid, H2O2 accumulation and oxidative damage parameters, such as TBARS and carbonyl content, were found to increase more significantly in the tetraploid as nutrient stress increased (Table 1). This finding indicates that under nutrient deprivation, tetraploids are less tolerant to stress. Similar patterns were also observed under drought (Table 2) and cold stress (Table 3) conditions. Among these three stress conditions, the octaploid exhibited greater tolerance during drought (2.5-fold increase) and nutrient (3-fold increase), than under cold stress (1.5-fold increase) when compared to the protein damage seen in the tetraploid (Tables 1, 2, 3). This suggests the tetraploid suffered greater oxidative damage than the octaploid during both stressful and stress-free conditions.

Table 1 ROS metabolism related parameters were compared between the tetraploid and octaploid tobacco leaves with different concentration of HS treatment
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Table 2 ROS metabolism related parameters were compared between the tetraploid and octaploid tobacco leaves with different concentration of PEG treatment
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Table 3 ROS metabolism related parameters were compared between the tetraploid and octaploid tobacco leaves under low temperature exposure
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Enzymatic and non-enzymatic antioxidant activities

Greater activity was observed in antioxidant enzymes, including SOD, CAT, APX and GR, in the octaploid plant leaves compared to the tetraploid; this difference was observed to increase when greater nutrient stresses were applied (Table 1). With regards to the non-enzymatic antioxidant capacities, such as the DPPH free radical scavenging capacity and Fe3+ reducing power, both lower IC50 values that indicate stronger DPPH scavenging capacities and higher FRAP values that indicate stronger Fe3+ reducing powers were measured in the octaploids compared to the tetraploids. Similar trends were also observed for stress increments and enzyme activities (Table 1) and for the other two stress conditions (Tables 2, 3). Additionally, a greater difference in non-enzymatic antioxidant capacity between the octaploid and tetraploid leaves was observed, but this was not found for antioxidant activities as the level of stress increased.

Antioxidant metabolite and redox values

Ascorbate and glutathione are two major antioxidant metabolites. The ratio of reduced to oxidized metabolites is commonly regarded as a sensitive indicator to evaluate the redox state of plant tissues (Tausz et al. 2004; Mittler 2002). Although total ascorbate and glutathione were both seen to increase with increased stress, the redox value showed a significantly greater decrease in the tetraploid, in comparison to the octaploid, irrespective of stress types (Tables 1, 2, 3). This suggests that under stress, antioxidant metabolites become more oxidized in the tetraploid than in the octaploid leaves.

Survival times and nonstructural carbohydrate content

Figure 2a shows a significant increase in survival time in octaploid plants than in tetraploid plants under exposure to drought conditions (survival increased by approximately 120%), nutrient deficit (increased by approximately 180%), cold (increased by 70%), shade (increased by 15%) and waterlogging stress (increased by 10%). Starch and soluble sugar content were also observed in the two ploidy levels of plant to evaluate the ‘deposite’ resource for antioxidant synthesis. The functional leaves (mid-positioned leaves) displayed higher soluble sugar and starch content than the fresh (upper position) or old (lower position) leaves (Fig. 2b). Higher amount of starch and sugar was observed in the octaploid than in the tetraploid, irrespective of growth stage.

Fig. 2
figure 2

Plant survival time under stress exposure and foliar NSC content under favorable condition. Survival time of two ploidy levels of tobacco plants was recorded under stress exposure (a); starch and soluble sugar content in the upper-, mid- and lower-positioned leaves under favorable condition (b). The data are the means of three independent experiments ± SE. Means followed by the same letter are not significantly different (Duncan’s multiple range test P < 0.05). NSC nonstructural carbohydrate

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Gas exchange and oxidative damage with ABA treatment

Compared to the controls, low levels of application of ABA were shown to slightly close the stomata and lower the Ci values, whilst promoting the Pn. Low-dose ABA also significantly protected both plants with different ploidy levels from oxidative damage. However, during exposure to high concentrations of ABA, severe decline of Pn, Ci and Gs, and a greater increase in oxidative damage was observed in the tetraploid than in the octaploid plants (Fig. 3). This suggests that the octaploid plant can perform photosynthesis better, under limited CO2 availability, and within a shorter duration of time where high-dose ABA can almost fully close the stomata.

Fig. 3
figure 3

Gas exchange and oxidative damage with ABA treatment. The Pn (a), lipid peroxidation (b), Ci (c), protein damage (d), Gs (e) and redox state, or GSH/GSSG ratio (f) under ABA treated conditions of the tetraploid versus the octaploid. The data are the means of three independent experiments ± SE. Means followed by the same letter are not significantly different (Duncan’s multiple range test P < 0.05)

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Discussion

Plant polyploidization is an interesting and widespread phenomenon in nature. The process has received increased attention recently due to the finding that polyploidization results in phenotype alteration at both physiological and morphological levels (Paucã-Comãnescu et al. 1999; Romero-Aranda et al. 1997; DeMaggio and Stetler 1971). It has been well-established that enhanced environmental stresses, such as cold, drought, UV and pathogen resistance, are found in polyploids in comparison to their diploid counterparts (Sugiyama 1998; Chandra and Dubey 2010; Niwa and Sasaki 2003).

In this experiment, tolerance to different extents of enhanced drought, nutrient and cold stress was also monitored in octaploid and tetraploid tobacco plants from the perspective of oxidative damage (Tables 1, 2, 3). In accordance, lower levels of oxidative damage and H2O2 accumulation were measured in the octaploids compared with the tetraploid (Tables 1, 2, 3). This low level of H2O2 accumulation may be due to the higher activity levels of antioxidant enzyme and stronger ROS scavenging capacity in octaploids than in tetraploids (Tables 1, 2, 3).

The results showed that a very high of non-enzymatic antioxidant capacity was observed in octaploid plants than tetraploids under stressful or stress-free conditions (Tables 1, 2, 3). It is easy to understand this higher accumulation of secondary metabolite in the slow-growing octaploid than in the fast-growing tetraploid, as the former has a longer growth stage (Fig. 1b). In addition, the non-enzymatic antioxidant compounds have a special function for ROS scavenging in comparison to the antioxidant enzyme because the latter becomes easily inactivated when the cell dies. Thus, the stronger non-enzymatic antioxidant may play a greater role in contributing to the enhanced stress tolerance of polyploids than in tetraploids.

However, the higher antioxidant enzyme activities were also observed in the ontogenesis-delayed octaploid in comparison to the fast-growing tetraploid counterpart under stressful or stress-free conditions (Tables 1, 2, 3). One possible explanation for this finding may be that higher accumulation of enzymatic antioxidant compound (or secondary metabolites, such as tocophenol and ascorbate) protect the antioxidant enzyme protein from oxidation by scavenging more OH−.

It is well known that ROS play a dual role in plants: at low concentrations, they act as a signal molecule involved in promoting the normal growth and development of plants (Foreman et al. 2003; Li et al. 2009b; Kim et al. 2007; Rodríguez et al. 2002) and, at high concentrations, they can cause plant senescence or even PCD (Prochazkova et al. 2001; Tiwari et al. 2002). Figure 1b shows that the octaploid has a low growth rate and biomass accumulation capacity; its delay in the ontogenesis phenomenon compared to the tetraploid was commonly attributed to the enhanced cell size and reduction of metabolic activity (Chen et al. 2007; Comai 2005; Kondorosi et al. 2000). Considering that the ROS play a critical role in plant development and growth, including in root elongation and leaf extension, one speculative possibility is that low levels of ROS and a more reduced redox state may also contribute to the delayed ontogenesis of the octaploid compared with the tetraploid.

Some researchers consider that, in the context of stress resistance, polyploids have a gene dosage advantage over the corresponding diploids, and some possible correlation between the enhanced stress resistance and increased antioxidant capacity of polyploids compared with their diploid counterparts has been reported (Chandra and Dubey 2010; Niwa and Sasaki 2003). However, the effects of the duplicate genes of polyploids are more closely correlated with developmental programs than with the response to environmental stresses, and, indeed, the duplicate genes can eventually be lost or have the tendency to mutate (Comai 2005; Chen 2010; Allario et al. 2011). It suggested that the contribution of gene dosage effect in polyploid plant for the increased stress resistance may not so much as we thought before, at least in the autopolyploid plant. In fact, there also exists an unequal elevation of varied stress resistance for polyploid in contrast to diploid. For example, in this experiment, there is only a slight increase of shade and waterlogging (compared with the intense increase of drought or nutrient stress) tolerance in the octaploid compared with the tetraploid tobacco (Fig. 2a).

Thus, the gene dosage theory does not appear to provide a completely satisfactory explanation for the stronger stress tolerance in polyploids compare with the corresponding diploids. Although the epigenetic theory can help to explain certain stress resistance advantages of polyploidy over diploidy in several plant species at the molecular level, the conclusive resolution of this problem awaits further study (Comai 2005; Chen 2010).

The acquisition of sufficient resources, such as CO2, light, water and nutrients, is very important for plants to survive under stressful conditions (Magyar et al. 2007; Neumann 2008; Niinemets and Valladares 2006; Chapin 1980). Gs, Pn and Ci are three parameters that can be used to evaluate plant photosynthesis performance. However, the photosynthesis and stomata conductance can both be profoundly affected by low temperature, drought or nutrient deficit stress (Nord and Lynch 2009; Stitt and Hurry 2002).

Some literature has reported that the polyploid has a low Gs, Ci and Pn under favorable conditions compared to their diploid counterparts (Xu et al. 2008; Romero-Aranda et al. 1997). It means that a stronger transpirate power is required for water and nutrient assimilation, and more CO2 is needed to perform the normal metabolic processes in diploid than in polyploidy plants. In fact, the stomata density reduction and low light transmittance of polyploids were strong evidence to support the low CO2 or light requirement in comparison to their diploid counterparts under stress-free conditions (Zhu et al. 2006; Ho et al. 1990; Romero-Aranda et al. 1997). However, a higher Pn and lower Ci were reported in the tetraploid compared to the diploid under stress conditions, such as drought conditions or high irradiance (Li et al. 2009a; Romero-Aranda et al. 1997). In this experiment, we also observed the Gs, Pn and Ci of the two ploidy levels of plants after treatment with ABA (Fig. 3). Compared to the control (water treatment), a low concentration of ABA (2 mg L−1) can protect the tobacco plant with enhancement of the Pn coupled with a slight reduction of the Gs and Ci values (Fig. 3). This finding suggested that higher levels of CO2 were required to maintain the higher Pn level of the tetraploid than the octaploid to satisfy its normal metabolic activity under slight stress conditions, such as a mild drought or exposure to salinity, which can trigger an increase in ABA increase. However, 20 mg L−1 ABA (such as severe drought or salinity exposure) can close the stomata of the two ploidy levels of tobacco plant and the Ci and Pn values were greatly decreased in the tetraploid and octaploid plants with very low Gn values (Fig. 3). This suggested that the “deposited” CO2 was consumed more rapidly in tetraploid than in octaploid plants, while at the same time the Pn was greatly reduced in the former due to a shortage of CO2.

We also determined the oxidative damage and redox parameter GSH/GSSG under low and high ABA exposure. Our data indicated that severe lipid peroxidation and protein damage combined with low GSH/GSSG value were observed in the tetraploid compared to the octaploid (Fig. 3). This further indicated that the tetraploid suffered greater damage under same resource availability conditions than the octaploid.

Interesting, we also can use the defense compound synthesis level to further explain the low resource requirement of octaploid than the tetraploid. Water, nutrients, CO2, and light all are indispensable resources for normal plant growth, and any deficiency-triggered oxidative stress would divert more resources from the normal metabolic pathways to the antioxidant response pathways (Baxter et al. 2007; Godon et al. 1998). Water or nutrient deficiency or a decrease in light or O2 availability would lead to a reduction in photosynthetic rate or changed metabolism pathway, or altered respiratory patterns respectively. Further, all these changes would result in a decrease in available energy (Neumann 2008; Tiwari et al. 2002; Baena-González and Sheen 2008); however, the impact of such changes on octaploids would be less marked than on tetraploids due to the fact that the former has a smaller resource requirement. From an antioxidant protection perspective, these stressors (drought, cold and nutrient stress) all can trigger ROS overproduction (Tables 1, 2, 3), and hence under these conditions, plants need more antioxidant compounds to scavenge them. Fortunately, slow-growing octaploids can not only accumulate more antioxidant compounds (especially the secondary metabolite) but can also synthesize them with a low energy cost due to their slow growing feature (Scheurwater et al. 2000). Hence, slow growers would be expected to suffer less severe drought, cold or nutrient stress than fast growers (Tables 1, 2, 3).

In contrast to tetraploids, it seems that octaploids use the non-protein antioxidant compounds (longer-lived) in preference to protein antioxidant (shorter-lived) activity as ‘deposite’ antioxidant power under normal conditions to counteract the latent stress challenge (Tables 1, 2, 3). In addition, a further factor to which we should pay attention is the higher stability of redox state which observed in octaploid than in tetraploid (Tables 1, 2, 3). One possibility is the higher content of non-enzymatic antioxidant compound can exist longer time and contribute more to the high steady of redox state of octaploid than tetraploid over the enzymatic antioxidant.

In addition, we can even from the resource investment perspective to understand the enhanced stress tolerance of octaploid than tetraploid. Carbohydrate is not a direct antioxidant compound (Couée et al. 2006), but the storage carbon pool is always considered as an important indicator to predict stress responsiveness (Niinemets 2010). Hence, we also monitored the nonstructural carbohydrate content in two ploidy level of plants under favorable conditions. Both soluble sugar and starch were observed higher in the slow growing octaploid than in the fast growing tetraploid, and it may be reasoned that octaploids should invest more resource (such as the carbohydrate) on the antioxidant compounds synthesis than in tetraploid (Fig. 2b).

Now, we can use the low resource requirement hypothesis, combined with antioxidant defense theory, to illustrate the polytolerance and unequal elevation of stress resistance and some other interesting but puzzling problems here of polyploids compared to diploid plants.

Compared with favorable conditions, the nature of the abiotic stress challenge is also an event that includes the reduction of resource availability, such as CO2 assimilation, nutrient element uptake, water gain and light harvest. Based on the low resource requirement hypothesis, we can not only understand the multiple tolerances that are displayed in polyploid plants in comparison to their diploid counterparts, but also understand the significant enhancement of the nutrient deficit, cold and drought stress tolerance in comparison to the slight enhancement of shade and waterlogging tolerance (Fig. 2a). As for the case of significantly decreased resources, the octaploid can be tolerant for a relative long duration of low nutrient and water/CO2 conditions in comparison to the tetraploid due to the fact that the former has retarded metabolic activity and these resources can be re-used in the plant tissues. However, as for the shade and waterlogging stress (Causin et al. 2009; Blokhina et al. 2003), the profound reduction of light availability (from sun plant living conditions to shade plant living conditions) or ATP production (oxidative and fermentative respiration generate 36–38 and 2–3 mol ATP mol−1 glucose, respectively) would markedly attenuate the advantage of low resource requirements of the octaploid compared to the tetraploid plants, even for short durations of stress. This may be the reason why many reports exist with regards to enhanced cold, drought, salinity, UV and nutrient deficits (Sugiyama 1998; Chandra and Dubey 2010; Niwa and Sasaki 2003; Saleh et al. 2008), but fewer regarding shade or waterlogging stress tolerance in polyploids compared to diploid plants. It is in accordance with the fact that fewer plants would tolerant more than one of drought, shade and waterlogging stress challenges (Niinemets and Valladares 2006).

This low resource requirement hypothesis can also well explain the fast-growing cultivated crop plants with reduced stress (such as drought, low nutrient, pathogen and pest) tolerance compared with their slow-growing wild diploid counterparts (Zhang and Kirkham 1994), due to the fast growers have a high resource requirement and invest more resource on the biomass synthesis but not on defense compounds (especial the antioxidant compounds) synthesis.

In conclusion, some important and interesting findings can be summarized in here. Firstly, enhanced drought, cold and nutrient stress tolerance were observed significantly in octaploid than in tetraploid; Secondly, higher antioxidant capacity and low resource requirement both contribute to the enhanced stress tolerance of octaploid than tetraploid. In addition, low levels of H2O2 and more reduced redox state may contribute to the delayed ontogenesis of octaploid over tetraploid.

This work would give us a novel insight to understand the polytolerance and unequal elevation of stress tolerance of polyploid, from both resource availability and antioxidant defense perspective (Fig. 4). Some problems, such as the potential relationship of the gene dosage effect and the resource requirement capacity in stress resistance, are still required further investigation.

Fig. 4
figure 4

A hypothetical diagram to illustrate the “low resource requirement hypothesis” described in the text. Polyploidization of the diploid slows plant growth rate and biomass accumulation under the same time interval. These traits confer the polyploids low resource requirement and high antioxidant compound accumulation. Both these properties contribute to the total ROS level decline in the polyploids compared to their diploid counterparts during exposure to stress. For further details, please refer to the text

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