Yield and Number of Tubers Produced per Plant
Drought has been shown to have a negative impact not only in diverse physiological processes including photosynthesis and respiration, but also on parameters of agronomic importance such as yield (Levy 2014; Yordanov et al. 2000). Average yields per plant differed significantly between treatments (P<0.05) (Fig. 1) with an average yield of 1059 g per plant for the well-watered control, whereas it was 611 g in the early bulking treatment (T1), and 813 g in the late bulking treatment (T2) (Fig. 1; Table 1). The average number of tubers produced per plant was also reduced (P<0.01) from 35 tubers in control plots to 21 in T1 and to 23 in T2, respectively (Fig. 1; Table 1).
Significant differences were also observed between varieties in the number of tubers produced (P<0.01) and yield (P<0.01) (Fig. 2). The average yield per plant in control plots ranged from 402 g for Michuñe Roja to more than 1.8 kg for Désirée, whereas the number of tubers ranged from 17 for Michuñe Blanca to 59 for Cabra Roja (Fig. 2; Table 1). Cabra Roja was the genotype most affected by early bulking stress (T1) achieving only 29% of the yield of the control treatment (Table 1). In contrast, the commercial variety Désirée performed well under T1 keeping 83% of the yield. The responses of the different varieties were also contrasting when the drought occurred during late bulking (T2). Cabra Roja was again the most penalised genotype achieving 37% of the yield of the control conditions (Table 1). The variety Désirée showed a yield comparable to the control conditions, despite producing less tubers (78%), confirming its known tolerance to drought stress.
The landrace Michuñe Roja performed well under drought 88 DAP maintaining 80% of the yield compared to well-watered conditions, even though its absolute yield was significantly lower compared to the other landraces. Concerning the treatment 110 DAP, the native genotypes Michuñe Azul and Quila performed well by maintaining respectively 93% and 89% of the yield of the control (in weight).
Impact of Drought on Bioactives
Drought treatments had a detrimental effect on yield and tuber production. However, they did not impact the concentration of bioactives and antioxidant activity in the tubers (Tables 1 and 2). As a result, the production of compounds estimated per area cultivated (ha) was significantly reduced (P<0.01) as shown in Table 3.
The percentage of dry matter in the tubers was significantly different between genotypes, both raw and boiled (P<0.01), with values that ranged in the boiled samples between 16% (Michuñe Azul) and 27% (Chona Negra) (Table 1).
Resistant and Non-resistant Starch
Genotypes differed significantly in their total starch content in both raw and boiled samples (P<0.01) (Table 2). In raw tubers, total starch content ranged from 34% of the dry weight (Michuñe Roja) to 54% (Chona Negra), while in boiled tubers, it ranged from 36% (Michuñe Azul) to 60% (Quila). On average, boiled samples displayed a higher total starch content than raw samples (+15%). The relationship between total starch of raw and boiled samples was significant (P<0.01) but with a relatively low R2 = 0.38.
Resistant starch was the main form of starch found in raw samples (81% of total starch), whereas non-resistant starch was predominant in boiled samples (89% of total starch) (Table 2). Similar values, between 71 and 87% of RS for raw potatoes, were reported by Bach et al. (2013). Starch content can vary generally between 10 and 19% of the fresh weight for commercial varieties (Bethke 2014; Schwärzel et al. 2016), whereas values in dry weight range from 61.5 to 75.8% DW, as reported for the varieties Imilla Negra and Kufri Bahar respectively (Negi and Nath 2002; Burlingame et al. 2009; Jiménez et al. 2009). Our measurements on raw potatoes showed starch values between 36 and 51% of the DW, which are significantly lower. Different cultivation practices, postharvest conditions and methodological analyses may be partially responsible for these contrasting values with literature. This is especially relevant when the natural diversity of an agronomic trait is being assessed. By subjecting all varieties to the same methodology, this study provides a good example of the diversity in starch content within Chilean potato landraces.
Resistant starch content was significantly different amongst genotypes (P<0.01), with values in raw samples between 25% DW (Michuñe Azul) and 44% DW (Montañera). This is in contrast with the uniformity observed of RS in commercial varieties as reported by Raatz et al. (2016) and highlights the importance of assessment of RS on native potatoes. Once boiled, resistant starch ranged from 4.5% DW for Montañera to 7% DW for Chona Negra, a landrace that was consistently on the higher range of resistant starch content compared to the other genotypes. The effect of boiling of tubers on resistant starch agreed with literature and no correlation could be found between the resistant and non-resistant starch contents of raw and boiled potatoes (R2 < 0.1) (Fig. 3e, f).
Total Phenolic Compounds
TPC content of raw tubers ranged from 0.8 mg GAE/g DW for the landrace Michuñe Blanca to 13.3 mg GAE/g DW for the commercial variety Désirée (Table 1). After boiling, the content of TPC decreased significantly for all genotypes (83% on average). With a TPC range from 67 to 1330 mg/100 g DW, our results are comparable to previous reports for raw samples’ TPC as reported by Kita et al. (2015) and Andre et al. (2007). Regarding boiled samples, values from 23 to 80 mg GAE/100 g in our samples were in a lower range than those reported by Xu et al. (2009) (80–224 mg GAE/100 g DW). Interestingly, the variety Désirée exhibited a higher TPC compared to the native genotypes in raw samples; however, most of these phenols (97%) were lost during boiling (Table 1). This commercial variety also behaved as an outlier in Inostroza-Blancheteau et al. (2018), with a chemical composition different from the Chilean landraces. The contrasting effect in TPC content after boiling between Chilean potatoes and the variety Désirée, added to the findings of Inostroza-Blancheteau et al. (2018), suggests that significant modifications in the chemical composition of tubers in modern varieties, compared to their original native varieties, may have taken place as a result of selection in breeding programs.
Considering only Chilean landraces, there was a strong correlation between TPC of raw and boiled samples (R2 = 0.65, P<0.01) (Fig. 3b). Additionally, TPC and anthocyanins of raw and boiled samples were highly correlated (R2 = 0.9 raw, R2 = 0.75 boiled, P<0.01) (Fig. 4a, b). The lower correlation in boiled samples suggests changes in the contribution of anthocyanins to the TPC in our samples. Interestingly, raw Désirée tubers had a high TPC content but very few anthocyanins. If polyphenols are the main contributors of AA in raw samples but are lost by leaching during boiling, it is possible that other compounds increase their contribution to the AA of boiled samples. For example, the thermic process has been shown to induce the hydrolysis of some glycosylated antioxidants, which will then be more active in their free form (Xu et al. 2007; Navarre et al. 2010; Andre et al. 2014).
Several authors describe potatoes as good sources of anthocyanins, especially native genotypes from South America (Brown et al. 2007; Lachman et al. 2009; Burgos et al. 2013; Tierno et al. 2015; Calliope et al. 2018). Anthocyanins in potato tubers have been reported in a range from 0 to 153 mg/100 g DW (Brown et al. 2007; Giusti et al. 2014), well within the range of the results from our study (0 to 97 mg/100 g DW).
Anthocyanin content was significantly different amongst landraces (P<0.01) ranging from 0 to 97.1 mg C3GE/100 g DW in raw samples and from 0 to 11 mg C3GE/100 g DW in boiled samples (Table 1). Flesh colour seemed to be a good indicator of tuber anthocyanin content (P<0.01) as described by Calliope et al. (2018) (Fig. 5b). Indeed, the dark blue–coloured genotype Chona Negra showed the highest content in anthocyanins, whereas the very light-coloured landrace Quila showed the lowest content (Table 1).
Anthocyanins decreased 84% on average in all genotypes after boiling. The ranking of the genotypes according to their content of anthocyanins did not change after boiling and the landraces Chona Negra and Cabra Roja showed the highest values (Table 1).
A high correlation was found between anthocyanin content of boiled and raw tubers (R2 = 0.9, P<0.01). Therefore, anthocyanin content in boiled tubers (y) could be well estimated from the raw content (x) as determined by the formula y = 0.448 + 0.128x (Fig. 3a).
ORAC was significantly different amongst genotypes (P<0.01) and between raw and boiled samples (P<0.01). However, it did not change significantly as a result of the drought treatments in the field (Table 1). Antioxidant activity in raw material ranged from 1380 μmol TE/100 g DW for Quila to 9913 μmol TE/100 g DW for Chona Negra. Boiling decreased ORAC on average 62% for all genotypes. The variety Désirée showed the lowest ORAC when boiled (554 μmol TE/100 g DW) while Chona Negra remained as the genotype with the highest ORAC (4177 μmol TE/100 g DW). Andre et al. (2007) reported ORAC values between 28 and 251 μmol/g DW for raw samples of 74 Andean potato cultivars. A similar range was observed by Navarre et al. (2011) and Brown et al. (2005).
Considering only the Chilean landraces, the correlation between ORAC raw and boiled was significant (P<0.01), although with a low R2 = 0.31 (Fig. 4c). This suggests that AA of raw potatoes is not a good predictor of antioxidant capacity of boiled material. This is especially relevant when health properties are inferred based on the antioxidant activity of raw tubers. There was a significant correlation of antioxidant activity with TPC and anthocyanins in tubers as reported by other studies such as Lee et al. (2016). In particular, a high correlation was found in raw samples between ORAC and TPC (R2 = 0.71), as well as between ORAC and anthocyanin content (R2= 0.67). The same trend was shown after boiling, with a R2 of 0.39 between ORAC and TPC and of 0.38 between ORAC and anthocyanins (Fig. 4c, d, e, f).