4. Chapter Four: Comparison of greenhouse and in-vitro screening of sweetpotato
4.4 Discussion
102
103 observations were reported by Rahimi et al. (2010) in their study of f leaf Ψw and relative WC under gradual drought stress in Plantago ovata and P. psyllium. Within five days after irrigation, the leaf Ψw was almost constant and varied between -0.7 and -1.0 MPa. However, there was a Ψw decline of -1.6 and -2.0 MPa after 8 days of irrigation and of -2.3 and -2.8 MPa after 10 days of irrigation for P. psyllium and P. ovata, respectively. The change of this Ψw was associated with the increase of solute concentration in the soil and resulted negative osmotic potential (Taiz and Zeiger 2006).
The control semi-solid MS medium with 30 g l−1 sucrose had a Ψw of -0.80 MPa. When this culture medium was supplemented with 0.4 M sorbitol and 0.012 M polyethylene glycol (PEG), the Ψw were -2.05 MPa and -1.30 MPa, respectively (Gopal and Iwama 2007). The dissolution of sugar in a solution of culture medium caused a Ψw decrease of the culture medium. In bananas, a culture medium supplemented with 0.09 M sucrose had a Ψw of - 0.37 MPa. When this culture medium was supplemented with 0.1 M and 0.5 M sorbitol, the Ψw shifted to -0.63 Mpa and -1.72 Mpa, respectively (Rukundo et al. 2012). These findings agree with the observation of this study in which under the in-vitro condition, the Ψw
decreased as the concentration of sorbitol increased at -0.07, -0.81, -1.35 and - 1.73 MPa for control (0.0 M), 0.2, 0.4, and 0.6 M sorbitol treatments, respectively. Both the greenhouse and in-vitro approaches showed the same trend; as the drought stress conditions increased, the Ψw decreased.
4.4.2 Effects of drought stress
4.4.2.1 Greenhouse experiments
Limited water availability at any crop growth stage causes unfavourable effects on growth.
These effects vary depending on the intensity of stress and the crop growth stage (Deblonde and Ledent 2001; Farooq et al. 2012). The results from this study corroborate with the above observations. There was a significant difference between plants grown on continuous irrigation and drought stress conditions (Figure 4.3). Plants grown under continuous irrigation showed high vegetative growth and dense green leaves.
In sweetpotato production a yield loss of more 60%, due to drought stress was recorded in South Africa (van Heerden and Laurie 2008). This loss was associated with a decreased of the aboveground biomass accumulation. Therefore, it was suggested that the genetic improvement for drought tolerance of sweetpotato has to consider these characteristics which include well-developed aboveground plant parts or vines (van Heerden and Laurie
104 2008). Saraswati et al. (2004) reported that plant biomass, main stem length, internode diameter, internode length, leaf number and area, and root weight decreased in response to water stress. Previous findings agreed with the results of this study. Plants grown on drought stress conditions revealed a loss of leaves and reduced growth. Continued irrigation showed the highest yield of storage root, vine and total biomass, weight of biggest root, root DMC and vine WC (Table 4.3). The effects of continuous irrigation were significantly different from other treatments (Table 4.3).
Previous studies showed that water regimes of 100, 80, 60, 40 and 30% of field capacity had significant effect on shoot growth and weight. The strongest effects were observed in plants grown on the 30% treatment where shoot fresh weight, shoot dry weight, and shoot length was reduced by 79, 7 and 76.0%, respectively, at the time of harvest (Van Heerden and Laurie 2008). In this study, highest reduction of growth and yield was observed on drought stress of three months. However, the effects of drought stress of three months were not significantly different from drought stress of two months (Table 4.3). This indicates that drought tolerance in sweetpotato genotypes could be investigated by applying one month of continuous irrigation for establishment followed by two months of drought stress under greenhouse conditions.
4.4.2.2 In-vitro tests
Effects of drought stress induced by 0, 15 and 25 % PEG in laboratory experiments caused a reduction of shoot, root biomass and plant height (Bayoumi et al. 2010). The increase of osmotic inducer in basal culture medium caused a decrease of callus induction and plantlet growth (Biswas et al. 2002). These findings agreed with results of the present study. T2
(control supplemented with 0.2 M sorbitol) were significantly different from T3 (control supplemented with 0.4 M sorbitol) and T4 (control supplemented with 0.6 M sorbitol) on FWG, DMC and WC (Table 4.6). The application of the same concentration of sorbitol (0.2 M) as an osmotic inducer on different varieties of banana plantlets showed that all varieties were negatively affected, but the degree of sensitivity varied significantly (Rukundo et al.
2012). The same observation was found in this study; the interaction between genotypes and treatment were significant for FWG, DMC and WC (Table 4.7). The control (T1) and control complemented with 0.2 M sorbitol (T2) did not show significant differences in fresh weight gain (Table 4.7). This indicates that the control supplemented with 0.4 M sorbitol could be the lowest concentration to be applied in screening for drought tolerance in sweetpotatoes under in-vitro condition suggesting more sorbitol concentrations in future studies.
105 4.4.3 Correlation between drought tolerance parameters during greenhouse
and in-vitro experiments
Selection for drought tolerance is complicated by the lack of fast and reproducible screening approaches, and constant water stress conditions to efficiently evaluate a large number of genotypes (Ramirez-Vallejo and Kelly 1998; Talebi et al. 2009). Thus, drought indices which provide a measure of drought based on yield and yield related traits under drought conditions in comparison to the control have been used for screening drought-tolerant genotypes (Mitra 2001). These indices are either based on drought resistance or susceptibility of genotypes (Talebi et al. 2009). Selection of different genotypes under stress conditions is one of the main tasks of plant breeders to develop stress-tolerant cultivars (Clarke et al. 1984). This study revealed that in all evaluated parameters under greenhouse and in vitro experiments (Table 4.9); only three parameters with positive correlations (vine weight and fresh weight gain, and total biomass) could be considered as major selection criteria to identify drought tolerant sweetpotato genotypes under greenhouse and in-vitro tests.