CHAPTER 4: RESPONSE OF SWEET POTATO (IPOMOEA BATATAS L.) CULTIVARS TO
4.4 Discussion and conclusions
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71 high number of leaves consequently increase the photosynthesis hub. Since the canopy represents the only source of biomass for subsequent partitioning to storage root, this would therefore, improve source sufficiency for yield attainment. Although the number of leaves was high, these cultivars did limit vine extension and branching which resulted into short bushy-growth. Bushy-growth translated to reduced leaf area index (LAI) which is characteristic of drought avoidance in plants (Blum, 2005). It is also possible that sweet potato plants may have developed a ramified root system (Jaleel et al., 2009) which allowed them to support above ground dry mass accumulation. This observation was similar to reports by Saraswati et al. (2004) that water stress reduced stem extension and internodes diameter of sweet potato cultivars; the authors did not mention any increases in leaf number but a decrease in LAI. Leaf size should be of main focus in sweet potato as earlier research reported that it had a negative correlation with apparent photosynthesis (Bhagsari and Brown, 1986).
Cultivars with small canopies, short stem length and small leaves were reported by Wilson (1982) to have the capability of giving higher yields than those with long stems and numerous broad leaves. This was later confirmed by Saraswati et al. (2004). Plants grown under 100%
ETc had low leaf number but longer vines and more branches. This did not however increase total biomass, instead total biomass was increased in plants grown under 30% ETc.
Storage roots yield recovered from the 30% ETc was higher than in 100% ETc even though the plants under 30% ETc had a lower LAI. Plants grown under 100% ETc may have been experiencing rank growth as reported in cotton (North Carolina State University 2012) where biomass partitioning favoured vegetative than storage root growth. At low water availability (30% ETc), the small canopy tended to partition assimilates more towards storage roots thus higher storage root yield. Generally, all yield parameters were higher at 30% ETc than 100%
ETc. Based on these results, the sweet potato cultivars were more drought tolerant. Reports by Mohankumar (2000) also concur with the fact that deficit irrigation strategies such as irrigating sweet potato plants once they had depleted 40% of available soil water tended to give higher yields. Withholding water until 60 days after planting (DAP) was also recommended in order to provide translocation of more photosynthates to the roots and the development of storage roots (Ekanayake et al., 1990). Another contributing factor to the high yield under water stress in this experiment could be the fact that water stress was constant throughout the growth cycle such that plants adapted to low supply of water and maximized its production. This means that if low levels of water are evenly distributed, sweet potato will give higher yields. While 30% ETc was a very low amount of water, the fact that it was
72 applied daily ensured there was some water as opposed to no water at all. Unlike in the field where plants will be subjected to slowly developing water deficit and sometimes subjected to near dryness. Under such conditions drought stress can strike the crop at the most vulnerable growth stage, thus affecting growth and subsequent yield. It would be advisable for farmers to alter their planting dates so that it coincides with even distribution of rainfall and the mid- summer drought coincides with the growth stage that requires a bit of water stress as reported by Mohankumar (2000). The yield results also suggest that these cultivars can be grown in marginal areas, as long as there is even distribution of the low water supply.
When comparing cultivars, it was observed that cultivar 199062.1 was high a yielding cultivar despite water stress. It also displayed the best growth characteristics in terms of branching and number of leaves. Moreover, it contained high CCI and responded to drought stress by reducing SC. These are some of the attributes that are highly appreciated when breeding cultivars for drought tolerance. Cultivar A40 was expected to give higher storage yield since photosynthesis was not substrate constrained due to higher SC results. However, enzyme kinetics does concur with the response of cultivar A40 that if there is excess substrate, enzymatic reactions are limited (Reed et al., 2010). In this case it is suspected that the high levels of CO2 became the source of inhibition to photosynthesis thus the low yields. Cultivar A45 on the other hand was mostly impartial to these treatments.
Physiological traits of the sweet potato plants reported in this experiment indicated that it is indeed drought tolerant. Stomatal conductance of these cultivars was very low, mostly below the 100 mmol m-2 s-1 threshold. To avoid metabolic impairment at such low SC values, the cultivars resorted to photorespiration. Growing these cultivars under 30% ETc gives similar results of RWC suggesting that the cultivars are drought tolerant. The cultivar (A40) with high SC but low CCI values indicated substrate inhibition for photosynthesis, which is why it gave lower yields. Water stress limitation to plants growth was to a lesser extent. Leaf number was the only parameter reduced by water stress. This consequently reduced its canopy indicating drought stress avoidance mechanism but gave higher yields than plants with long stems and numerous broad leaves grown under 100% ETc. This indicates that these cultivars are suitable for production under marginal areas as long as there is even distribution of the low water supply. Optimum water supply (at 100% ETc) encourages rank growth on these cultivars, this encourages famers to alter their planting dates such that the stages of growth where plants have to be exposed to water stress coincides with mid-summer drought to break
73 the rank growth syndrome and increases storage root yield. Cultivar 199062.1 was more tolerant to drought and gave higher yield than the other two cultivars.
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