Sweetpotato [Ipomoea batatas (L.) Lam] is a herbaceous, perennial plant that is grown mainly for its edible storage roots, although the leaves and tips are consumed as a green vegetable (Villareal, 1982; Woolfe, 1992). The storage roots are high in carbohydrates, beta-carotene (orange-fleshed cultivars), vitamins B1 (thiamine) and C, iron (Fe), and phosphorus (P) and dietary fiber (Hill et al., 1989; Woolfe, 1992). The leaves are high in vitamins B2 (riboflavin), Fe, and P. Because of its versatility, high energy concentration, nutritional composition, and the potential of using the tender shoots as a green vegetable, the crop is among the list of staple and vegetable crops selected by the National Aeronautics and Space Administration (NASA) to provide food for space travelers on extended missions (Salisbury and Clark, 1996; Tibbitts and Alford, 1982; Waters et al., 2002).
The absorption of light by plants is a component in regulating plant growth and development. In addition to irradiance, daily light period also influences the growth and yield of sweetpotatoes. As part of the Exploration Life Support System program, NASA testing has focused on light period responses of candidate species, the rationale being that if plant growth and storage root yield could be increased with longer daily light periods, the total crop area required to sustain humans in a life support system could be reduced (Wheeler, 2006).
Porter (1979) reported greater storage root yield and McDavid and Alamu (1980) reported that longer light periods at similar total irradiance promoted leaf expansion and larger leaf area among plants grown at light periods between 11.5 and 12.5 h compared with 8-h or 18-h periods. In contrast, Biswas and Mukhopadhay (1989) reported significant increases in the number of storage roots among plants exposed to short light periods (9 h), whereas vine yields were greater under longer daily light periods (16 h). Bonsi et al. (1992) grew sweetpotatoes under 24-h or 12-h light periods in soilless culture and reported greater biomass production with continuous light compared with plants grown at 12-h light periods, which had fewer but larger leaves. Previous studies on the interaction of irradiance and N:K ratio on sweetpotato responses have shown that irradiance exerted a greater effect in increasing storage root yield than did N:K ratio (Mortley et al., 1993).
Although longer light periods have enhanced storage root yield and some cultivars are more sensitive than others (Mortley et al., 1996), the question still remains as to whether enhanced growth responses are the result of increased daily light period or the increase in the number of photosynthetic photons to which the plants are exposed irrespective of daylength effects.
Our objective was to evaluate the effect of irradiance and daily light period on storage root yield and leaf elemental concentration of two sweetpotato cultivars grown hydroponically using the nutrient film technique (NFT).
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