Beetroot is a cool weather crop, produced for its leafy greens and roots. It is rich in active compounds such as carotenoids, glycine, betaine, betacyanines, folates, betanin, polyphenols, and flavonoids (de Zwart et al., 2003; Dias et al., 2009; Vali et al., 2007). Beetroots can be consumed together with other vegetables in the form of salads, or used commercially as a dye to color processed food. The roots of beetroot are also used as a nutrient supplement and as a canned product.
Nitrogen is one of the most important minerals in determining plant growth and crop yield. It is a component of proteins, enzymes, and vitamins in plants as well as many structural, genetic, and metabolic compounds (chlorophyll and amino acids) in plant cells required for biomass synthesis (Petek et al., 2012).
Most farmers apply N fertilizer to increase crop yields. Insufficient supply of N can decrease plant N content, reducing photosynthate production (Boussadia et al., 2010) and thus, reduce plant growth and decrease quality of harvestable materials (Mikkelsen and Hartz, 2008). An increase in plant growth characteristics such as plant height, shoot dry matter, and leaf area index in most crops was reported to be due to N application (Najm et al., 2013). The optimal application of N fertilizers positively impacts production. However, oversupply of N does not always lead to increased yield, and it might actually result in reduced growth and yield. Excessive application of N fertilizers leads to delayed maturity and competition between sink (tubers) and source (leaves), and occasionally lower yields (Najm et al., 2013). Excessive N application does not only delay plant maturity, but limits the formation of storage organs, especially for crops where roots and tubers are harvested. This results in lower yields and can reduce the quality of storage organs in terms of flavor and physical characteristics due to low sugar content, lower acidity, and reduced firmness (Johnson, 2014). The nutritional value of the crop will also be negatively affected by causing high nitrate accumulation (Rahn, 2000).
Leaves are responsible for providing the plant with photosynthates during the entire growing season. However, leaf removal can negatively affect plant growth, yield, and quality of beetroot. Isutsa and Mallowa (2013) reported that pumpkin (Cucurbita moschata) leaf and fruit yields were affected by leaf-harvesting intensity. Excessive leaf removal will result in a reduction in the production of photo-assimilates required for plant growth (Isutsa and Mallowa, 2013; Kiozya et al., 2001), thus affecting the yield negatively.
There is a demand for beetroot leaves as well as roots for consumption due to their nutritional value (Dias et al., 2009; Vali et al., 2007). Beetroot leaves are generally harvested at the end of the growing season together with the roots. South Africa is also promoting the intake of nutritious leafy vegetables to alleviate nutritional deficiencies, thus reduce disease especially in rural communities (Faber et al., 2007). In spite of the importance of beetroot as a leaf and root crop, there is limited information on procedures for leaf harvesting that growers should follow to maximize yield and quality of beetroot, unlike for other crops, such as pumpkin (Isutsa and Mallowa, 2013) and sweetpotato [Ipomoea batatas (Kiozya et al., 2001)].
There is also limited information on how leaf harvest percentage will affect beetroot production and how N management may modify that effect. Beetroot has the ability to generate new leaves, and the percentage of how many leaves could be harvested early without reducing harvestable yields (leaves and roots) at the end of the growing season is not known. An understanding of the plant’s response to leaf harvest and N application is important in developing improved cultivation practices for increased beetroot yield and nutritional quality. Therefore, the aim of this study was to determine the effect of N application and leaf harvest percentage on the yield and nutritional quality of roots and leaves of beetroot.
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