Understanding agronomic factors that affect the initiation and development of sweetpotato (Ipomoea batatas (L.) Lam.) storage roots: The role of nitrogen fertilisation and organic soil amendments

Sweetpotato (Ipomoea batatas (L.) Lam.) is a major root crop that is widely grown throughout the world. In Australia, sweetpotato is one of the few very profitable and rapidly expanding vegetable crops. The sweetpotato industry is experiencing remarkable growth, with rapid growth in recent decades. However, storage root (SR) size and shape are important for consumer acceptance in developed countries, including Australia, where SRs with an excessively large or small size are rated as low quality with low commercial value. This determines the marketable yield and profit of growers. Agronomical factors have been reported to influence initiation and development of SRs, which affects the size, number and evenness of SRs over time. Environmental factors cannot be manipulated on a large scale. Therefore, this project focused on the influence of nitrogen (N) fertilisation and some local organic amendments to these processes. Also, the chemical changes inside the plants, including soluble sugar and starch, as well as the accumulation of N, were examined during SR initiation. Preliminary experiments were conducted using different soilless cultures to investigate the development of SRs. Three growing methods, nutrient film technique, fine sand and washed (coarse) river sand, were utilised. Results demonstrated that sweetpotato could be grown in the nutrient film technique systems as some SRs formed in the system. However, the observations of SR initiation and development were obstructed as SRs developed under the nutrient film and formed odd shapes. Similarly, fine sand culture was not suitable as it resulted in delayed SR formation and promotion of lignified roots. Washed river sand culture was suitable for SR initiation and development, and was utilised in further studies to examine the initiation of SRs. Results suggested that both deficient (N0) and high (N200) rates of N inhibited the formation of primary cambium at 10 days after transplanting (DAT) and then anomalous cambium (AC) at 21 DAT. Therefore, the initiation of SRs was delayed in those N conditions. Both N0 and N200 treatments had a significantly lower SR rate at 21 DAT whereas both 50 and 100 mg/L N treatments promoted the formation of SRs during this period. Application of N at 100 mg/L was optimal for SR formation as suggested by the highest percentage of SRs during 21 and 56 DAT. In this experiment, treatment N200 had the highest percentage of roots with cambium development in the earliest observation and the highest percentage of initiated SRs between 21 to 56 DAT. This treatment also had the highest starch accumulation in roots during the first 35 DAT. However, sweetpotato required more N after SR formation as indicated by faster growth, higher N acquisition, highest efficiency of N use after 35 DAT and higher carbohydrate accumulation in roots in the N200 treatment. This study indicates that moderate N fertilisation level should be maintained for a few weeks to promote SR formation, and then further N fertiliser should be applied to improve SR development. A pot trial was conducted for Orleans variety to evaluate the effects of different N fertilisation timings on SR initiation, the accumulation of non-structural carbohydrates in plants and the acquisition of N in sweetpotato during the formation of SRs. Both no and delayed N application till 14 DAT inhibited the formation of regular vascular cambium (RVC) and AC during the early stage of adventitious root development. Then, those treatments promoted the lignification of stele cells, resulting in a significantly higher rate of lignified roots. However, plants supplied with N within the first week after transplanting demonstrated significantly higher rates SRs and lower rates of lignified roots. Furthermore, earlier N application promoted plant and root growth as indicated by higher biomass and SR weight, more non-structural carbohydrate and N accumulation in plants, and higher N recovery efficiency. The study indicated that moderate N should be available in soil before or on planting day to promote SR initiation. A pot experiment was set up to investigate the impacts of poultry manure (PM) and sugarcane trash (SCT) on available soil N and total N in dermosol soil and then examined the effects of these products on the initiation of Orleans sweetpotato SRs. The PM treatments increased soil available N and total N whereas SCT application had significantly lower available N in soil. The SCT application at both rates promoted SR initiation and reduced lignification compared to PM and chemical fertilisers. By contrast, both PM applications inhibited the initiation of SRs and promoted lignified roots. Application of PM at 66 tons ha-1 enhanced vine growth and reduced root growth. Both SCT applications maintained shoot growth and promoted root growth. This indicates that SCT should be used for sweetpotato to promote SR initiation and high PM rates should not be applied to sweetpotato as they inhibit SR initiation. This result is in line with our first pot experiment and demonstrate soil organic amendment affects sweetpotaot SR initiaion by changing soil available N. Our study provides agronomic indication that moderate N supply level should be maintained from planting for a few weeks to promote SR initiation, and then another application of N supplied to improve SR development. Soil organic amendments can affect SR initiation due to their impacts on N availability in soil. Therefore, the type and rate should be carefully considered before application.