ABSTRACT This study evaluated the effects of the rates and timings of prohexadione-calcium (prohexadione-Ca) application on shoot growth, tuber yield, and quality of two potato cultivars (Agata and Mondial). An experiment with each cultivar was carried out in a greenhouse using a randomized block design with four replications. Treatments consisted of a control and single or sequential foliar applications of prohexadione-Ca at the rate of 50 or 100 g ha-1 (50 g ha-1 applied once, 50 g ha-1 applied twice, 50 g ha-1 applied three times, 100 g ha-1 applied once, 100 g ha-1 applied twice, and 100 g ha-1 applied three times). Prohexadione-Ca application changed the distribution pattern of tuber size in both cultivars. A rate of 100 g ha-1 prohexadione-Ca was applied three times between the stages of tuber initiation and tuber bulking, making the haulm length of the plants short for a long time. Prohexadione-Ca application did not alter the tuber yield and quality of the cultivar Agata, but 50 g ha-1 applied three times or 100 g ha-1 applied twice increased tuber yield in the cultivar Mondial. Only in the cultivar Mondial, 50 g ha-1 prohexadione-Ca applied three times increase tuber dry matter, starch, and protein contents.

ABSTRACT The excess of nitrogen (N) causes the overgrowth of sweet potato foliage, increasing the self-shading and reducing the root yield. Therefore, a combined N and paclobutrazol (PBZ) application can reduce the vegetative overgrowth and benefit the sweet potato yield. The purpose of this study was to evaluate the plant growth, yield, uptake and removal of N by sweet potato plants fertilized with N and treated with PBZ during two planting seasons. The treatments consisted of four N rates (0, 50, 100 and 200 kg·ha–1), three forms of PBZ application (PBZ-10: PBZ applied at 10 days after side-dressing N fertilization [DASNF]; PBZ-25: PBZ applied at 25 DASNF, and PBZ-10 + 25: PBZ applied at 10 plus 25 DASNF), and a control (without PBZ application). Paclobutrazol application temporarily reduced the length of the main branches of sweet potato planted in the rainy season, but it did not reduce the shoot biomass of plants in both planting seasons. A single application of PBZ at 10 DASNF increased the yield of fresh storage roots in the two planting seasons, but, during the rainy season, the increases were greater when two applications were carried out. The application of 50 kg·ha–1 N in the rainy season was sufficient to reach the maximum yield of fresh storage roots, but, in the dry season, the N fertilization increased the N uptake without benefiting the root yield.

ABSTRACT Sweet potato crops take up large amounts of nutrients, especially nitrogen. In low-fertility soils, the addition of nitrogen (N) increases the sweet potato yield. Green manure may be an alternative method for improving soil quality and supplying nutrients to this crop. This study aimed to evaluate the plant’s nutritional status and the amount of nutrients taken up and removed by sweet potato plants subjected to green manure and mineral N fertilization. The experiment was carried out in the field for two growing seasons using a randomized block design in a split-plot scheme with four replications. The plots consisted of a control treatment (spontaneous weeds) and the previous cultivation of Crotalaria spectabilis and Mucuna aterrima. The subplots consisted of four N rates (0, 50, 100, and 200 kg ha-1) that were applied to the sweet potato. The species M. aterrima is more suitable for use as green manure in the sweet potato than C. spectabilis. Nitrogen application rates promoted a greater increase in the biomass of the storage root, nutrient uptake, and removal in the sweet potatoes unfertilized with green manure. In the sweet potato fertilized with M. aterrima, mineral N supply in excess (above 50 kg ha-1) increases the nutrient uptake and removal without a significant increase in the biomass of the storage root. In the sweet potatoes unfertilized with green manure, high rates of N (greater than 120 kg ha-1) must be applied to obtain the utmost biomass of the storage root, nutrient uptake and removal.