2017 ASHS Annual Conference

Tropical Plant & Soil Sciences 610 (Plant Mineral Nutrition in the Tropics) was taught during Spring 2016 as a hybrid course (asynchronous online sessions combined with a face-to-face laboratory). Three teams of graduate students conducted aeroponic studies on sweetpotato [Ipomoea batatas (L.) Lam. var. batatas ‘Okinawan’] for six weeks in the greenhouse at the University of Hawaii (lat. 21°18’N long. 157°48’W), beginning on 29 January 2016 and harvested on 11 March 2016. The first team examined effects of six levels of nitrogen (0, 1, 2, 3, 4, and 5 mM N) in two blocks; the second team studied effects of four levels of potassium (0, 1.6, 3.2 and 6.4 mM K) in three blocks; and the third team looked at effects of four levels of phosphorus (1.0, 4.8, 9.7, and 19.0 mM P) in three blocks. The experimental design followed a randomized complete block design. Two cuttings of sweetpotatoes (with at least three nodes) were placed into 27.4 L polypropylene storage containers with 6 L of nutrient solution and a venturi mister. At harvest, plants were separated into leaves, stems, and roots, and then leaf areas, root lengths, and fresh and dry weights were determined. In addition, leaves were analyzed for nutrient concentrations. Increasing N levels in solution resulted in increasing fresh weights of sweetpotato leaves, stems, and roots from 0 to 3 mM N, and then decreasing fresh weights as levels of N increased. Interestingly, partitioning of fresh weight biomass to roots decreased linearly as levels of N increased. Also, at the 0 mM N level, roots exhibited purplish coloration, perhaps indicating initiation of storage root formation. Increasing K levels (from 0 to 6.4 mM K) resulted in increasing fresh and dry weights of plants, and total leaf area. Increasing K levels up to 3.2 mM K resulted in increasing total root lengths followed by a decrease at the highest K level. Increasing P levels above 1.0 mM resulted in P toxicity effects with significantly decreased fresh and dry weights of roots and leaf areas. Results of this laboratory experiment demonstrated that response of root growth to various nutrients differed from those of leaves or stems, and that ignoring the ‘hidden half’ of plants could result in different conclusions regarding optimal levels of nutrients.