Dry-season deficit irrigation increases agricultural water use efficiency at the expense of yield and agronomic nutrient use efficiency of Sacha Inchi plants in a tropical humid monsoon area

Results showed the growth and root to stem mass ratio had lower sensitivity responded to DI, probably owing to their extremely low root mass fraction and seasonal short-term effect of DI on leaf photosynthetic traits. Irrigation affected the seasonal variations in seed size, seed oil concentration and seed yield, depending on the harvest date; whereas, with constant mean seed size and mean seed oil concentration across irrigation and fertilization treatments, the total seed and seed oil yield over the growing seasons were largely determined by the seed numbers per unit area. The soluble sugar and nitrogen storages as the active process, are related to effective flower formation, fruit (seed) development and enhance productivity of Sacha Inchi plants, which was indicated by the positive relationships between total seed yield and total nitrogen pool in the vegetative tissues or sugar pool in stems across all treatments. Fertilization increased total seed and seed oil yield, but no interaction between irrigation and fertilization was found. Compared with DI100, DI50 and DI20 had significant lower total seed yield, especially under the fertilized condition, although having higher agronomic water use efficiency (WUEagr, yield divided by irrigated water applied) but lower agronomic nutrient use efficiency (NUEagr, increased yield divided by fertilization rate). As a water-demanding crop species, Sacha Inchi plants under DI100 with the similar values to full irrigation had the highest total seed yield and NUEagr, but at the expense of water use efficiency. The maximum seed yield and maximum WUEagr, or maximum WUEagr and maximum NUEagr of Sacha Inchi plants are not compatible because the negative relationships existed between each of them. The polynomial regression relationships between total seed or seed oil yield and relative evapotranspiration could help to develop appropriate water-saving techniques for Sacha Inchi plantation in the tropical humid monsoon region.