Quinoa: A super or pseudo-super crop? Evidences from evapotranspiration, root growth, crop coefficients, and water productivity in a hot and semi-arid area under three planting densities

Quinoa is well known for its great ability to tolerate water stress. However, very little information is available about its potential growth under full irrigation particularly in hot and semi-arid regions. In this experiment, a newly-released quinoa (cv. Q5) bred for hot and dry regions was grown under three planting densities (PDs) of 150,000, 185,000, and 270,000 in the drainable lysimeters, south of Iran. The highest and lowest grain yields were observed in the middle and low PDs of 3.65 Mg ha(-1) and 2.86 Mg ha(-1), respectively. Quinoa showed very high crop evapotranspiration (ETc) and transpiration (T) rates. ETc and T varied in the range of 1448-1687 mm, and 777-1228mm among the PDs, respectively. These high values resulted in high single crop coefficients (Kc) that overall varied between around 1 and 2.4 during the growing season. The basal crop coefficient (Kcb) of the dual Kc (Kc = Kcb + Ke) reached about 1.9 and 1.2 in the high and low PDs, respectively, indicating high transpiration capacity. The main reasons of high Kc and Kcb were high soil evaporation rate due to very frequent irrigations of 3-4 days and soil wetting, and the prevailing regional sensible heat advection that increased transpiration. It was concluded that quinoa has a specific physiological systems that transpire continually for allowing better leaf cooling at high temperature, which results in high water use. Moreover, a vigorous root system that extended down to 1.2 m with high root length densities in the deep layers (RLD > 1 cm cm(-3)) helped quinoa to supply the water use. This extensive root system down to 1.2 m could help to increase irrigation interval and reducing soil evaporation. However, the effect of PD on the root length and root mass was mainly observed in the top 40 cm, below which its effect diminished and root length and root mass were nearly identical among the PDs. Overall, it is concluded that quinoa Q5 is a super crop that not only can tolerate water stress, but also can potentially grow well and produce acceptable grain yield in the hot and semi-arid areas. Adapting appropriate PD and irrigation management such as drip irrigation (surface and subsurface), mulching, increasing irrigation interval attributed to the deep rooting system, and water-saving irrigation managements would substantially reduce soil evaporation and increase water productivity.