Carbohydrates as a Nitrogen Use Efficiency Indicator in Cassava

Cassava, a root crop that feeds numerous people worldwide, is a traditional root crop that requires standards for physiology-based fertigation (nutrient application through irrigation), especially under intensive cultivation. Accordingly, the effects of 10, 40, 70, 100, 150, or 200 mg L^-1 nitrogen (N) fertigation on growth and root yield of cassava were studied in relation to carbon and water relations. It was observed that from 10 to 70 mg N L^-1 cassava increased photosynthesis and transpiration, but only to support the canopy’s growth. At 100 mg N L^-1 cassava reached a threshold of sugar in the leaves (~47 mg g^-1), began to accumulate starch, and supported higher root yields. At 200 mg N L^-1 the canopy became too demanding and the plants had to restrain transpiration, reduce photosynthesis, loose carbohydrates, and lower root yields. We concluded that there are three phases of cassava’s response to N that influence root yields: 1) growth that does not support root yields at low N, 2) productive N application, and 3) excessive use of N that depresses root yields. While traditional assessments of N in the leaves failed to exhibit these responses, a simple and inexpensive carbohydrates analysis can determine whether N application is inefficient (possibly reducing farmers’ profits) or is detrimental to root yields and a risk to the environment.