A project undertaken at the Papua New Guinea National Agricultural Research Institute, and supervised by Joel Pilon
Acridity, the itchiness trait in taro (C. esculenta) including other species in the aroid family probably evolved with cultivation systems, fertility status of soil and abiotic stresses pose by environmental interactions. The challenge of minimizing acridness in taro to acceptable limits is been championed by the agrarian societies over many years, yet requires scientific rigor in better understanding the physiological mechanisms involved, and thereby expedite the crops improvement in agronomy, breeding and food processing.
Figure 1. NARI breeding line (NARI TARO 2) that has moderate acridity level, tested on non water logged land.
Figure 2. Numkowe a Local TARO Cultivar with less moderate acridity level, tested on non water logged land.
This important research funded by APSF grant – No 1701 was conducted between July 2017 and October 2018 at PNG National Agricultural Research Institute Momase Regional Centre, Lae, Papua New Guineae. The research involved evaluating taros grown under flooding, rainfed and ridge/furrow with distinct soil water holding capacity, and observing the effect these production systems had on taro oxalate synthesis. In addition to that, oxalate response to increasing levels of N-fertilizer (urea) was assessed. The project endeavored to deliver fundamentally on methodologies and techniques of the above factors which are thought to be responsible for influencing the oxalate levels in taro.
A series of four studies were conducted. The first study revealed that taro cultivated under flooding system accumulated more oxalates than the other two. Further, oxalate distribution along the corm segments (from top-middle-bottom) was not uniform between the different cultivation systems and does suggest that cultivation systems uniquely influence the distribution of oxalates. Insoluble oxalates constitute more in the corms and vary with cultivars.
Moreover, doing the sensory evaluation on the corms, panellists were able to blindly agree to the oxalate trends observed above through independent lab procedures. Ridge cultivation is shown to be the most preferred system with high acceptability score for the taro corms.
With the second research study, taro oxalate was quantified following periodic N-fertilizer applications at corm development and corm maturity stages during two cropping periods and under different N-fertilizer rates. The work showed that N-fertilizer did not influence oxalate build up during corm development and the maturity stages. However, N drastically reduced total and insoluble oxalate and very less for the soluble oxalate. Cropping year significantly has effects on total oxalates and soluble oxalates but not insoluble oxalates. The present result is explained by ammonium (NH4+) activity in plants which may have served as a negative signal to inhibit oxalate accumulation.
The third study took into consideration the oxalate level under drought as it is thought to accumulate and degrade under such conditions. The unveiling of oxalate levels under drought conditions coupled with back up information on drought responsive traits in taro is important. Hence, this study assessed oxalate under different drought induce condition with the aim of quantify oxalate content at different soil field capacity. The study revealed about 60% reduction in the three oxalate parameters compared to their normal condition.
The important highlight of the novel research conducted in Papua New Guinea has been to understand and to confidently apply quantification methodologies to determine taro oxalate influenced by cultivation systems, N-levels and moisture deficit situations. Cultivation practices affects levels of oxalate hence consumer preferences. Oxalate concentration is known also to be affected by slightly reducing total, soluble and insoluble oxalates. N applied as urea significantly reduced total and insoluble oxalates but did not have an effect on the soluble oxalates and under drought condition taro oxalate level slight decreased. Taro can accumulate or degrade oxalate level within their minimum and maximum limits
This research paves a pathway for more work into understanding the level of influence farming practices had on oxalate concentration in taro corms. Moreover in-depth knowledge on the metabolic pathway of taro oxalate synthesis would indeed be a prerequisite for better understanding of the complexities surrounding its biological functions and thereby presents opportunities for genetic improvement in the line of optimising taro to consumer acceptable levels.