Interactive Effects of N Form and P Concentration on Growth and Tissue Composition of Hybrid Napier Grass (Pennisetum purpureum × Pennisetum americanum)

Abstract

This study aimed to assess effect of nitrogen (N) form and phosphorus (P) level on the growth and mineral composition of hybrid Napier grass. Experimental plants were grown with different N forms (NO₃^-, NH₄NO₃, and NH₄⁺; 500 µM) and P concentrations (100 and 500 µM) under greenhouse conditions for 42 days. Growth rate, morphology, pigments, and mineral nutrients in the plant tissue were analysed. At the low P concentration, the better growth was found in the plants supplied with NH₄⁺ (relative growth rate (RGR) = 0.05 g·g^-1·d^-1), but at the high P concentration, the NH₄⁺-fed plants had 37% lower growth rates and shorter roots and stems. At the high P level, the NH₄NO₃^--fed plants had the highest RGR (0.04 g·g^-1·d^-1). The mineral nutrient concentrations in the plant tissues were only slightly affected by N form and P concentration, although the P concentrations in the plant tissue of the NO₃^--fed plants supplied with the high P concentration was 26% higher compared to the low P concentration plants. The N concentrations in the plant tissues did not vary between treatments. The results showed that the optimum N form for the plant growth and biomass productivity of hybrid Napier grass depends on P level. Hybrid Napier grass may be irrigated by treated wastewater containing high concentrations of N and P, but future studies are needed to evaluate biomass production and composition when irrigating with real wastewater from animal farms.

Keywords: N–P interactions; Pennisetum; ammonium; energy crop; nitrate.