Effects of Irradiation with Light of Different Wavelengths on Nitrate Reductase Activity in Roots of Zea mays L.

Abstract

The influence of light on nitrate reductase activity in root tips of Zea mays excised from seedlings grown in darkness was studied. Results obtained from an in vitro method were similar to those obtained using in vivo assay, but in vitro activity was higher than in vivo activity. The optimum pH for in vitro activity was found to be 7.5 and the K m value amounted to 8.04 × 10 −4 M N 3 − . The in vivo assay revealed no pH optimum and the K m value was 9.30 − 10 −2 M NO 3 − . Upon irradiation with white light in root tips the activity of nitrate reductase was increased, dependent on illumination and the period of light application. Red light (650 nm) and a nitrate supply also resulted in an increase in nitrate reductase activity. The maximum activity was attained 8 h after the beginning of light application or nitrate supply, the highest efficiency is obtained in a combination of red light plus nitrate. Light treatment did not affect the nitrate content of the root tips but after nitrate supply their nitrate content was higher. Root tips irradiated with far red light (725 nm; 15 h) showed the highest level of enzyme ,activity, whereas the wavelengths 575 and 658 nm led to the lowest level. The activity of nitrate reductase again increased under light of the short wavelength range of the visible spectrum. The results suggest that a photoreceptor system is involved in the light regulation of the nitrate reductase activity in roots. A high irradiance response of nitrate reductase activity may be mediated by phytochrome.