Nitrogen assimilation enzyme activities in witchweed (Striga) in hosts presence and absence
1996
Igbinnosa, I. | Thalouarn, P.A.
N fertilizers suppress witchweed plant growth and development, thus reducing the severity of parasite attack and increasing host yield simultaneously. However; the underlying physiological mode of N action occurring within the parasite cells remains largely unknown. This study aims at screening for the effects of N forms and different growth conditions on some N assimilation enzymes in witchweed seedlings grown aseptically without host plant, and in pots with host plants. Results show that supply of N in NH4+ or urea forms resulted in 83 to 92% reduction in nitrate reductase activity (NRc), compared with control. Increasing NO3(-) concentrations from 0 mM to 100 mM, led to a corresponding increase in NRc in giant witchweed. NRc of giant witchweed seedlings grown under light and dark cycles were about 270 times higher than seedlings grown in continuous darkness. A combination of NH4+ and NO3(-); resulted in increased giant witchweed NRc, compared with NR4+ or NO3(-) supplied singly. Highest shoot development and NRc was at NH4+ and NO3(-) ratio 1:1, followed by ratios 1:3, 3:1, 0:1, and 1:0, respectively. Addition of N in soils resulted in increased NRc, followed by rapid deterioration and death of giant witchweed plants. NRc, GSc, and GDHc in witchweed, maize, cowpea, and tobacco were affected by diurnal fluctuations with higher enzyme activities occurring during the day than at night. Higher GSc than GDHc suggests that NH4+ assimilation occurs mainly through the GS pathway in witchweed plants. NRc and GDHc were two and four tines higher in giant witchweed grown in aseptic media without host plant, than that grown in potted soils with host plants. These findings provide insight into the physiological mode of N action and their implications on witchweed control.
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