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Section 14: Novel Applications in Nitrogen Fixation


'Botany Dept., Univ. of Cape Town, P/B Rondebosch 7701, Cape Town, South Africa 2Dept of Agronomy and Range Science, Univ. of California, Davis, USA

1. Introduction

Bacteria in the Rhizobiaceae (i.e. rhizobia) affect fundamental processes in plants through the use of powerful molecules. Some of these compounds such as the phytohormone IAA (Law, Strijdom 1989; Hirsch et al. 1997) have been known for a long time. These bacterial molecules include nodulation factors or lipochito-oligosaccharides (Lopez-Lara et al. 1995; Prome, Dermont 1996; Smith et al. this volume). Another signal compound that has been identified from the culture filtrate of Sinorhizobium meliloti is lumichrome (Phillips et al. 1999). This molecule enhances root respiration in alfalfa (Volpin, Phillips 1998) and can trigger an increase in net carbon assimilation and plant growth (Phillips et al. 1999). The aim of this study was to test the effect of lumichrome on the growth of tropical grain legumes and cereals under glasshouse conditions.

2. Materials and Methods

Legumes used in this study included cowpea (Vigna unguiculata (L) Walp) and soybean (Glycine max). The cereals sorghum (Sorghum bicolour (L) Moench) and maize {Zea mays) were also tested. In all experiments, surface-sterilized seed material was sown in the sterile 1 liter pots containing vermiculite and watered every second day with a nutrient solution containing 1 mM NH4NO3, and the antibiotics ampicillin (125 mg l"1) and rifampicin (10 mg l"1).

Lumichrome effects on plant growth were tested by treating the roots or seeds with the compound. The treatments applied included soaking sterile seeds for 2 h in 5 nM lumichrome, or applying 0, 5 or 50 nM lumichrome to emerging seedlings of the test plants three times a week. Plants developing from seeds soaked in 5 nM limichrome were watered with lumichrome-free solution. The pots were initially covered with transparent, clear plastic wraps that were removed after 6 d. The plants were harvested at various time intervals and separated into shoots and roots for dry matter determination after oven drying at 70°C for 48 h.

3. Results and Discussion

Culturing cowpea seedlings for 11 d either after soaking the seeds in 5 nM lumichrome or watering seedlings with 5 nM concentration resulted in 35^11% increase (P < 0.05) in shoot dry matter and 36^19% increase in total biomass (Figure 1). The trifoliate leaf dry matter also increased significantly by 70% with lumichrome application to cowpea due to increase in leaf size and area. However, treatment with 50 nm lumichrome resulted in decreased plant biomass. When watered with 5 nM lumichrome, soybean also showed a significant 17% increase in unifoliate leaf area, 76% increase in trifoliate and 30% increase in total leaf area per plant. This resulted in an overall 10% increase in soybean plant biomass relative to control.

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