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This article in AJ

  1. Vol. 73 No. 1, p. 9-12
     
    Received: Apr 16, 1979


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doi:10.2134/agronj1981.00021962007300010003x

Nodulation and N Fixation of Field-Grown California Cowpeas as Influenced by Well-Irrigated and Droughted Conditions1

  1. Robert M. Zablotowicz,
  2. Dennis D. Focht and
  3. Glen H. Cannell2

Abstract

Abstract

The cowpea, (Vigna unguiculata (L.) Walp.), is considered a drought-tolerant legume. The N fixation process, however, is sensitive to water stress. This study was conducted to describe the development of nodulation and N fixation of field-grown Californian cowpeas as influenced by stage of growth and drought. California No. 5 blackeyes were evaluated for nodulation and N fixation (as measured by acetylene reduction assay) utilizing four strains of Rhizobium sp. applied as seed inoculum. Two water regimes were utilized in this study: 1) the well-watered regime was irrigated when the soil matric potential reached -0.6 bars at a depth of 20 cm, 2) the draughted regime received sufficient irrigation to allow good seed emergence, followed by a 42-day period free of rain or irrigation. Maximum nodulation of 175 mg/plant was observed at flowering in the well-watered plots versus 15 mg/plant in the draughted plants. Following this development stage, there was no significant difference in nodulation between water regimes. Maximum total nitrogenase activity was 22.5 and 18.8 µmoles of ethylene/plant/hour, respectively at flowering and pod-fill in the well-watered plants. The dry regime by contrast achieved a maximum nitrogenase activity of 5.3 and 4.5 µmoles ethylene/plant/hour at pod-fill and pod yellowing stages of maturity, respectively. Consequently, there was a significant reduction in total nitrogenase activity (mmoles of ethylene/plant/growing season) averaged over four Rhizobium strains from 14.5 in the well-watered to 3.7 in the drought ed regime. This decrease in N fixation capacity did not decrease seed yields of draughted plants below those of well-watered plants.

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