| Abstract | Pea plants (Pisum sativum L. cv. Trapper) were inoculated and grown in controlled-environment chambers at two irradiance levels. Shoot and root dry weights and nitrogen contents, total leaf and stipule areas, and rates of C₂H₂ reduction were determined during growth in different treatments of NH₄NO₃ addition. Although overall growth increased with irradiance, the growth responses to combined nitrogen addition were similar at both light levels. Two phases of early vegetative growth were identified by their different responses to combined nitrogen. During the first phase, low levels of NH₄NO₃ greatly increased the relative growth rate, the growth per unit leaf area, and the percentage of nitrogen in the tissues. This indicated a period of nitrogen stress which lasted only until the 3rd week. Over the next 2 weeks, combined nitrogen increased the relative growth rates to a lesser extent, primarily through an increased partitioning of assimilates to shoot development. This distribution effect was rapidly reversible on changing nutrient conditions. The early stimulation of leaf area development by addition of combined nitrogen during these two growth phases resulted in greater capacity for symbiotic fixation after NH₄NO₃ was removed. |
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