The nitrogen balance of Raphanus sativus x raphanistrum plants. II. Growth, nitrogen redistribution and photosynthesis under NO₃⁻ deprivation

Abstract. Wild radish plants deprived of, and continuously supplied with solution NO⁻₃ for 7 d following 3 weeks growth at high NO⁻₃ supply were compared in terms of changes in dry weight, leaf area, photosynthesis and the partitioning of carbon and nitrogen (NH₂‐N and NO⁻₃‐N) among individual organs. Initial levels of NO⁻₃‐N accounted for 25% of total plant N. Following termination of NO⁻₃ supply, whole plant dry weight growth was not significantly reduced for 3 d, during which time plant NH₂‐N concentration declined by about 25% relative to NO⁻₃‐supplied plants, and endogenous NO⁻₃‐N content was reduced to nearly zero. Older leaves lost NO⁻₃ and NH₂‐N, and roots and young leaves gained NH₂‐N in response to N stress. Relative growth rate declined due both to decreased net assimilation rate and a decrease in leaf area ratio. A rapid increase in specific leaf weight was indicative of a greater sensitivity to N stress of leaf expansion compared to carbon gain. In response to N stress, photosynthesis per unit leaf area was more severely inhibited in older leaves, whereas weight‐based rates were equally inhibited among all leaf ages. Net photosynthesis was strongly correlated with leaf NH₂‐N concentration, and the relationship was not significantly different for leaves of NO₃⁻‐supplied compared to NO⁻₃‐deprived plants. Simulations of the time course of NO⁻₃ depletion for plants of various NH₂‐N and NO⁻₃ compositions and relative growth rates indicated that environmental conditions may influence the importance of NO⁻₃ accumulation as a buffer against fluctuations in the N supply to demand ratio.