Clough, Elizabeth Charlotte Mary; (1993) Ecological aspects of nitrate utilization in woody plants. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

High levels of foliar nitrate reductase (NR) activity have been demonstrated in fast-growing pioneers (e.g Sambucus nigra: 1601.9 pkat g-1 fwt.), while climax species exhibit low foliar activities (Carpinus betulus: 16.1). Pioneer species show varied seasonal patterns of activity with a tendency for early seasonal flushes, often corresponding to periods of maximum light availability. High NR activities are associated with high concentrations of alanine and glutamate, while high concentrations of asparagine correlate with low NR activities. Studies investigating patterns of metabolic control suggest that asparagine may regulate foliar NR activity in the climax species Quercus petraea, possibly through end-product repression. Diurnal variations in NR activity, photosynthetic rates, transpiration rates and amino acid concentrations exist in woody pioneers. Maximal rates of nitrate reduction correspond with high photosynthetic photon flux densities. Foliar NR activities of climax species do not fluctuate diurnally, suggesting that these species assimilate nitrate (or an alternative nitrogen source) in their roots. Comparative studies of field-grown seedlings from shaded and open habitats indicate that open-grown pioneers are able to maximise leaf nitrate reduction by taking advantage of high light availability and higher levels of nitrate generally found in open habitats. Pioneer seedlings were intolerant of shade. Climax species showed a tendency towards root nitrate assimilation, even in open habitats, suggesting that little energetic advantage is gained from leaf assimilation. 15N studies demonstrate uptake and utilization preferences by climax and pioneer species for the forms of inorganic nitrogen which would tend to prevail in their usual habitats. The preferential uptake of 15N-nitrate by pioneers, and the partitioning of the majority of label to leaves helps to explain high levels of foliar NR activity exhibited by these species. Preferences for 15N-ammonium exhibited by climax species suggests that they gain a significant advantage from root ammonium assimilation.

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