Lodging is a serious problem in dry pea production in North‐West Europe. Therefore, lodging and its effects on yields were studied in five field‐experiments with two cultivars with two different canopy structures, both as single crop and in various mixing weight ratios. In 1985, mechanically simulated lodging decreased yield by about 45% in both the conventional type cv. Rondo and the semi‐leafless type cv. Solara, by reducing both the number of seeds and seed weight. Natural lodging brought about the same effect in ‘Rondo’ but in ‘Solara’, however, effects were less because of delayed lodging. In 1986, 1987 and 1988, mixtures of the conventional cv. Finale and Solara showed delayed lodging compared to ‘Finale’ single and tended to produce higher yields than the single crops of both cultivars. Polynomial analysis of the yields of all mixing experiments showed maximum yields at 53%‘Solara’ (quadratic analysis) or 67%‘Solara’ (cubic analysis). Thus, mixing high yielding, lodging sensitive conventional cultivars with lower yielding but lodging resistant semi‐leafless cultivars may improve yield in dry peas.

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH~, NO~, or both) and level of N supply (4 mM N, and zero-N following a period of 4 mM N). A distinct preference for either NH~ or NO 3 could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NOr and free NH 4. Especially the accumulation of free NH~ was unusually large.Generally, relative growth rate (RGR) was highest with a combination of NH 4 and NO~. Compared to mixed N supply, RGR of NO 3-and NH4-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH~ supply probably resulted from direct or indirect toxicity effects associated with high NH~ and/or low Ca 2+ contents of tissues. NRA in NO~ and NH4NO 3 plants was very similar with maxima in the leaves of ca 40 ~mol NO2-g-~ DW h -1. 'Basal' NRA levels in shoot tissues of NH 4 plants appeared relatively high with maxima in the leaves of ca 20/~mol NO 2 g-~ DW h-~. Carboxylate to organic N ratios, (C-A)/Norg, on a whole plant basis varied from 0.2 in NH 4 plants to 0.9 in NO 3 plants.After withdrawal of N, all accumulated NO~ and NH 4 was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50-80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -N H 4 plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/Norg ratios increased to values around unity.Possible difficulties in interpreting the (C-A)/Norg ratio and NRA of plants in their natural habitats are briefly discussed.

The growth of nitrate-supplied and dinitrogen-fixing pea plants was studied in a pot experiment with a sandy soil in a pH-H ? O range from 3.4 to 5.6. Optimum growth in both treatments occurred at pH 5.0. At low pH, N 2 -plants yielded significantly less than NO 3 -plants. Planting of nodulated seedlings did not enhance yield in comparison with sowing in inoculated soil, indicating that nodulation was not the most sensitive process in restricting yield. Comparison of the nitrogen contents of shoots of planted and sown N 2 -plants allowed the suggestion that the synthesis of nitrogenous compounds was also not limiting yield.At low pH, root growth was severely reduced in dinitrogen-fixing plants in comparison with nitrate-supplied plants. This difference could be explained by the influence of the form of nitrogen 493

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