Griffith G S, Cresswell A, Jones S, Allen D K
Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion SY23 3EB, UK.
J Exp Bot. 2000 Nov;51(352):1879-92. doi: 10.1093/jexbot/51.352.1879.
Ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) have contrasting responses to soil mineral N availability and clover has the ability to fix atmospheric N(2) symbiotically. It has been hypothesized that these differences are the key to understanding grass-clover coexistence and vegetative dynamics in pastures. However, the whole plant response of clover and ryegrass to mineral N availability has not been fully characterized and inter-cultivar variability in the N-handling dynamics of clover has not been assessed. A detailed experimental study to address these issues was undertaken. For all clover cultivars and ryegrass, mass specific mineral N uptake rates (of whole plants) were similar saturating functions of mineral N availability. For all clover cultivars total N assimilation rates, whole plant C : N ratios and root : shoot ratios were independent of mineral N availability. Clover growth rates were also independent of mineral N availability except for a slight (<10%) reduction at very low N availability levels. Specific N(2) fixation rate (whole plant) was precisely controlled to ensure fixation balanced the deficit between mineral N uptake and the total N assimilation required to maintain constant whole plant C : N ratio. There was always a deficit between N uptake and the total N assimilation required to maintain C : N ratio. Consequently, some N(2) fixation remained engaged even at high mineral N availability levels. All inter-cultivar variation in N(2) fixation dynamics could be attributed to variations in growth rate. Clover mass specific growth rate declined as plant size increased. Ryegrass specific growth rate, whole plant C : N ratio and root : shoot ratio were dependent on N availability. Increased N availability led to increased growth rate and decreased C : N and root : shoot ratios. Specific growth rate was also dependent on plant size, growth rate declining as plant size increased. It is concluded that clover inter-cultivar variation in field performance is unlikely to be a consequence of variation in N-handling characteristics. Inter-cultivar differences in growth rate are likely to be a much more important source of variation.
黑麦草（Lolium perenne L.）和白三叶草（Trifolium repens L.）对土壤矿质氮有效性的反应截然不同，且三叶草具有通过共生作用固定大气中N₂的能力。据推测，这些差异是理解牧场中禾本科-三叶草共存及植被动态的关键。然而，三叶草和黑麦草对矿质氮有效性的整株植物反应尚未得到充分表征，且三叶草氮处理动态的品种间变异性也未得到评估。为此开展了一项详细的实验研究以解决这些问题。对于所有三叶草品种和黑麦草，（整株植物的）质量比矿质氮吸收速率是矿质氮有效性的相似饱和函数。对于所有三叶草品种，总氮同化率、整株植物碳氮比和根冠比均与矿质氮有效性无关。除了在极低氮有效性水平下略有（<10%）降低外，三叶草的生长速率也与矿质氮有效性无关。特定N₂固定速率（整株植物）受到精确控制，以确保固定作用平衡矿质氮吸收与维持恒定整株植物碳氮比所需的总氮同化之间的不足。氮吸收与维持碳氮比所需的总氮同化之间始终存在不足。因此，即使在高矿质氮有效性水平下，仍有一些N₂固定作用在进行。所有三叶草品种间N₂固定动态的差异都可归因于生长速率的差异。随着植株大小增加，三叶草的质量比生长速率下降。黑麦草的特定生长速率、整株植物碳氮比和根冠比取决于氮的有效性。氮有效性增加导致生长速率提高，碳氮比和根冠比降低。特定生长速率也取决于植株大小，随着植株大小增加，生长速率下降。研究得出结论，三叶草品种间田间表现的差异不太可能是氮处理特性差异的结果。品种间生长速率的差异可能是一个更为重要的变异来源。
