Perennial ryegrass (Lolium perenne L.) is an economically important resource in New Zealand, particularly for its use as a pasture grass. Nitrogen fertiliser is frequently applied after defoliation by grazing and, when assimilation by the plant is limited, causes detrimental impacts on the surrounding environment. The main water-soluble storage carbohydrates in L. perenne are fructans, which are primarily located in the sheath tissue of the plant. Fructans are remobilised following defoliation. Carbon plays a crucial role in nitrogen uptake and assimilation, and it has been suggested that nitrogen use efficiency (NUE) in the early stages of plant re-foliation following defoliation (grazing) is greater in plants that have greater concentrations of stored carbohydrate. This study investigated the potential interaction between the fructan concentrations and NUE within three cultivars of ryegrass, two of which, Expo and Aber Magic, were regarded as high sugar grasses. To manipulate the fructan stores further, half the plants were grown in cooler conditions, which was hypothesised to increase fructan content. NUE was determined using 15N supplemented urea.

The cooler environmental treatment led to greater fructan accumulation in the plant base (sheath) material of all three cultivars, with neither ‘high sugar’ cultivar having significantly more fructan accumulation than the standard New Zealand variety, Nui. Total nitrogen in harvested sheath (g) matched the pattern seen for fructan, again with no significant difference between cultivars. In both sheath and root tissue, there was a trend for increased uptake of the 15N in those plants with the greater fructan levels, which was statistically significant for the ‘Nui’ plants.

In conclusion, the relationship between increased carbon stores and increased nitrogen uptake was confirmed. There was no evidence that the putatively ‘high fructan’ cultivars displayed higher fructan contents in this experiment, but a strong effect of the environmental treatments may have masked any differences between the cultivars. Nonetheless, the cool temperature response displayed here does indicate that the strongest impact of higher fructan content on NUE may be expressed in cooler months during the growing season.