Morphological compatibility of white clover and perennial ryegrass cultivars grown under two nitrate levels in flowing solution culture

Abstract

The effects of nitrate (NO3-) supply on shoot morphology, vertical distribution of shoot and root biomass and total nitrogen (N) acquisition by two perennial ryegrass (Lolium perenne L.) cultivars (AberElan and Preference) and two white clover (Trifolium repens L.) cultivars (Grasslands Huia and AberHerald) were studied in flowing nutrient culture. Cultivars were grown from seed as monocultures and the clovers inoculated with Rhizobium. The 6-week measurement period began on day 34 (grasses) and day 56 (clovers) when the NO3- supply was adjusted to either 2 mmol m-3 (low nitrogen, LN) or 50 mmol m-3 (high nitrogen, HN). These treatments were subsequently maintained automatically. Plants were harvested at intervals to measure their morphology and N content. Cultivars of both species differed significantly in several aspects of their response to NO3- supply. In the grasses, the LN treatment increased the root : shoot ratio of AberElan but did not affect the distribution of root length in the root profile. In contrast, this treatment changed the root distribution of Preference compared with HN, resulting in a larger proportion of root length being distributed further down the root profile. The morphology of white clover Grasslands Huia was for the most part unaffected by the level of NO3- supply. In contrast, AberHerald exhibited different growth strategies, with LN plants increasing their stolon weight per unit length at the expense of leaf production, leaf area and stolon length, whereas HN plants showed reduced stolon thickness, greater leaf area production and stolon length per plant. Cultivars with different morphological/physiological strategies in response to NO3- supply may be of value in the construction of 'compatible mixtures' aimed at reducing oscillations in sward clover content by extending the range of conditions that allow balanced coexistence of species to occur.

Fig. 1. Morphology of two perennial ryegrass cultivars (Preference, closed circles; AberElan, open circles) under two levels of NO₃^– supply (HN, dashed lines; LN, continuous lines) in flowing nutrient solutions. The vertical arrow denotes the time of the defoliation harvest. Values are treatment means (± s.e.) of measurements on four replicate vessels per cultivar at each harvest and are expressed on a per plant basis. Each vessel contained 12 plants.

Fig. 2. Leaf area (cm²) measured within 2·5 cm strata up the leaf profile on perennial ryegrass plants growing in flowing nutrient solutions. Values are means (± s.e.) expressed on a per plant basis for cultivars Preference (filled circles) and AberElan (open circles) at H5 under HN (A) and LN (B) treatments. Measurements were made on four replicate vessels per cultivar, each containing 12 plants.

Fig. 3. Root length (cm) measured within 2·5 cm strata down the root profile on perennial ryegrass plants growing in flowing nutrient solutions. Values are means (± s.e.) expressed on a per plant basis for cultivars Preference (closed circles) and AberElan (open circles) at H5 under HN (A) and LN (B) treatments. Measurements were made on four replicate vessels per cultivar, each containing 12 plants.

Fig. 4. Morphology of two white clover cultivars (AberHerald, closed squares; Grasslands Huia, open squares) under two levels of NO₃^– supply (HN, dashed lines; LN, continuous lines) in flowing nutrient solutions. The vertical arrow denotes the time of the defoliation harvest. Values are treatment means (± s.e.) of measurements on four replicate vessels per cultivar at each harvest and are expressed on a per plant basis. Each vessel contained six plants.

Fig. 5. Root length (cm) measured within 2·5 cm strata down the root profile on white clover plants growing in flowing nutrient solutions. Values are means (± s.e.) expressed on a per plant basis for cultivars AberHerald (filled squares) and Grasslands Huia (open squares) at H5 under HN (A) and LN( B) treatments. Measurements were made on four replicate vessels per cultivar, each containing six plants.

Fig. 6. Total nodule numbers at H1, H2, H3, H4 and H5 measured on white clover plants growing in flowing nutrient solutions. Values are means (± s.e.) expressed on a per plant basis for cultivars AberHerald (filled squares) and Grasslands Huia (open squares) under two levels of NO₃^– supply (HN, diagonally hatched columns; LN, cross‐hatched columns). The vertical arrow denotes the time of the defoliation harvest. Measurements were made on four replicate vessels per cultivar at each harvest. Each vessel contained six plants.

Fig. 7. Daily rates of net NO₃^– uptake by (A) two white clover cultivars (AberHerald, filled squares; Grasslands Huia, open squares), and (B) two perennial ryegrass cultivars (Preference, closed circles; Aberelan, open circles) under two levels of NO₃^– supply (HN, dashed lines; LN, continuous lines) in flowing nutrient solutions. The vertical arrow denotes the time of the defoliation harvest. In each case uptake was measured on a single culture unit initially containing 144 (clover) or 288 (grass) plants.