Mats Höglind
Swedish University of Agricultural Sciences, Department of Crop Production Science, P. O. Box 7043, S-750 07 Uppsala, Sweden
Introduction
Materials and methods
Results
Discussion
References
The persistency and spread of white clover depend to a large extent on the maintenance of an adequate population of growing points and on the replacement of those lost, for example as a result of extreme environmental conditions. The size of the growing point population is also sensitive to management, one of the most important factors being nitrogen (N) fertilization.
Generally, the application of mineral N results in a reduction in the number of white clover growing points in mixed swards (Dennis and Woledge, 1987; Davies and Evans, 1990), although the extent of this effect can vary considerably and the underlying mechanisms are not fully understood.
The reduction in the number of growing points may be partly a result of decreased light availability in the lower parts of the sward, caused by the stimulating effect of N on grass growth. However, there are indications that the number of growing points begins to decrease already very early after N application, before the occurrence of any significant shading effects (Dennis and Woledge, 1987). This means that factors other than light intensity must also be involved, a probable factor being the red (660 nm) to far red (730 nm) photon flux ratio of the light (R:FR ratio). Thus, artificial shading experiments (Thompson and Harper, 1988) have shown that not only light intensity but also the R:FR ratio can influence white clover branching. Furthermore, it can be expected that the R:FR ratio changes very early after N application, as a result of the grass leaves turning greener. However, the effects of mineral N application on sward light intensity and light quality are not very well studied.
In the following experiment, the effects of N fertilization on sward light climate and white clover growth and morphology were studied during a summer regrowth in a mixed grass/white clover sward.
The experiment was carried out in 1994 in Uppsala (59°49'N, 17°39'E), on a two-year-old mixed sward of white clover (cv. Sandra), meadow fescue (cv. Svalöfs Sena) and timothy (cv. Alexander) grown on sandy soil (78% sand, 15% loam, and 7% clay). The sward had been fertilized with 30 kg P ha-1 and 100 kg K ha-1 in the spring each year, but had not been given any N since establishment. A first cut was taken on 8 June and a second on 8 July. Three days after the second cut, half the sward received 70 kg N ha-1 as calcium nitrate (N1), while the other half received no N (N0).
At intervals of three to seven days after N fertilization, the following observations were made in the field: the height of clover and grass leaves, measured with a rod, the proportion of incident radiation reaching ground level, measured with a Delta-T Devices Sunfleck Ceptometer and the R:FR ratio of this light, measured with a Skye Instruments Ltd SKP 2200 instrument connected to a 6 mm fibre-optic probe. In addition, the number of white clover growing points, the dry weights of white clover stolons and leaves, and the dry weight and number of grass shoots were determined from five or six 86.5 cm2 turves removed from each plot on the following occasions: just before N fertilization, after two, and after four weeks of regrowth.
The experimental layout was a completely randomized-block design with six replicates and a plot size of 1.5 × 5 m. The sward was irrigated three times during the course of the experiment, with about 30 mm each time.
Light measurements
The amount of light penetrating to ground level as well as the R:FR ratio of this light decreased with time in both treatments (Table 1). From day nine onwards after N application, i.e at all measuring dates except the first, light penetration was significantly smaller in the N1 than in the NO treatment. At all measuring dates, the R:FR ratio was significantly lower in the fertilized than unfertilized plots (Table 1).
Table 1. Proportion of PAR penetrating to ground level and the R:FR ratio of this light
|
|
Days after N fertilization |
||
|
3 |
71 |
14 |
|
|
Proportion of PAR reaching ground level |
|||
|
N0 |
0.41 |
0.18 |
0.06 |
|
N1 |
0.39 |
0.11 |
0.02 |
|
s.e.m. and signif. of difference |
0.02 ns |
0.02* |
0.01** |
|
R:FR ratio at ground level |
|||
|
N0 |
0.72 |
0.66 |
0.17 |
|
N1 |
0.57 |
0.46 |
0.10 |
|
s.e.m. and signif. of difference |
0.02** |
0.03** |
0.01** |
1 PAR: day 9
Growing point density and height of leaves
The number of white clover growing points increased during the first two weeks of regrowth, at more or less the same rate for both treatments (Figure la). Thereafter, during the following two weeks, the growing point density changed little overall in the N0 sward while it fell in the N1 sward, resulting in significant differences on the last sampling date.
Figure 1. Numbers of white clover growing points (a) in the N0 (---) and N1 (- - -) treatments
Figure 1. Numbers of white clover growing points (b) in the N0 (---) and N1 (- - -) treatments
Fertilizer N caused a rapid increase in the height of both white clover (Figure 1b) and grass (not shown), with significant treatment effects observed already three days after N application.
Dry weights of grass shoots and clover stolons and leaves
At final harvest, four weeks after N application, grass shoot dry weights were 359 and 139 g m-2 (p<0.01) in the N1 and N0 swards, respectively, while the corresponding values for dry weight of clover leaves were 112 and 138 g m-2 (p<0.05). On the same date, stolon weight was about 125 g m-2 in both treatments.
The results show that white clover responds very quickly to increased levels of mineral N in the soil, the earliest response found here being a rapid increase in height. This rapid response, which was observed to take place within days after N application, is probably a photomorphogenic response, triggered by the reduced R:FR ratio of the light reaching the plants, in turn probably caused by the effects of N on grass growth and colour. Shading experiments (Thompson and Harper, 1988) showing that petiole elongation is strongly influenced by the R:FR ratio of the light, and the fact that the N induced laminae elevation observed here started at a time when only the R:FR ratio but not the light penetration had decreased as a result of N application, supports this hypothesis.
The earlier observations (Dennis and Woledge, 1987) indicating that the number of growing points in white clover is reduced already very early after N application, before any significant shading effects has occured, could not be verified in the present study; while N-related differences in sward light penetration were already observed one week after N application, the number of growing points did not begin to diverge until between one and three weeks later. The seemingly contradictory results between the present and the earlier study is probably an effect of the different light measurement methods used, the flat sensor used in the present study allowing for differences in light penetration to be detected at earlier regrowth stages and lower sward heights than the instrument used by Dennis and Woledge (1987).
Even if the number of growing points did not begin to diverge before any differences in shading had occurred between the fertilized and unfertilized plots, the existence of an earlier effect of N on white clover branching, induced by the reduced R:FR ratio of the light, cannot be excluded. Thus, although not measured here, early changes in assimilate partitioning to the leaves at the expense of the branches might have taken place. Results of Robin et al. (1992), who found that white clover can respond very fast to light quality changes by altering its assimilate distribution pattern, support this hypothesis. The question is being studied further in an ongoing assimilate partitioning experiment.
DAVIES, A. and EVANS, M. E. (1990). Effects of spring defoliation and fertilizer nitrogen on the growth of white clover in ryegrass/clover swards. Grass and Forage Science. 45, 345-356.
DENNIS, W. D. and WOLEDGE, J. (1987). The effect of nitrogen in spring on shoot number and leaf area of white clover in mixtures. Grass and Forage Science. 42, 265-269.
ROBIN, C. H., VARLET-GRANCHER, F., GASTAL, F., FLENET, F. and GUCKERT, A. (1992). Photomorphogenesis of white clover (Trifolium repens L.): phytochrome mediated effects on 14C partitioning. European Journal of Agronomy, 1, 235-240.
THOMPSON, L. & HARPER, J. L. (1988). The effect of grasses on the quality of transmitted radiation and its influence on the growth of white clover Trifolium repens. Oecologia, 75, 343-347.
