EFFECTS OF SEED SIZE AND EMERGENCE TIME ON SUBSEQUENT GROWTH OF PERENNIAL RYEGRASS

Transcription

1 Phytol (980) 84, EFFECTS OF SEED SIZE AND EMERGENCE TIME ON SUBSEQUENT GROWTH OF PERENNIAL RYEGRASS BY ROBERT E. L. NAYLOR School of Agriculture, The University, Aberdeen {Accepted 2 January 979) SUMMARY The seed weight of individual perennial ryegrass seeds was measured prior to recording their date of emergence and subsequent growth in experimental populations raised in the glasshouse and sown at different initial densities. Seed size did not correlate with time to emergence although lighter individuals had a slightly lower probability of emergence. Seed size had no statistically significant effect on subsequent tillering up to 27 weeks, or on the dry wt attained by this time. The time from sowing to emergence was significantly related to subsequent tillering of the grass plants for a number of weeks in populations sown at different densities. After some 20 weeks the effect of emergence time became non-significant. This coincided with the onset of tiller mortality in the population. Even after 27 weeks individual emergence time was inversely related to dry wt but was not related to mean tiller weight. INTRODUCTION Seed size or weight is often the least plastic component of a plant's response to stress, particularly in those species in which the terminal meristem does not itself produce the inflorescence. In the Gramineae an early phase of indeterminate growth gives way, after any necessary vernalization and photoperiodic requirements are met, to a deterministic phase. Environmental conditions during inflorescence formation may influence the number of grains formed but, after the apical meristem of the tiller ceases to give rise to more spikelet initials, further environmental changes may affect the size of the reproductive products. In practice, stress conditions may not lead to lower seed size hecause in some species assimilates produced prior to flowering may be remobilized and transported to the inflorescence (e.g. Cock and Yoshida, 972, for rice). Despite this, variation in seed size of grasses can and does occur [e.g. Anslow (964) with ryegrass; Kaufmann and McFadden (960) with barley]. The agronomist is interested in seed size as a relatively easily determined parameter which is assumed to be a sound predictor of the success of establishment and the Vigour of the crop. Consequently most work has concentrated on the relationships between the average seed size, and average values for various germination, establishnient, and growth parameters, and samples of different seed sizes have been compared. lhe fact that such mean values may be misleading due to the non-normality of the distribution of various parameters has been discussed by Naylor (976). Little work has concentrated on the effect of different seed size within rather than between populations, and this paper examines such effects in Lolium perenne (perennial ' yegrass) X/8O/O2O33--O6 $02.00/0 980 The New Phytologist

2 34 R- E. L. NAYLOR MATERIALS AND METHODS Individual seeds of perennial ryegrass, (cv. S23) were weighed and then precision sown 5 mm deep at 24, 48, and 96 seeds per 34 x 2 cm seed tray. This is equivalent to about 600, 200 and 2400 plants per m^; the agricultural rate is about 200/m2. The seeds were sown as three rows of eight (A), three of 6 (B), and either three of 32 (C) or as six rows of 6 (D). One set of trays was sown on 8 August 973 and another on 2 November 973. The trays were kept in a glasshouse and watered regularly. They received supplementary lighting from banks of warm white fluorescent tubes to give a 6 h daylength. The temperature of the greenhouse was maintained above 2 C. The emergence of each of the 528 seedlings was recorded. The period from sowing to emergence was called the emergence time. Subsequently the number of tillers on each individual was counted at weekly intervals. After 27 weeks the trays were destructively harvested to obtain individual values of dry weight of green m.aterial and of dead material. The effects of seed weight and emergence time were assessed by regressing weekly individual tiller numbers and the data obtained at the final harvest on seed weight and on emergence time. RESULTS The distribution of seed weights of perennial ryegrass was normal with a mean of 2-30 mg and standard deviation 0-66 mg. Lighter seeds had a lower percentage emergence (Table ). The correlation of seed size with emergence time was not significant for any of the trays separately, nor for the whole population of 528 seeds. Thus, while heavier seeds had a slightly higher final emergence than lighter ones, they did not seem to be more vigorous. This is reinforced by observing further growth; the correlation of seed size with weekly counts of number of tillers per plant was not significant on any occasion, or at any density. The effect of emergence time on subsequent tiller production varied with time (Tables 2, 3). During the early weeks of growth, before and just after tillering commenced, the number of tillers was independent of emergence time, i.e. the correlation, and the slope of the regression line were both non-significant. Later there were significant negative correlations of emergence time and plant size. In the trays sown in November which were observed for a longer period of time, although the slope of the regression line got steadily steeper, it was in fact not statistically significantly differrent from a line of zero slope. Table. The emergence of perennial ryegrass seed of different weights Seed weight (mg) < No. of seeds No. not emerging Figures in parentheses are from small samples. Non-emergence (%) (00) (6-7)

3 Seed size emergence and growth of Lolium 35 * * * * * * * * * * * * * Q \ ugus e OOOO^-i'-^'-''^'--''-''-''-'-^ I I I I I I I I I I I I I * * *» ras 2.^ )-2 o 0) 6OOOOOOOOCCOO I I I I I I I I I I I I I 0 * w O S It O O T-t y t ^ ' I I I I I I I I I I I I * * Cu o ^ o c 62 m 6 -^ <a I I I l l in P< C p 00 O X V " tn ft,<z

4 3i6 R. E. L. NAYLOR en I I O O I I I I I I I I I I I I I I i a. -^ cioooood>ooooo c>ooc>oo I I I I I I I I I I I I I I I I I I I mm s a. " T^ T^ a tn ro OOOOOOOOOOOOOOOOOOO 6 m be : O O O O ^ T- I I I I I I I. a o

5 Seed size emergence and growth of Lolium 37 The effect of density was to limit the maximum number of tillers per plant and thus the slope of the regression line relating tiller number to emergence time. The differences between treatments A, B and C reflect differences not only in density but also in spatial arrangement of individuals. Comparison of A and D reveals differences due to density alone, the pattern is the same but interplant distance is halved, while comparison of C and D shows effects due to different sowing patterns at the same overall density. Initial seed weight was not correlated with the dry weight either of the whole plant, or the live portion or the dead portion, or with mean tiller dry weight. However, the emergence time was a good predictor of total and live dry weight of individuals, accounting for over 50% of their variation. Mean tiller dry wt was not correlated with emergence time. DISCUSSION Many authors have studied the relationship of average seed weight with germination. For a range of dicotyledonous species differences in mean seed size between samples appear to influence emergence time or germination time but not final percentage germination, for example, Edwards and Hartwig (97) with soya beans, Beveridge and Wilsie (959) with alfalfa, and Robinson (974) with sunfiower. Observations on 2 strains of 5 species of North American grasses, similarly showed that although average seed size of a sample had little effect on final percentage germination, there were significant differences in seedling vigour both between and within species: samples of larger sized seeds had more vigorous seedlings, emerged sooner, and subsequently grew faster (Kneebone and Cremer, 955). Bean (973) quotes work with 2 certified samples of each of Lolium perenne (perennial ryegrass) cv. S23 and L. multiflorum (Italian ryegrass) cv. S22 in which different 000-seed weights were significantly positively correlated with subsequent seedling dry weight. In the present experiment using a single commercial sample of perennial ryegrass, lighter seeds did have a lower probability of emerging but, of the seeds that emerged, seed size was not correlated with emergence time. It is likely that the commercial seed source used had been screened to remove the majority of small seeds and the lack of correlation of seed size with speed of germination may represent the efficiency with which this was done. Other experiments with commercial samples of wheat (cv. Cappelle Desprez) and barley (cv. Golden Promise) gave similar results as did samples from two provenances of Pinus contorta. One must conclude that any selection for rate of seedling growth should be based on actual measurements of this rather than by using seed weight as an indicator. The variation in emergence time that did occur was strongly associated with subsequent growth over a number of weeks. Ross and Harper (972) have also shown the advantage conferred on the individual plant by earliness in emergence. The advantage was realized not simply as a greater period from growth to harvest, but also in a higher position in the dominance hierarchy of the population. This hierarchy based on order of emergence was in their experiments, which lasted less than 8 weeks, only slightly modifi.ed by individual initial seed weight and early growth rate. However, one effect of competition was to cause deviations from the relationship of length of growing period with individual plant weight. In the present experiments, on populations sown at widely different densities, tillering was observed for 8 and 27 weeks after sowing, and dry wt was measured after 27 weeks, and it was apparent

6 3i8 R.E.L. NAYLOR that the relationship between emergence date and subsequent tiller number was only a transient one. The residual variation, after removal of that contributing to the regression, was large in later weeks, accounting for up to 97 % of the total variation. This variation was not only due to simple competition effects resulting in variation between plants but, in addition, mortality of tillers and of whole plants took place. It has been shown that this response may result in changes in the structure of the population resulting in non-normal distributions of plant size, particularly in the direction of positive skewness (Naylor, 976; Mohler, Marks and Sprugel, 978). The dominance hierarchy becomes even more extreme with a few heavy, well-tillered individuals, and many small individuals, but the order of the hierarchy changes and there is no significant relationship between initial seed size and plant size. It was concluded that within the grass population sampled, individual seed size was a poor predictor of emergence time and of subsequent tillering and production. Emergence time was a sound predictor of subsequent tillering until the onset of plant mortality in the population but, thereafter still continued to be correlated with individual dry wt. ACKNOWLEDGEMENTS I am grateful to Mr A. D. McKelvie, Head of the Agricultural Botany Division for help and encouragement, and to Mrs M. Jamieson for performing most ably the bulk of the data collection. The statistical analyses were carried out at the University of Aberdeen Computer Centre. REFERENCES ANSLOW, R. C. (964). Seed formation in perennial ryegrass. II. Maturation of seed. Journal of the British Grassland Society, 9, 349. BEAN, E. W. (973). Seed quality: its variation, control and importance in breeding and varietal assessment. Report of the Welsh Plant Breeding Station 972, 94. BEVERIDGE, J. L. & WiLSiE, C. P. (959). Influence of depth of planting, seed size, and variety on emergence and seedling vigour in alfalfa. Agronomy Journal 5, 73. COCK, J. H. and YOSHIDA, S. (972). Accumulation of "C-labelled carbohydrate before flowering and the subsequent redistribution and respiration in the rice plant. Proceedings of the Crop Science Society o Japan, 4, EDWARDS, C. J. & HARTWIG, E. E. (97). Effect of seed size upon rate of germmation in soya beans. Agronomy Journal, 63, 429. KAUFMANN, M. L. & MCFADDEN, A. D. (960). Tbe competitive interaction between barley plants grown from large and small seeds. Canadian Journal of Plant Science, 40, 623. KNEEBONE, W. R. & CREMER, C. L. (955). The relationship of seed size to seedling vigour in some native grass species. Agronomy Journal, 47, 472. MOHLER, C. L., MARKS, P. L. & SPRUGEL, D. G. (978). Stand structure and allometry of trees during self-thinning of pure stands. Journal of Ecology, 66, 599. NAYLOR, R. E. L. (976). Changes in tbe structure of plant populations. Journal of Applied Ecology, 3, 53. ROBINSON, R. G. (974). Sunflower performance relative to size and weigbt of acbenes planted. Crop Science, 4, 66. t.i- u t Ross, M. A. & HARPER, J. L. (972). Occupation of biological space during seedling establishment. Journal of Ecology, 60, 77.

7