ABSTRACT

The objective of this study was to obtain basic information that could be used to develop management recommendations to improve the overwintering survival of oilseed rape. The influences of air temperature, sowing date, seeding rate, and nitrogen application on leaf growth and development and dry matter accumulation of winter oilseed rape during vegetative growth in autumn were studied. Leaf area and dry weight correlated with growing degree day (GDD) and the number of days after germination. The 5th leaf reached its maximum area after 552oC GDD were accumulated. Effects of stand density and preplant N application were observed when plants accumulated more than 350oC GDD. Leaf dry weight did not respond to N applied before 300oC GDD had been accumulated. As population density increased, plant dry weight decreased, especially on early-sown plots. However, leaf weight per area unit increased as population density increased. When the mean daily temperature dropped below 9oC, the accumulation of leaf dry matter content was accelerated. However, when the mean daily temperature rose above 9oC again, this process declined.

INTRODUCTION

A strong relationship exists between leaf growth and development of winter oilseed rape and environmental factors (Daniels et al., 1986). Under favorable conditions, fast growing varieties can possess six to seven fully expanded leaves by 1 month after seeding. However, under unfavorable conditions, plants may develop only one to two fully expanded leaves during the same period (Daniels and Scarisbrick, 1984). When the seeding date is delayed, the dry weight and leaf area are reduced (Sidlauskas, 1997). Nitrogen (N) fertilization at seeding increases plant leaf area with early seeding dates (Scott et al., 1973; Allen and Morgan, 1972, 1975) but has little or no effect on the leaf area with late seeding dates (Mendham et al., 1981). Increased plant population density also can cause a reduction in leaf area and plant dry weight (Ogilvy, 1984; Mendham et al., 1981). As with leaf area, N applied in the autumn increases the plant dry weight (Mendham et al., 1981), but the effect can be influenced by weather conditions during the vegetative growth period (Leach et al., 1994).

The objective of this study was to determine the effects of air temperature, plant population density, sowing time, and presowing N application on leaf growth and development as well as the aboveground dry weight of oilseed rape under Lithuanian agrometeorological conditions. The basic information obtained will be used to help develop management recommendations to improve the overwintering survival of oilseed rape.

MATERIAL AND METHODS

The experiments were carried out in 1993-1996 at the Lithuanian Institute of Agriculture in Dotnuva-Akademija on a light loam soddy-gleyic soil containing 0.11 to 0.16% of total N, 193 to 297 mg kg-1 soil P2O5 and 117 to 221 mg kg-1 K2O. The pH ranged from 6.8 to 7.0 when measured in 1m KCl. Ammonium nitrate, single granular superphosphate and potassium chloride were broadcast at seeding. 'Accord' was planted on three dates (early, mid and late August), and three seeding rates (3, 6, and 9 kg ha-1) were used for each date. From emergence until the end of vegetative growth in late autumn, 10 plant samples were collected from each plot every week, and leaf area and aboveground dry weights were measured. Weather records were obtained from the Agrometeorological Station on site. The baseline temperature used for growing degree day (GDD) computations was 5oC. Regression and correlation methods were used for statistical analyses. The data from 1995 were chosen for this paper because they best show the influence of mean daily air temperature and tested agronomic factors on aboveground dry weights of winter oilseed rape.

RESULTS AND DISCUSSION

Leaf Area

Leaf area expansion Leaf area was dependent on the order of emergence. The first leaf reached maximum area when 300oC GDD were accumulated and dropped off after 442oC GDD or 49 days after emergence (Figure 1). The second leaf dropped off at 484oC GDD or 56 days after emergence. Plants accumulated 552oC GDD from emergence until the end of vegetative growth. Only five leaves reached maximum leaf area during vegetative growth in autumn. The area of each leaf correlated with GDD and the number of days from emergence until the end of vegetative growth. Correlation coefficients of all leaves were highly significant (Table 1).

Figure 1. Effect of GGD on the growth of 1st - 10th leaves of winter oilseed rape.

Table 1. Intercepts (a) and coefficients of regression (b and c) and of correlation (r) for the relationship between 1 - 10th leaf area (cm2) and GDD (>5oC) and duration of vegetative growth of winter oilseed rape in autumn (days).

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GDD Duration .

Leaf a b c r a b c r

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1st 0.9 0.064 0.027 0.84** -0.8 0.678 -0.007 0.84**

2nd -5.0 0.131 -0.0001 0.84** -12.8 1.775 -0.016 0.77**

3rd -44.1 0.407 -0.0003 0.89** -46.2 3.503 -0.025 0.84**

4th 34.5 0.258 0.0001 0.89** -37.6 2.339 0.002 0.89**

5th 11.1 -0.032 0.001 0.95** -22.9 0.251 0.038 0.89**

6th 69.0 -0.541 0.001 0.89** 28.1 -2.777 0.073 0.89**

7th 227.9 -1.255 0.002 0.71* 48.0 -3.409 0.073 0.95**

8th 703.5 -3.331 0.004 0.77* 534.8 -20.45 0.214 0.89*

9th 397.5 -1.977 0.003 0.84* 313.9 -12.81 0.139 1.00**

10th 712.4 -3.377 0.004 0.89* 639.0 -23.16 0.215 0.95**

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*, ** Significant at the 0.05 and 0.01 levels, respectively.

Leaf area. Growing degree day had a significant influence on plant leaf area (y= -29.76 + 0.094x + 0.003x2; r=0.91**), as did the number of days from plant emergence (y= 64.26 - 6.97x + 0.24x2; r=0.89**).

Seeding date. Seeding date also had a significant influence on leaf area. The earliest established stands grew well and produced large plants with a larger leaf area by late October. Plants established early made steady progress in leaf area production in spite of replacing older leaves when they dropped off. Plants established later just maintained their weight and leaf area.

Plants density. The effect of plant density on leaf area of winter oilseed rape was negligible and became evident only when plants accumulated more than 400oC GDD. This may have been caused by severe competition between plants. At the end of vegetative growth, the larger leaf area per plant, as well as greater dry matter, was detected at the lowest plant density. The differences in plant leaf area between the medium and high densities were negligible, even at the end of vegetative growth. However, the leaf area per plot tended to increase with increasing plant density.

Nitrogen. Preplant N application had a small effect on plant leaf area. A more noticeable effect was detected only when plants accumulated more than 300oC GDD. Late-sown plants did not exhibit any response to autumn-applied N in leaf area or dry matter content. The main factor limiting the growth of late sown-plants was low air temperature.

Dry Weight

GDD and the number of days after emergence. During the initial 35-40 days after emergence, the weights of plants from the earliest seeding dates increased slowly. At 385oC GDD, average dry weight was 0.63g per plant. Plant dry weight started to increase rapidly at 400oC GDD and continued until the end of vegetative growth, especially at 500oC GDD. Similar tendencies for variation in plant weight were observed for the second and third sowing dates, but plant dry weight was lower when the sowing date was delayed. Average plant weights were 7.61g for the first sowing date, 5.16g for the second date, and 1.97g for the third date. A rapid weight increase in the plants for later planting dates was observed with much lower accumulations of GDD. The dry weight of plants from the second sowing date began increasing rapidly at 292oC GDD, and plants of the third seeding date began increasing at 199oC GDD. In every case, regardless of the planting date, 35-40 days passed from emergence until the beginning of the rapid increase in plant weight.

Mean daily temperature. When mean daily air temperature (MDT) was above 9oC, the weight of winter oilseed rape plants increased slowly. However, when MDT was below 9oC, a rapid increase in plant dry weight began (Figure 2). The lower peak near 11.4oC demonstrates the first period of rapid increase in plant weight, which was caused by a 2- week spell of colder weather in late September - early October when the MDT dropped below 9oC. The most substantial plant weight increase during the cool period is seen most clearly in winter oilseed rape from the first planting date. When MDT rose above 9oC, the increase in plant dry weight declined markedly, irrespective of the sowing date. However, in the second half of October when MDT dropped below 9oC again, a very strong plant weight increase occurred in all the three planting date treatments. The left peak shows this increase. Accord responded to the MDT transition at 9oC, regardless of the sowing date. This growth increase most likely was related to the intensive preparation of plants for the coming winter.

Nitrogen. Although the autumn during the experimental period was long and warm, no substantial differences in plant dry weight related to preplant N fertilization were observed. This may have been influenced by a relatively high mineral N content in the arable layer (8.2 to 14.2 mg kg-1 soil). Only when the plants accumulated more than 300oC GDD was a slight tendency toward weight increase in N-fertilized plants revealed. The response to preplant N fertilization occurred only for the earliest sown winter oilseed rape. Plants sown at the second sowing date accumulated only 360oC GDD.

Stand population density. A significant correlation between the number of plants per unit area and per plant dry matter was observed only for the first planting date. The

Figure 2. The influence of mean daily temperature on plant dry weight of winter oilseed rape.

postsowing period of the second planting date was warm and dry. Stand emergence was

uneven and may explain why no significant correlation was observed. The plants from the third planting date were relatively small at the end vegetative growth, and competition between them was slight and did not have much effect on plant weight. With an increase in plant number per area, plant weight was reduced, and the weight reduction was more obvious with bigger plants. When the number of plants per area unit increased by one plant, average plant weight declined by 0.07 g. Stand population density had the largest effect on the earliest sown winter oilseed rape. However, increasing stand density had the opposite effect on total dry matter content per unit area. An increase in stand density resulted in an increase in dry matter content. As plant number increased by one plant per area unit, the dry matter content at the end of vegetative growth increased by as much as 4.13g (y=45.08+4.13x; r=0.795). Therefore, an increase in stand density reduced single plant weight, but increased total dry matter content.

CONCLUSIONS

Growth of winter oilseed rape during the autumn, in terms of both leaf area and dry weight of the aboveground vegetation, depends mostly on the accumulation of GDD, number of days after emergence, and mean daily temperature. Plant population density and preplant nN application affected rape leaf area and dry weight of aboveground vegetation only for the earliest planting date. Results obtained from this study will help to understand the effects of these factors and their interactions on growth and development of winter oilseed rape during the vegetative period in autumn. The ultimate goal of this type of research is to improve overwinter survival of winter oilseed rape through management practices.

REFERENCES

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7. Ogilvy, S.E. 1984. The influence of seed rate on population, structure and yield of winter oilseed rape. In Aspects of Applied Biology. Agronomy, physiology, plant breeding and crop protection of oilseed rape. 6: 59-67.

8. Scott, R.K., E.A. Ogunremis J.O. Ivins, and N.J. Mendham. 1973. The effect of fertilizers and harvest date on growth and yield of oilseed rape sown in autumn and spring. J. Agric. Sci., Camb. 81: 287-293.

9. Sidlauskas, G. 1997. Effects of temperature and agronomic factors on the dry weight of winter oilseed rape during the vegetative growth in autumn. In: Prospects for Rational Use of the Lithuanian Climate and Soil Potential. Proceedings of the Scientific Conference, Vilnius. pp. 51-58.