Emergence of sugar beet seedlings from under different obstacles

Eur. J. Agron., \993, 2(3), 2\3-221

Emergence of sugar beet seedlings from under different obstacles N. Souty and C. Rode INRA, Unite de Science du Sol, Centre de Recherches d'Avignon, BP 91, F 84143 Montfavet Cedex, France.

Accepted 12 August 1993.

Abstract

The emergence of sugar beet through two types of seed beds, one encrusted and the other made up of obstacles independent of the underlying layer, was studied in the laboratory. Relationships were established between the physical and mechanical characteristics of these obstacles and the growth force of the seedlings measured beforehand, which made it possible to develop a predictive model of the percentage and kinetics of emergence. If there was a crust at the surface of the seedbed, the calculated probability of emergence was estimated from statistical distribution of hypocotyl forces by using a mechanical model of rupture with bending and fracture. The emergence percentage was lower than 50 per cent as soon as the resistance of the crust was greater than 0.2 N and the maximum emergence percentages were reached after about 40 hours. If the seedlings were under a seedbed with clods, one of two processes were involved in emergence : the seedling lifted the clod or it grew under the clod to escape. It appeared that the seedlings were able to lift obstacles the weight of which was greater than the growth force because the hypocotyl moved under the obstacle. To obtain the moment of the clod weight equal to the moment of the seedling force, the probability of heaving was determined after about 6 hours by using the statistical distribution of the forces developed during this period. The percentage of heaving was lower than 50 per cent when the weight of the obstacle was greater than 0.1 N. Emergence (heaving and escape) was greatly reduced if the seedling encountered an obstacle with a diameter greater than 3 cm. Key-words: seedbed, mechanical resistance of crusts, obstacle weight, growth force of the hypocotyl, emergence, emergence percentage, emergence kinetics, model.

INTRODUCTION The percentage and kinetics of seedling emergence vary significantly with environmental factors, seed qualities and soil properties. Emergence is reduced by the presence of crusts which can form naturally under the effect of rain followed by drying out by sun or wind. This harmful effect can be reduced either by methods of preparing soils, removing the risk of subsequent formation of crusts or by selecting varieties of plants capable of rapidly exerting considerable growth forces. Goyal (1982) conducted an exhaustive bibliographical review on all the problems of emergence linked to the presence of crusts. In the case of sugar beet (Beta vulgaris L.), several studies have been conducted on the influence of soil characteristics on emergence (Stout et al., 1956; /SSN //61-0301/93/03/$ 4.00/ © Gauthier-Villars - ESAg

Johnson and Law, 1967). Studies on the control of the planting phase (Scott et al., 1974) and on more successful crops through improving seed quality, tillage equipment and sowing conditions (Richard, 1988 ; Sebillotte and Servettaz, 1989; DUff et al., 1990) were carried out and tend to lead to means of diagnosis and forecasting. The present study, based on experiments conducted in the laboratory, was carried out to develop a model to predict the percentage and kinetics of the emergence of sugar beet seedlings in the presence of obstacles in the seedbed: crusts and heavy obstacles. To develop a model, it was essential to establish relationships between the characteristics of the obstacles and the characteristics specific to the seedling. The growth force of the hypocotyl was measured and compared to the force which the seedling had to develop in order to emerge, depending on whether it

214

was placed in contact with mechanical obstacles which were either penetrable and linked to the sublayer (superficial crusts), or heavy and independent of the sub-layer (clods, crust fractions, small stones). MATERIALS AND METHODS All experiments were conducted at 20°C ± 0.5 °C in darkness. The growth force (see below) was measured over a period of 24 hours on seedlings of different ages. The emergence of 98-hour-old seedlings with hypocotyl length about 1 to 2 cm, was studied to simulate the deep seedbed usually selected for sugar beet culture. Seed Sugar beet seeds of the monogerm cultivar Vega, not coated and between 3.25 and 4.25 mm in diameter and weighing between 13.0 and 16.9 mg, were used. The standard conditions of the I.S.T.A. (International Seed Testing Association) also used by Richard (1988) were provided for germination. The water and mineral nutrition used was determined and tested according to Saglio and Pradet (1980). The seedlings were sampled approximately 3, 4, 5 and 6 days after sowing (that is: 80, 98, 122 and 162 hours). These ages were situated within three phases of the redistribution of the seed reserves in darkness determined for carrot and sugar beet by Durr et al. (1990). The seminal reserves were transferred from seed to seedling during the first four days after sowing and then these reserves were redistributed inside the seedling. The cotyledon reserves were transferred to the hypocotyl which grew until the sixth day and whose growth subsequently diminished and stopped after the ninth day. The experiments were not carried out on hypocotyls older than 162 hours because they were too long and too thin to be handled without being damaged. Measurement of the growth force of the sugar beet hypocotyJ We used the apparatus previously used for root strength and coleoptile strength measurements (Souty, 1987; Souty and Stepniewski, 1988; Bouaziz et aI., 1990; Souty et at., 1992). This apparatus gave the relative deformation of an elastic steel beam by a force. The pre-germinated seed was put in a container filled with wet sand (20 per cent water content) and the hypocotyl, put in a glass tube to avoid buckling, was in contact with the surface of the gauge stuck in the middle of a steel beam. In an elastic medium there was a linear relation between the force exerted (F) and the relative deformation of the beam (~a/a) : F = ex (~a/a); the coefficient ex depended on the

N. Souty and C. Rode

length, the width and the thickness of the beam and the elasticity coefficient of steel; it was determined by previous calibration. In experiments with seedlings, values of relative deformation of the beam were obtained and used to calculate the force exerted by the growing hypocotyl. Measurement of emergence from beneath obstacles Penetrable obstacles: superficial crusts

The method of Souty et al. (1992) for obtaining homogenous and reproducible disks of known thickness and water content was used. The disks were made from a saturated soil paste (grain size 2 mm) ; the particle size distribution of the soil, from the north of the Paris Basin, being 21.9 per cent clay, 72.1 per cent silt and 6.0 per cent sand. The resistances to penetration and/or breaking (F) were measured on control crusts (of well-known thickness and water content) with an apparatus perfected by Guerif (1988) and slightly modified for these experiments. The tip of the penetrometer was cylindrical and had a 3 mm diameter. The force needed to break a crust depends on the thickness (e) and the moisture (W in g per 100 g dry soil). The effect of crust thickness was taken into account by using (F/e 2), the modulus of rupture, which was a function of soil water content. The best relationship between (F/e 2 ) and W was: F/e 2 (MPa)

= 0.00176*W2 (r

=-

0.106*W + 1.629 0.9398)

The soil disk was put on the surface of a container filled with wet sand (20 per cent water content) which contained the pregerminated seed. The top of the hypocotyl coincided with the upper sand surface, so it was directly in contact with the middle of the internal crust face; an aluminium sheet between sand and crust avoided a modification of crust water content and only a small hole through the sheet allowed the hypocotyl to enter the crust. The crust was kept in place by a metallic ring at the sand surface to avoid being moved by the seedling (Souty et al., 1992). Thus the hypocotyl could only emerge by breaking or perforating it. An aluminium sheet between sand and crust avoided a modification of crust water content: a small hole through the sheet allowed the hypocotyl to enter the crust. Seedling emergence was studied during 4 or 5 days. Heavy obstacles not bound to the underlying layer

These obstacles were simulated by metallic and plastic calibrated disks. Four surface areas were used: 4.1, 7.8, 15.9 and 30.2 cm 2• The masses corresponded to the weights: 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40 and 0.50 N for the two larger surface areas and 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25 N for the Eur. 1. AR ron.

215

Emergence of sugar beet seedlings

two smaller surface areas. In addition, for the 4.1 cm2, disks weights of 0.30 and 0.40 N were used. For each weight, five disks were made; in total 180 disks were used.



F(sugar beet)

= (F/e 2 ) (1

+ v) (3/2 1t)

(114 (ria? -

Table I shows the average measurements of force according to age. The force of a 98-hour-old seedling (0.16 N, standard deviation = 0.14) was the greatest: the null hypothesis test enabled us to conclude that it

30

seedling weight: 13-17 mg 98-hour-old seedlings

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(1)

Equation 1 enabled us to obtain the maximal tensile stress which appeared on the lower side of a crust broken by the penetrometer. If a growing hypocotyl was able to break a similar crust in the same way, the stress appearing on the upper side of the crust would be the same. However, equation (I) was modified because we assumed the load borne by the plate was distributed over an ellipse (the hypocotyl has a hook configuration). The two axes band h of this ellipse were measured on 59 seedlings 98 hours old; the mean values were b = 0.693 mm (standard deviation = 0.146 mm) and h = 1.783 mm (standard 0.246 mm). The relationship between deviation Vol. 2. n° 3 - 1993

(3)

There was considerable variability in the maximum growth force of hypocotyls although seeds were chosen to have the same mass and the seedlings were the same age. The values of the growth forces of 98hour-old seedlings were distributed from 0 to 0.60 N (Figure I).

5

where 2a is the bearing of the plate (m), e is the plate thickness (m), r the radius of the load (m), v is the Poisson coefficient and F is the force used for breaking (N).

=

= 0.7304*F(penetrometer)

Maximum growth force of hypocotyls of various ages

Because the sugar beet hypocotyl is hook-shaped, its emergence through the crust was assumed to occur by breaking rather than by penetration.


(2)

RESUL TS AND DISCUSSION

Theoretical considerations

This theory allowed the calculation of the maximal tensile stress
= (F/e 2 ) (I + v) (3/2 1t) (1/4 ({h/2a)2 - 10g e C',!b*{h/2a))

The equality of the two tensile stresses (1) and (2) gave the relationship:

For studying emergence, heavy disks were placed on the surface of the pot containing the seedling so the seedling was the centre of the disk. The different heavings were measured with a cathetometer (an optical instrument allowing the measurement of vertical distance between two points) over several hours (24, 48 and 120 hours). We assumed that there was a heaving if the value measured was at least 0.03 cm (which was the maximum sensitivity of the cathetometer). The experiment was repeated three times to obtain 15 results for each pair: surface area - weight.

The classical theory of thin circular brittle plates of Timoshenko and Woinowsky-Krieger (1961) can be used to relate the force exerted by the seedling to the resulting distribution of stress and strain in a crust. The soil crust around the seedling can be considered as a circular brittle plate. Some conditions were assumed: the crust was isotropic and elastic, the plate thickness was small compared with the plate dimensions, the plate was large compared with the seedling dimensions, the load which bent the plate was uniformly distributed over a known surface area.

and F exerted by the hypocotyl became:

0.2

0.3

F(N)

0.4

0.5

Figure 1. Sugar heet seedling force frequency.

Table 1. Mean values and standard deviations of the growth forces of the sugar heet hypocotyl at different ages. Age (hour)

Number seedlings

F (N)

80 98 122 162

52 58 69 52

0.12 0.16 0.10 0.08

Standard deviation (N)

0.08 0.14 0.06 0.065

0.6

216

N. Souty and C. Rode

was significantly greater than the force of 162-hourold seedlings (p = 0.001) and 122-hour-old seedlings (p = 0.05) ; it was less significantly greater (p = 0.08) than the force of 80-hour-old seedlings.

in Figure 3. There was a good agreement between this calculated curve and the observed emergence, in the range of resistance of crusts from 0 to 0.8 N. The differences observed may be explained by a change with time in the size of the hypocotyl hook under the obstacle (principally an increase in the large axis h of the ellipse), causing an increase in the multiplicative coefficient of the force exerted by the penetrometer (equation 3) and thus a shift in the theoretical curve towards lower values of the abscissa (Figure 3). The lack of a good contact, at the beginning of the experiment, between the hypocotyl and the crust may also contribute to a shift in the theoretical curve towards lower values of the abscissa since when the seedling comes into contact with the crust, it will be older than 98 hours (Figure 3). Nevertheless, the curve of the force frequency of 98-hour-old seedlings enabled us to fix a threshold for crust resistance (0.2 N) below which the probability of emergence was at least 50 per cent.

These results were in agreement with the discovery of three distinct phases in the pre-emergence life of sugar beet and carrot seedlings (Durr et at., 1990): from the time course of mobilizing seed reserves one may suppose that, as far as the sugar beet was concerned, its force reached a maximum at the age of 4 days and then decreased. The representation of the cumulative percentage of seedlings the force of which was greater than a given value, indicated the biological variability within a batch of seedlings apparently identical in all aspects (Figure 2). This variability in the characteristics of the hypocotyl was responsible, to a large extent, for the differences observed in the emergence percentages and kinetics; these curves could therefore express the variation of the percentage of emergence.

Figure 4 shows several kinetics curves: (i) of seedlings emerging under crusts of known resistance and (ii) of seedlings reaching, under an impenetrable elastic obstacle (a steel beam), a force enabling them to go through these crusts (in accordance with equation 3). We noticed (i) a similarity between the experimental emergence rate and the rate at which the force developed, (ii) a difference, constant in time, of between 9 to 15 hours depending on the resistance of the crusts, for the emergence kinetics ; this amount of time probably corresponds to the time necessary for the seedling to break through the crust.

Emergence through the crusts We conducted the experiment under optimal conditions and so we limited ourselves to monitoring the emergence of 98-hour-old seedlings, which exerted the greatest force. The curve in Figure 2 (98 hours) representing the cumulative percentage of seedlings developing a given force, can be transformed using equation 3 into a curve giving the percentage of seedlings capable of breaking the crusts of which the resistance is expressed by the force exerted by the penetrometer.

A logarithmic fitting of the kinetics curves of the development of force between 0 and 25 hours was satisfactory (r2 at least equal to 0.9526) ; a lO-hour

The theoretical percentage of emergence according to the resistance of the crusts is shown by the curve

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Figure 2. Cumulative percentage of sugar beet seedlings having a force greater than a given value.

0

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0.5

0.6

0.7

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0.9

F(N) Penetrometer

Figure 3. Comparison of experimental emergence with theoretical emergence,' x and *,' bending hypothesis and hook configuration,' b = 0.693 mm, h = 1.783 mm. +,' bending hypothesis and hook configuration,' b = 1.5 mm, h = 3 mm. Eur. 1. Agron.

Emergence of sugar beet seedlings

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4. Force development kinetics,' -x-x-x-xcurve,' A = 0.02 N, B = 0.05 N, C = 0.08 N, Emergence kinetics for four different crust resistances,' .,+.,+ .. +. shifted curve,' A = 0,03 N, B = 0.07 N, C = 0.01 N, D

Figure

shift in the case of emergence through crusts whose resistance was at most equal to 0.07 N, and a 14-hour shift if the resistance was greater than 0.07 N made it possible to describe emergence through crusts. Lifting heavy obstacles The period of survival of a seedling under an obstacle, that is the possibility for this seedling to grow using the seed's reserves, has its limits. The seedling's ability to lift the obstacle but also to keep it lifted so that it can manage to get free of it, is an essential factor for satisfactory growth. We define the result of these two processes as emergence. We did not notice, even over a period of 5 days, any sudden fall of obstacles, but only, occasionaly, a decreasing value of heaving towards a stable value. Furthermore, we did not observe any plant escaping from an obstacle by growing horizontally under the obstacle parallel to the surface of the sand. First process

The percentage of seedlings capable of lifting obstacles in the range of 0.01 to 0.50 N was independent of the surface area of the obstacle and decreased Vol. 2, nO 3 - 1993

50

D = 0.15 N. = 0.2 N.

as the obstacle's weight increased; however this tendency was less marked for the observations carried out after 120 hours. Nevertheless, even for an observation period of 24 hours;, the experimental points representing the percentage of seedlings heaving the various obstacles were situated, on the whole, above the curve of hypocotyl force frequency (Figure 5). The superiority of the heaving emergence over force frequency was probably a result of a shift of the points of application of the hypocotyl force and the weight of the obstacle. This shift could have been caused on the one hand by the hook being initially badly placed under the obstacle (it was difficult to locate the centre of the ellipse and to bring it into contact with the centre of the disk), and on the other hand by the hypocotyl's spreading out over time by slipping on the lower surface of the obstacle. Mechanically, at the instant of heaving, the moments of force were equal (Figure 6). If the points of application of the weight of the obstacles and the hypocotyl force were the same, the two moments were equal: Fh*DI2 = Po*DI2

(D

= diameter

of the disk)

218

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.-II----~-·-·t=-_==J 0.4

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Under these conditions, the hypocotyl needed to exert a force which was less than the weight of the obstacle; for example, about 50 per cent of seedlings exerted a force of 0.15 N (curve of Figure 5) and 50 per cent of seedlings were able to lift obstacles of mass in the range of 0.15 N to 0.05 N.

p

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Figure 6. Respective positions of the points of application of the weight (P) of the disk and the force (F) of the hypocotyl. D is the diameter of the disk and x is the shift of the hypocotyl force from the centre of the disk.

If the point of application of the force of the hypocotyl was shifted by a distance x from the centre of the obstacle, the relationship became:

Fh *(D12 + x) = Po* D12 or Fh = (P o* D12)/(D12 + x)

Since it is impossible to attribute a given value representing the shift from the point of application of the force of the seedling, it seemed wiser to reduce these shifts by reducing the time at the end of which the observations on heaving were made. This was why we considered the number of seedlings having heaved an obstacle after about 6 hours (minimum observation period for the four series of obstacles) and traced the force frequency developed by the seedlings at the end of this period (Figure 7). We found a satisfactory agreement between the frequency curve and points representing the percentage of seedlings having heaved obstacles. Figure 7 showed that the obstacles greater than 0.20 N were heaved either very little (about 10 per cent of seedlings) or not at all during the first 6 hours and that at least 50 per cent of the seedlings heaved obstacles of a weight at most equal to 0.10 N during this same time. Second process

During the experiments, we observed that the percentage of seedlings capable of escaping from heaved Eur. 1. Agron.

Emergence of sugar beet seedlings

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obstacles, that is capable of emerging as previously defined, decreased with the increase in weight and surface area of the obstacle. Although no seedling had emerged in 24 hours, several seedlings (maximum 50 per cent) had escaped from obstacles of smaller surface areas and of a weight at most equal to 0.20 N in 48 hours; however, it was necessary to wait 120 hours for a maximum of 25 per cent of the seedlings to escape from large obstacles. The length of the experiments: 48 hours under obstacles with a surface area of 4.1 and 7.8 cm 2 and 120 hours under obstacles with a surface area of 15.9 and 30.2 cm 2, was probably too short to reach the maximum percentage of emerging seedlings. For wheat (the force of which is greater than that of sugar beet) Bouaziz (1991) noticed a maximum emergence after 5 days under blocks of 4 cm 2 and 0.11 N or 9 cm 2 and 0.25 N ; but it was only after 10 days that emergence was at its maximum under the blocks of 16 cm 2 and 0.45 N or of 25 cm 2 and 0.75 N.

30.2 cm 2). Emergence from under these large surface areas required a further 2 to 4 days due to a greater distance under the obstacle: 2.25 and 3.10 cm instead of 1.15 and 1.57 cm. These results enabled us to conclude that obstacles larger than 3.14 cm in diameter greatly reduced the chances of the sugar beet seedlings emerging.

Certain tendencies appeared on the kinetics curves during the periods studied (Figure 8). For obstacles of the same weight, the emergence characteristics (percentage and rate) were identical under the two smallest obstacles (4.1 and 7.8 cm 2). Emergence from under the small obstacles was greater than that obtained under the two largest obstacles (15.9 cm2 and

The emergence model developed in this paper is essentially based on the relationship between the biological variability of the force developed by the growing hypocotyl and either the resistance of the surface layer in the first case envisaged (crusts), or the weight of the free obstacle in the second case (heavy elements).

Vol. 2. n° 3 - 1993

CONCLUSION We pointed out in the introduction that the experiments carried out simulated two simple classes of seedbeds: one in the process of drying out after deterioration by the rain, the other aggregated where the binding forces of the structural elements were nil or very small. Each of these classes corresponded to a specific mechanical process and the modelling of the emergence kinetics was an essential tool for estimating the success in establishing a stand of seedlings.

220

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The growth forces of sugar beet were low compared with forces of other species and depended on the seedling age. The emergence of sugar beet through the crusts could be explained by a rupture with bends and fracture of the crust, whose kinetics were relatively well represented by the growth kinetics of the seedling force. The factor of the transfer of the hypocotyl force to the resistance of the crust (represented by the force necessary for a penetrometer to break or penetrate into the crust) enabled us , using the curve of force frequency , to obtain the variation of the maximum emergence percentage depending on the resistance of the crust. If the resistance reached 0.5 N, no emergence was possible and a threshold of 0.2 N could be fixed, below which a maximum emergence percentage of at least 50 per cent was obtained within about 40 hours . The curve of force frequency developed by a sugar beet stand during a limited period (about 6 hours) could be used as a basis for predicting a minimum percentage of seedlings able to lift an obstacle of a given weight. We therefore eliminated or at least reduced the variation of the moment of force of the seedling due to the morphological modifications of the hypocotyl. Nevertheless, we must not overlook the fact that because of the morphological modifications of the seedlings over time, the probabilities of heaving were multiplied by a factor l.5 to 2 if the duration of observations of lifting was extended beyond

this minimal period (Figure 5). The percentages obtained in this way after a limited period, constituted low values which then increased over time because of the shift in the point of application of the hypocotyl force. We noted that in 6 hours no seedling heaved a weight greater than 0.2 N and that the weight of the aggregated element must not exceed 0.10 N in order that at least 50 per cent of the seedlings might heave it. This first mechanism was a stage in the survival of the seedling, but full emergence including heaving and escaping the obstacle was very limited, even for weights lower than 0.10 N, if the diameter of the obstacle exceeded 3.1 cm (2* 1.57 cm). We have therefore at our disposal several limit values of criteria such as crust resistance and the sizes of clods or free pieces of crusts, essential for judging the quality of a seedbed or for organizing the preparation of a seedbed according to the type of soil and the growth force of the seedling. These threshold values could therefore be useful in modelling and would make it possible to introduce the effects of the seedbed quality on the final maximum emergence percentage.

REFERENCES Bouaziz A., Souty N. and Hicks D. (1990). Emergence force exerted by wheat seedlings. Soil Tillage Res. , 17, 211-219. Eur. 1. Ag ron.

Emergence of sugar beet seedlings

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