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Seasonal effects on the cropping-cycle of lettuce in glass-houses during the winter
Scientia Horticulturae, 11 (1979) 371--377
371
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
SEASONAL E F F E C T S ON THE CROPPING-CYCLE OF LETTUCE IN GLASSHOUSES D U R I N G THE WINTER
D. KLAPWIJK
Glasshouse Crops Research and Experiment Station, Naaldwijk (The Netherlands) (Accepted for publication 4 July 1979)
ABSTRACT Klapwijk, D., 1979. Seasonal effects on the cropping-cycle of lettuce in glasshouses during the winter. Scientia Hortic., 11 : 371--377. Current information about the duration of the lettuce cropping-cycles in glasshouses during the winter is presented as linear functions which provide a simple model for crop planning. The data are also plotted against the date in the middle of each cropping-period, in order to show as accurately as possible the effect of growing-conditions on the duration of the cropping-cycle. Plotting the data in this way is based on the concept that the croppingcycle depends mainly on seasonal radiation, presented by two-sided linear functions with a maximum value around the shortest day (21 December). The a m o u n t of radiation appears to be the dominant factor which determines the length of the cropping-cycle.
INTRODUCTION
In the production of butterhead lettuce under glass in winter, the croppingcycle is regarded as the basis of crop planning, usually in relation to the planting-date. However, the planting-date does not provide any reliable information a b o u t the effects of the season on the cropping-cycle as the season has yet to start. By the same token, there would be little sense in examining the cropping-cycle on the harvest date as by this time the seasonal effects have passed completely. Therefore, a better basis is required for the interpretation of the seasonal effects on the cropping-cycle. The seasonal effects axe determined to a large extent by the level of radiation. The rate of crop growth is reduced by decreasing radiation. The process is not proportional, as the plant tends to make better use of the available light when it is weaker. In other words, in winter the radiation level is reduced, and though the amount of light required for each gramme increase in plant weight is reduced, the rate of growth is decreasing. MATERIALS AND METHODS
Two stages of growth are recognised in the commercial production of butterhead lettuce. In the first place, there is the plant-raising period. The y o u n g
372 plants grown in soil-blocks are usually supplied to the grower by specialised plant propagation nurseries. The second stage starts with planting out in the cropping-houses and finishes when the lettuce is harvested. In order to study the effects of the season on the cropping cycle, use was made of data supplied by propagation nurseries, the Agricultural Economic Research Institute (L.E.I.) and the Research Station at Naaldwijk. For all the crops for which data were supplied, there were also daily radiation data available from meteorological Kipp solarimeter readings at Naaldwijk. The cropping cycles obtained from commercial plant raisers were plotted against the beginning and end dates of the cycle concerned, and regression equations were computed. For the positive parts data were used until the m a x i m u m croppingcycle, and for the negative slope the rest of the data were used, including the maximum cycle. When the cropping-data were plotted against the date mid-way between the beginning and end of the cropping-cycle (middle date), the separation between the data for the positive and negative slope was laid at a middle date of 21 December. If light is the dominant growth factor, the two lines should meet each other at that point. No attention was paid to the cultivars because of the very small differences existing between them during the winter. None of them exhibits any reaction to the shift in day-length during the season. Day-length is only of interest because it is correlated with the a m o u n t of light-energy per day. No data were available about the light transmission of the glasshouses in the commercial holdings from which cropping-cycles were collected. The holdings were sampled at random by the Agricultural Economic Research Institute. Most of the greenhouses in the Westland area, however, are up to date. The heating-capacity was not equal in all cases. This implies temperature differences when outside temperatures were low. Another source of variability in the length of the cropping-cycle are the small differences in initial weight and in weight at harvest. RESULTS T h e seasons and the p l a n t raising- and cropping-cycles. -- In commercial practice, an empirical relationship is used between the season and the cropping-cycle to predict harvest date. This relationship has been described by Van der Hoeven and Groenewegen (1970) and Van Esch (1976). Figure 1 shows the relationship according to Van Esch. Another relationship between the cropping-cycle and the season could be computed on the basis of the cropping-recommendations given by the lettuce-plant raisers to their customers. This is a linear relationship, also shown in Fig. 1. The cultural recommendations concerning sowing, planting- and harvestdates supplied by the lettuce-plant raisers during the 1977/78 season also show a linear relationship, with the season for the propagation period (Fig. 2). The pattern is shown in the figure for both the sowing-date and the planting-date.
,oo
373
Days
80
60
40 20
,
2'0 ,4'0
1/9
6'0
1/10
8'0 ,;ao 1~
1/11
1112
1~0 ,1~0 ,~p 2ao rio Day numb*r
111
112
113
1/4 Planting outdate
Fig. 1. Cropping-cycle of lettuce from planting o u t until harvest, p l o t t e d against the date of planting o u t (1/9 = 1 September, etc.), according to Van Esch (1976) (flowing line) and according to r e c o m m e n d a t i o n s f r o m lettuce plant raisers (dots). C o m p u t e d functions: 2 < x< 58:y= 1.16 x + 37.4 r=0.982; 58 < x < 210 : y = - - 0 . 4 0 6 x + 124 r = 0.984.
Days 70 Middle date 60 •
5O
S
o,"
°l
w
I
°° o
o
~
•
0% Planting ~o date
40 3O lie
20
0
10
1~9
20
i
,:o
1110
1/11
80
i
16o ,
1112
,
1/1
14o 1~o I i
112
1/3
2'0o 2;O Day °umber i
114 Date
Fig. 2. Plant-raising cycle of lettuce on plant-raising nurseries, f r o m sowing until dispatch. C o m p u t e d functions p l o t t e d against the sowing-date: 10 < x < 86 : y = 0.683 x + 4.82 r = 0.986 86 < x < 2 0 6 : y = - 0 . 3 8 3 x + 94.1 r=0.996; plotted against the middle date: 18 < x < 1 1 6 : y = 0.514 x + 3.58 r = 0.993; 116 ~< x ~< 214 : y = --0.471 x + 1 1 6 r=0.994; plotted against the planting-date: 26 < x < 1 4 7 : y = 0.408 x + 2.62 r=0.995; 147 < x < 2 2 2 : y = --0.616 x + 1 5 1 r=0.989. Cropping-cycle and the middle date. -- Figure 2 shows that neither the sowing date nor the planting date demonstrate the annual radiation cycle. D e p e n d i n g o n t h e t i m e s c a l e c h o s e n , t h e m a x i m u m p l a n t - r a i s i n g c y c l e is o b t a i n e d w i t h a s o w i n g - d a t e o n 2 3 / 1 1 o r a p l a n t i n g - d a t e o n 2 4 / 1 . H o w e v e r , if t h e
374
plant-raising periods are plotted against a date mid-way between sowing and planting out (middle date), then the maximum plant-raising cycle practically coincides with the depth of winter (Fig. 2). This relationship is also found for the cropping-period, as can be demonstrated b y plotting the data of the linear relationship between the season and the cropping-cycle in Fig. 1 against the middle date.
The cropping-cycles of commercial lettuce production in winter. In publications of the Agricultural Economic Research Institute (L.E.I.) (Anonymous, 1974, 1975, 1976), the cropping-cycles obtained on a large number of lettuce nurseries during the 6 winter months of 3 years are described. The relationships between the middle date and the cropping-cycle for each season have been c o m p u t e d and the results are the following regression equations, where x = day number after 31 August and y = cropping cycle (days) from planting out until harvesting: -
Autumn
1973 1974 1975
Spring
1974 1975 1976
25 ~
-
22.4 21.1 24.8
r=0.890 n = 3 3 r=0.924 n = 3 8 r=0.926 n = 5 6
x +191 x +150 x +155
r=0.919 n = 3 9 r=0.854 n=55 r=0.826 n = 5 3
The average of the relationships of the 3 years is shown in Fig. 3, where the relationship is also shown of the observations recorded at the Efford Experimental Horticulture Station (Anonymous, 1972, 1973) in the south of England during the 1972/73 season. Days 100
1",~~a
80
nd
60 z.O 20
2'o ~o 6'0 8~ ,~o ,I~o 12o rio ~o 2bo riO Day °o~be, 1/9
1~10
1/11
1112 211121/1
112
113
11/. Middle date
Fig. 3. C r o p p i n g - c y c l e o f l e t t u c e f r o m p l a n t i n g o u t u n t i l h a r v e s t , p l o t t e d a g a i n s t t h e m i d d l e date. R e g r e s s i o n e q u a t i o n s : H o l l a n d , average f o r t h e s e a s o n s 1 9 7 3 / 7 4 , 1 9 7 4 / 7 5 a n d 1 9 7 5 / 7 6 (see a b o v e ) : 16 ~< x < 1 1 2 : y = 0 . 7 1 2 x + 23.4 r= 0.913 n = 127; 1 1 2 ~< x ~< 227 : y = --0.523 x +161 r= 0.842 n = 147; Efford (U.K.), for the season 1 9 7 2 / 7 3 (Harnett, 1975): 44 ~< x ~< 1 1 2 : y = 0.726 x + 5.44 r=0.965 n= 27; 112 < x < 1 9 4 : y = --0.603 x + 152 r= 0.968 n = 27.
375 DISCUSSION T h e l i n e a r r e l a t i o n s h i p b e t w e e n c r o p p i n g - c y c l e a n d season. - - The best startingpoint for a discussion of the relationship between the cropping-cycle of lettuce and the season is the plant-raising stage (Fig. 2), since this is more standardised and shorter than the cropping-stage. Also, during the plant-raising stage there are hardly any disruptions as a result of diseases and disorders. In addition, ideas a b o u t the plant-raising cycle have remained unchanged for a considerable time. There are hardly any differences between the results obtained by Van der Hoeven and Groenewegen (1970) and the cultural recommendations issued in 1977 (Fig. 2). The relationship between the cropping-cycle and the season is shown as a complicated curve by Van der Hoeven and Groenewegen (1970) and also by Van Esch (1976) (Fig. 1). However, taking all the data together, it becomes apparent that the relationship (between the end of September and the beginning of April) is bilinear. As already quoted, the data in the L.E.I. publications (Anonymous, 1974, 1975, 1976) a b o u t the stage between planting and harvesting show rather a wide variation. Nevertheless, Fig. 3 showed that the relationship between season and cropping-cycle may also be regarded as linear for the data of the second cropping-stage. For prediction of cropping-cycles one may use the linear function, because at least 80% of the variability is explained by this equation. This is much simpler than the complicated curves of Van der Hoeven and Groenewegen (1970) and Van Esch (1976). T h e use o f t h e m i d d l e data. - - For a description of the effects of the season on the cropping-cycle, it is obvious that the use of the start or the end of this cycle as the basis gives a distorted view. It is for this reason that ,the description has been based on the date in the middle of the relevant period (Figs. 2 and 3). The same choice was made earlier by Klapwijk and De Lint (1975a) in their work on the growth of young t o m a t o plants. In the latter case, the middle of the growing-period coincided with a certain weight, since the starting-point of the work was the growing-period between 0.1 and 10 g fresh weight. It had been shown already that the relative growth rate during this growth stage is constant. It seemed sensible to use the same approach for lettuce, be it with the provisos mentioned under "Materials and Methods". This m e t h o d may be used for different vegetative and developmental stages in different plant species. By using the middle date, the maximum cropping-cycles coincide fairly accurately with 21 December (Fig. 3). Considering the annual radiation cycle, this is a satisfactory result in every respect. Should a different growth factor be of great value for winter growth, the maximum cropping-cycle should be situated before or after 21 December, but even if the maximum cropping-cycle on 21 December is determined by another growth factor in correlation with radiation, it is suitable to use the middle date. It will demonstrate the relation between plant growth and growth conditions during the relevant stage. The data presented for Efford by Harnett (1975), plotted in Fig. 3, also show
376 a maximum cropping-cycle on a middle date of 21 December, although the cropping-cycles are shorter than in Holland. In Denmark, Christoffersen (1973) also collected data on cropping-cycles under lightAimiting conditions. These data fit nicely to the bilinear functions presented. However, it cannot be proved that the maximum cycle in Denmark will also be found at 21 December, since midwinter observations are t o o few. E f f e c t s o f radiation on the cropping-cycle. -- If the cropping-cycle is plotted
against the date in the middle of that period (Fig. 3), it becomes clear that the maximum cropping-cycle coincides with the date of minimum radiation (21 December). This indicates that radiation is the most important factor which determines the length of the cropping-cycle. This is valid for the data from plant raisers a b o u t propagation and cropping as well as for all data of the Agricultural Economic Research Institute (Anonymous, 1974, 1975, 1976), which were derived from a great range of commercial holdings with all variabili ty in cropping-practices included. Irrespective of the different sources of variation, however, about 70% of the variability is explained by the equations used. The temperature will not be of great importance since most growers will try to keep temperatures more or less constant. If it may be assumed from the cropping-results that the maximum croppingcycle coincides definitely with the middle date of 21 December, it may also be assumed that the bilinear character of the relationship is maintained, with the minimum around 21 December when the cropping cycle is shortened by improved growing-conditions. However, the level, and to a lesser extent the slope, will change as in the case of Efford versus Holland (Fig. 3). This change has also been recorded for young t o m a t o plants by Klapwijk and De Lint (19755). CONCLUSION The series of data on the cropping-cycles of lettuce under glass which have been published, and are used mainly for crop planning, may all be described with regard to the season as linear functions, providing a simple model for crop planning. If one plots each cropping-cycle against the middle date of that period, a straight line may be drawn through the points. One straight line runs from the end of summer to 21 December, and another from mid-winter (21 December) to spring. The spread around this two-sided linear function is slight and no greater than where it was deemed necessary to draw flowing lines through the data. This flowing-illustration of the relationship between the cropping-cycle and the season means that the extreme values for the very long cropping-cycles are levelled off too much. All available data show a maximum cropping-cycle on the middle date of 21 December. It is concluded that seasonal radiation is the dominant factor affecting the duration of the cropping-cycle, but even if other seasonal growth factors play a role, one gets a good impression of the effect by plotting the duration of the cropping-cycles in the middle of the periods concerned.
377 ACKNOWLEDGEMENTS
Thanks are due to all those who made available the necessary data for this study. Thanks are also expressed to P.J.A.L. de Lint for his critical reading of the manuscript.
REFERENCES Anonymous, 1972, 1973. Efford Experimental Horticulture Station. Annu. Rep., 1972 and 1973. Anonymous, 1974, 1975, 1976. Overzicht van opbrengsten van sla in het Westland, 1973/ 74, 1974/75 and 1975/76. L.E.I., The Hague, nos. 632, 649, 672. Christoffersen, A., 1973. Programmeret dyrkning af salat i hus. Statens Fors~bgsvirksomhed i Plantekultur 1125, beretning: 669--688. Esch, I-L van, 1976. Plant- en oogsttijden bij sla. Groenten Fruit, 32: 143. Harnett, R.F., 1975. Study of glasshouse type and orientation. Acta Hortic., 46: 209--215. Hoeven, A.P., van der and Groenewegen, J.H., 1970. Zaai-, plant- en oogsttijden bij sla. Tuinderij, 10: 567--570. Klapwijk, D. and De Lint, P.J.A.L., 1975a. Growth rates of t o m a t o seedlings and seasonal radiation. Neth. J. Agric. Sci., 23: 2 5 9 - 2 6 8 . Klapwijk, D. and De Lint, P.J.A.L., 1975b. Growth and development of young t o m a t o plants. Acta Hortic., 51: 147--161.
371
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
SEASONAL E F F E C T S ON THE CROPPING-CYCLE OF LETTUCE IN GLASSHOUSES D U R I N G THE WINTER
D. KLAPWIJK
Glasshouse Crops Research and Experiment Station, Naaldwijk (The Netherlands) (Accepted for publication 4 July 1979)
ABSTRACT Klapwijk, D., 1979. Seasonal effects on the cropping-cycle of lettuce in glasshouses during the winter. Scientia Hortic., 11 : 371--377. Current information about the duration of the lettuce cropping-cycles in glasshouses during the winter is presented as linear functions which provide a simple model for crop planning. The data are also plotted against the date in the middle of each cropping-period, in order to show as accurately as possible the effect of growing-conditions on the duration of the cropping-cycle. Plotting the data in this way is based on the concept that the croppingcycle depends mainly on seasonal radiation, presented by two-sided linear functions with a maximum value around the shortest day (21 December). The a m o u n t of radiation appears to be the dominant factor which determines the length of the cropping-cycle.
INTRODUCTION
In the production of butterhead lettuce under glass in winter, the croppingcycle is regarded as the basis of crop planning, usually in relation to the planting-date. However, the planting-date does not provide any reliable information a b o u t the effects of the season on the cropping-cycle as the season has yet to start. By the same token, there would be little sense in examining the cropping-cycle on the harvest date as by this time the seasonal effects have passed completely. Therefore, a better basis is required for the interpretation of the seasonal effects on the cropping-cycle. The seasonal effects axe determined to a large extent by the level of radiation. The rate of crop growth is reduced by decreasing radiation. The process is not proportional, as the plant tends to make better use of the available light when it is weaker. In other words, in winter the radiation level is reduced, and though the amount of light required for each gramme increase in plant weight is reduced, the rate of growth is decreasing. MATERIALS AND METHODS
Two stages of growth are recognised in the commercial production of butterhead lettuce. In the first place, there is the plant-raising period. The y o u n g
372 plants grown in soil-blocks are usually supplied to the grower by specialised plant propagation nurseries. The second stage starts with planting out in the cropping-houses and finishes when the lettuce is harvested. In order to study the effects of the season on the cropping cycle, use was made of data supplied by propagation nurseries, the Agricultural Economic Research Institute (L.E.I.) and the Research Station at Naaldwijk. For all the crops for which data were supplied, there were also daily radiation data available from meteorological Kipp solarimeter readings at Naaldwijk. The cropping cycles obtained from commercial plant raisers were plotted against the beginning and end dates of the cycle concerned, and regression equations were computed. For the positive parts data were used until the m a x i m u m croppingcycle, and for the negative slope the rest of the data were used, including the maximum cycle. When the cropping-data were plotted against the date mid-way between the beginning and end of the cropping-cycle (middle date), the separation between the data for the positive and negative slope was laid at a middle date of 21 December. If light is the dominant growth factor, the two lines should meet each other at that point. No attention was paid to the cultivars because of the very small differences existing between them during the winter. None of them exhibits any reaction to the shift in day-length during the season. Day-length is only of interest because it is correlated with the a m o u n t of light-energy per day. No data were available about the light transmission of the glasshouses in the commercial holdings from which cropping-cycles were collected. The holdings were sampled at random by the Agricultural Economic Research Institute. Most of the greenhouses in the Westland area, however, are up to date. The heating-capacity was not equal in all cases. This implies temperature differences when outside temperatures were low. Another source of variability in the length of the cropping-cycle are the small differences in initial weight and in weight at harvest. RESULTS T h e seasons and the p l a n t raising- and cropping-cycles. -- In commercial practice, an empirical relationship is used between the season and the cropping-cycle to predict harvest date. This relationship has been described by Van der Hoeven and Groenewegen (1970) and Van Esch (1976). Figure 1 shows the relationship according to Van Esch. Another relationship between the cropping-cycle and the season could be computed on the basis of the cropping-recommendations given by the lettuce-plant raisers to their customers. This is a linear relationship, also shown in Fig. 1. The cultural recommendations concerning sowing, planting- and harvestdates supplied by the lettuce-plant raisers during the 1977/78 season also show a linear relationship, with the season for the propagation period (Fig. 2). The pattern is shown in the figure for both the sowing-date and the planting-date.
,oo
373
Days
80
60
40 20
,
2'0 ,4'0
1/9
6'0
1/10
8'0 ,;ao 1~
1/11
1112
1~0 ,1~0 ,~p 2ao rio Day numb*r
111
112
113
1/4 Planting outdate
Fig. 1. Cropping-cycle of lettuce from planting o u t until harvest, p l o t t e d against the date of planting o u t (1/9 = 1 September, etc.), according to Van Esch (1976) (flowing line) and according to r e c o m m e n d a t i o n s f r o m lettuce plant raisers (dots). C o m p u t e d functions: 2 < x< 58:y= 1.16 x + 37.4 r=0.982; 58 < x < 210 : y = - - 0 . 4 0 6 x + 124 r = 0.984.
Days 70 Middle date 60 •
5O
S
o,"
°l
w
I
°° o
o
~
•
0% Planting ~o date
40 3O lie
20
0
10
1~9
20
i
,:o
1110
1/11
80
i
16o ,
1112
,
1/1
14o 1~o I i
112
1/3
2'0o 2;O Day °umber i
114 Date
Fig. 2. Plant-raising cycle of lettuce on plant-raising nurseries, f r o m sowing until dispatch. C o m p u t e d functions p l o t t e d against the sowing-date: 10 < x < 86 : y = 0.683 x + 4.82 r = 0.986 86 < x < 2 0 6 : y = - 0 . 3 8 3 x + 94.1 r=0.996; plotted against the middle date: 18 < x < 1 1 6 : y = 0.514 x + 3.58 r = 0.993; 116 ~< x ~< 214 : y = --0.471 x + 1 1 6 r=0.994; plotted against the planting-date: 26 < x < 1 4 7 : y = 0.408 x + 2.62 r=0.995; 147 < x < 2 2 2 : y = --0.616 x + 1 5 1 r=0.989. Cropping-cycle and the middle date. -- Figure 2 shows that neither the sowing date nor the planting date demonstrate the annual radiation cycle. D e p e n d i n g o n t h e t i m e s c a l e c h o s e n , t h e m a x i m u m p l a n t - r a i s i n g c y c l e is o b t a i n e d w i t h a s o w i n g - d a t e o n 2 3 / 1 1 o r a p l a n t i n g - d a t e o n 2 4 / 1 . H o w e v e r , if t h e
374
plant-raising periods are plotted against a date mid-way between sowing and planting out (middle date), then the maximum plant-raising cycle practically coincides with the depth of winter (Fig. 2). This relationship is also found for the cropping-period, as can be demonstrated b y plotting the data of the linear relationship between the season and the cropping-cycle in Fig. 1 against the middle date.
The cropping-cycles of commercial lettuce production in winter. In publications of the Agricultural Economic Research Institute (L.E.I.) (Anonymous, 1974, 1975, 1976), the cropping-cycles obtained on a large number of lettuce nurseries during the 6 winter months of 3 years are described. The relationships between the middle date and the cropping-cycle for each season have been c o m p u t e d and the results are the following regression equations, where x = day number after 31 August and y = cropping cycle (days) from planting out until harvesting: -
Autumn
1973 1974 1975
Spring
1974 1975 1976
25 ~
-
22.4 21.1 24.8
r=0.890 n = 3 3 r=0.924 n = 3 8 r=0.926 n = 5 6
x +191 x +150 x +155
r=0.919 n = 3 9 r=0.854 n=55 r=0.826 n = 5 3
The average of the relationships of the 3 years is shown in Fig. 3, where the relationship is also shown of the observations recorded at the Efford Experimental Horticulture Station (Anonymous, 1972, 1973) in the south of England during the 1972/73 season. Days 100
1",~~a
80
nd
60 z.O 20
2'o ~o 6'0 8~ ,~o ,I~o 12o rio ~o 2bo riO Day °o~be, 1/9
1~10
1/11
1112 211121/1
112
113
11/. Middle date
Fig. 3. C r o p p i n g - c y c l e o f l e t t u c e f r o m p l a n t i n g o u t u n t i l h a r v e s t , p l o t t e d a g a i n s t t h e m i d d l e date. R e g r e s s i o n e q u a t i o n s : H o l l a n d , average f o r t h e s e a s o n s 1 9 7 3 / 7 4 , 1 9 7 4 / 7 5 a n d 1 9 7 5 / 7 6 (see a b o v e ) : 16 ~< x < 1 1 2 : y = 0 . 7 1 2 x + 23.4 r= 0.913 n = 127; 1 1 2 ~< x ~< 227 : y = --0.523 x +161 r= 0.842 n = 147; Efford (U.K.), for the season 1 9 7 2 / 7 3 (Harnett, 1975): 44 ~< x ~< 1 1 2 : y = 0.726 x + 5.44 r=0.965 n= 27; 112 < x < 1 9 4 : y = --0.603 x + 152 r= 0.968 n = 27.
375 DISCUSSION T h e l i n e a r r e l a t i o n s h i p b e t w e e n c r o p p i n g - c y c l e a n d season. - - The best startingpoint for a discussion of the relationship between the cropping-cycle of lettuce and the season is the plant-raising stage (Fig. 2), since this is more standardised and shorter than the cropping-stage. Also, during the plant-raising stage there are hardly any disruptions as a result of diseases and disorders. In addition, ideas a b o u t the plant-raising cycle have remained unchanged for a considerable time. There are hardly any differences between the results obtained by Van der Hoeven and Groenewegen (1970) and the cultural recommendations issued in 1977 (Fig. 2). The relationship between the cropping-cycle and the season is shown as a complicated curve by Van der Hoeven and Groenewegen (1970) and also by Van Esch (1976) (Fig. 1). However, taking all the data together, it becomes apparent that the relationship (between the end of September and the beginning of April) is bilinear. As already quoted, the data in the L.E.I. publications (Anonymous, 1974, 1975, 1976) a b o u t the stage between planting and harvesting show rather a wide variation. Nevertheless, Fig. 3 showed that the relationship between season and cropping-cycle may also be regarded as linear for the data of the second cropping-stage. For prediction of cropping-cycles one may use the linear function, because at least 80% of the variability is explained by this equation. This is much simpler than the complicated curves of Van der Hoeven and Groenewegen (1970) and Van Esch (1976). T h e use o f t h e m i d d l e data. - - For a description of the effects of the season on the cropping-cycle, it is obvious that the use of the start or the end of this cycle as the basis gives a distorted view. It is for this reason that ,the description has been based on the date in the middle of the relevant period (Figs. 2 and 3). The same choice was made earlier by Klapwijk and De Lint (1975a) in their work on the growth of young t o m a t o plants. In the latter case, the middle of the growing-period coincided with a certain weight, since the starting-point of the work was the growing-period between 0.1 and 10 g fresh weight. It had been shown already that the relative growth rate during this growth stage is constant. It seemed sensible to use the same approach for lettuce, be it with the provisos mentioned under "Materials and Methods". This m e t h o d may be used for different vegetative and developmental stages in different plant species. By using the middle date, the maximum cropping-cycles coincide fairly accurately with 21 December (Fig. 3). Considering the annual radiation cycle, this is a satisfactory result in every respect. Should a different growth factor be of great value for winter growth, the maximum cropping-cycle should be situated before or after 21 December, but even if the maximum cropping-cycle on 21 December is determined by another growth factor in correlation with radiation, it is suitable to use the middle date. It will demonstrate the relation between plant growth and growth conditions during the relevant stage. The data presented for Efford by Harnett (1975), plotted in Fig. 3, also show
376 a maximum cropping-cycle on a middle date of 21 December, although the cropping-cycles are shorter than in Holland. In Denmark, Christoffersen (1973) also collected data on cropping-cycles under lightAimiting conditions. These data fit nicely to the bilinear functions presented. However, it cannot be proved that the maximum cycle in Denmark will also be found at 21 December, since midwinter observations are t o o few. E f f e c t s o f radiation on the cropping-cycle. -- If the cropping-cycle is plotted
against the date in the middle of that period (Fig. 3), it becomes clear that the maximum cropping-cycle coincides with the date of minimum radiation (21 December). This indicates that radiation is the most important factor which determines the length of the cropping-cycle. This is valid for the data from plant raisers a b o u t propagation and cropping as well as for all data of the Agricultural Economic Research Institute (Anonymous, 1974, 1975, 1976), which were derived from a great range of commercial holdings with all variabili ty in cropping-practices included. Irrespective of the different sources of variation, however, about 70% of the variability is explained by the equations used. The temperature will not be of great importance since most growers will try to keep temperatures more or less constant. If it may be assumed from the cropping-results that the maximum croppingcycle coincides definitely with the middle date of 21 December, it may also be assumed that the bilinear character of the relationship is maintained, with the minimum around 21 December when the cropping cycle is shortened by improved growing-conditions. However, the level, and to a lesser extent the slope, will change as in the case of Efford versus Holland (Fig. 3). This change has also been recorded for young t o m a t o plants by Klapwijk and De Lint (19755). CONCLUSION The series of data on the cropping-cycles of lettuce under glass which have been published, and are used mainly for crop planning, may all be described with regard to the season as linear functions, providing a simple model for crop planning. If one plots each cropping-cycle against the middle date of that period, a straight line may be drawn through the points. One straight line runs from the end of summer to 21 December, and another from mid-winter (21 December) to spring. The spread around this two-sided linear function is slight and no greater than where it was deemed necessary to draw flowing lines through the data. This flowing-illustration of the relationship between the cropping-cycle and the season means that the extreme values for the very long cropping-cycles are levelled off too much. All available data show a maximum cropping-cycle on the middle date of 21 December. It is concluded that seasonal radiation is the dominant factor affecting the duration of the cropping-cycle, but even if other seasonal growth factors play a role, one gets a good impression of the effect by plotting the duration of the cropping-cycles in the middle of the periods concerned.
377 ACKNOWLEDGEMENTS
Thanks are due to all those who made available the necessary data for this study. Thanks are also expressed to P.J.A.L. de Lint for his critical reading of the manuscript.
REFERENCES Anonymous, 1972, 1973. Efford Experimental Horticulture Station. Annu. Rep., 1972 and 1973. Anonymous, 1974, 1975, 1976. Overzicht van opbrengsten van sla in het Westland, 1973/ 74, 1974/75 and 1975/76. L.E.I., The Hague, nos. 632, 649, 672. Christoffersen, A., 1973. Programmeret dyrkning af salat i hus. Statens Fors~bgsvirksomhed i Plantekultur 1125, beretning: 669--688. Esch, I-L van, 1976. Plant- en oogsttijden bij sla. Groenten Fruit, 32: 143. Harnett, R.F., 1975. Study of glasshouse type and orientation. Acta Hortic., 46: 209--215. Hoeven, A.P., van der and Groenewegen, J.H., 1970. Zaai-, plant- en oogsttijden bij sla. Tuinderij, 10: 567--570. Klapwijk, D. and De Lint, P.J.A.L., 1975a. Growth rates of t o m a t o seedlings and seasonal radiation. Neth. J. Agric. Sci., 23: 2 5 9 - 2 6 8 . Klapwijk, D. and De Lint, P.J.A.L., 1975b. Growth and development of young t o m a t o plants. Acta Hortic., 51: 147--161.