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Effect of different day and night temperature regimes on greenhouse cucumber young plant production, flower bud formation and early yield
Scienti~ Horticulturae, 53 ( 1993 ) 191-204
191
Elsevier Science Publishers B.V., A m s t e r d a m
Effect of different day and night temperature regimes on greenhouse cucumber young plant production, flower bud formation and early yield Svein O. G r i m s t a d and Endre Frimanslund ~ Agricu,'tural University of Norway, Department of Horticulture, PO Box 22, N-1432•s-NLH, Norway (Accepted 22 July 1992)
ABSTRACT Grimstad, S.O. and Frimanslund, E., 1993. Effect of different day and night temperature regimes on greenhouse cucumber young plant production, flower bud formation and early yield. Scientia Hottic., 53: 191-204. The effects of different day temperature (DT) and night temperature (NT) combinations on the growth and development of young cucumber plants during propagation were investigated. The temperature combinations gave differences (DIF) between DT and NT ranging from - 12 to + 12°C. After propagation the plants were transferred to a greenhouse and grown at a constant DT and NT set point of 21 °C for 8 weeks after propagation. The experiment was carried out in early spring under natural light conditions. The rate of development was dependent on the temperature regime. DT lower than NT inhibited growth .and development of the young plants, except for leaf unfolding rate which depended on the average daily temperature (ADT) only. Both an increase in ADT and DIF enhanced internode length. However, increasing the ADT by only raising DT affected the elongation to a larger extent than an equal raise of DT and NT. For flower bud development an increase in negative values of DIF reduced the nurrtber of flower buds to a greater extent than an increase in positive values. The c ptimum propagation temperature, based on earliness (first harvest) and early yield was approximately 25 ° C. Raising the ADT from 15 to 25 °C reduced the time to first harvest by 1.6 days per 1 °C and increased the average total yield during the first 8 weeks after transplanting by 0.54 kg m -2 ° C - 1. DIF had no effect on earliness nor the yield, but improved fruit quality. Keywords: Cucumis sativus; flowering; fruit quality; temperature; yield. Abbreviations: ADT = average daily temperature; CT = constant temperature; DIF = difference between I3,T and NT; DT = day temperature; NT = night temperature.
Correspondence to: S.O. G r i m s t a d , Saerheim Research Station, N-4062, Klepp St., Norway. ~Present address: Ullensvang Research Station, N-5774 Lofthus, Norway.
© 1993 Elsevier Science Publishers B.V. All rights reserved 0 3 0 4 - 4 2 3 8 / 9 3 / $ 0 6 . 0 0
192
S.O. GR1MSTAD AND E. FRIMANSLUND
INTRODUCTION
Propagation of cucumber has considerable commercial importance for both the seedling producer and the greenhouse cucumber grower. At the time of transplanting, the cucumber plant should be vigorous, close to flowering, and not too tall. Cucumber growers want plants with only one or two fruits per node so only two flower buds should be formed (Anonymous, 1980). Maturity of a plant is often correlated with a large plant. However, if plants grow too tall, they become troublesome to maintain during propagation, transport and planting. One way to grow shorter plants is to apply a growth retardant (S.O. Grimstad, unpublished results, 1990). This is not legal on vegetables in many countries today because of the potential danger of chemical residues. Traditionally, young plants are cultivated at a day temperature (DT) somewhat higher than the night temperature (NT). It has been known for many years that stem elongation is thermoperiodic in many species (Went, 1944, 1953; Kristoffersen, 1963 ). Mature plant height increases when plants are grown with day temperature higher than night temperature in both short day and long day plants (Moe and Heide, 1985; Erwin et al., 1989). Erwin et al. (1989) found that the relationship between DT and NT influenced final plant height to a greater extent than DT, NT or ADT in Lilium longiflorum. Calvert (1964), Hussey (1965) and Klapwijk and Wubben (1978) reported a reduction in growth of young tomato plants with an inverse temperature regime. Hurd and Graves (1984) and De Koning (1988), however, found that total tomato yield was not influenced by the temperature regime, but mainly by the temperature integral. The same was found for the yield of sweet pepper (Hand and Hannah, 1978 ) and cucumber ( Slack and Hand, 1983 ). Negative DIF temperature regimes may be a strategy for the seedling producer to substitute growth retardants. Negative DIF temperature regimes may also result in profitable energy savings when thermal screens are used in winter (Leatherland, 1986). The objective of the present research was to quantify thermomorphogenic responses in young plants of greenhouse cucumbers. This is being done in a general attempt to find a way to control plant height without increase in flower buds, delay in time to first harvest, decrease in yield, or reduction in fruit quality. MATERIAL AND METHODS
Cucumber seeds (Cucumis sativus cultivar 'Farbiola') were sown separately in 12 cm plastic pots (0.671 ) filled with fertilized and limed Norwegian peat moss (Horalux) on 22 February and germinated at 25 + 1 °C. The seedlings received supplemental light (85/zmol m - 2 s - l ) for 20 h per day with
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
193
cool white fluorescent lamps (Philips TLD 33 ) after germination. Ten days after ,,;owing (when the cotyledons were horizontal), seedlings were selected for uniformity, whereafter they were placed in a daylight phytotron with temperature setpoints of 15, 18, 21, 24, or 27 °C and grown under ambient light conditions for 3 weeks (latitude 5 9 ° 5 6 ' N ) . Plants were placed on trolleys with a plant density of approximately 20 plants m -2 and moved among the five temperature sections twice a day to provide factorial DT and NT treatments; with 25 plants per treatment. To obtain the same transportation stress, plants held at constant temperature were moved from one location to another within the compartment. The 12-h DT period was defined from 07:00 to 19:00 h when the plants received the high irradiance of sunlight and the NT in the low light period from 19:00 to 07:00 h. Sunrise and sunset at the beginning of the experiment was algproximately 07:00 and 18:00 h, and at the end of the experiment approximately 06:00 and 18:45 h, respectively. The average daily global radiation during the propagation period was 19.2 mol m -2 day -1. Temperature variations in the phytotron compartments were < 0.5 °C deviation from the set point. The vapour pressure difference (zlX) was kept at 4.7 g m -3. No CO2 enrichment was given during propagation. The plants were watered regularly after emergence with a complete nutrient solution at an EC level of 1.0 mS cm -1 . The solution was composed of a complete fertilizer (Red Superba), calcium nitrate and potassium sulphate and contained: 158 N, 40 P, 244 K, 38 Mg, 116 Ca, 54 S, 2.0 Fe, 1.1 Mn, 0.20 Cu, 0.30 Zn, 0.33 B and 0.03 Mo mg 1Twenty plants were removed from the phytotron and planted in two 12 m X 1"7m greenhouses to observe subsequent development at the end of propagation, i.e. 21 days after cotyledon expansion. All plants were grown at constant 21 °C DT and NT set point under natural light conditions (average 36.8 mol m - 2 day- 1). The ventilation temperature was set at 25 ° C. Root temperature was maintained at 20-22 °C with substrate heating. Pure supplementary CO2 ( 1000 [tl 1-1 ) was given daily from planting, except when the ventilator opening exceeded 5 cm. Vapour pressure difference (•X) in the greenhouse was at m a x i m u m 5.6 g m -3 in the day and not less than 1.9 g m -3 during; the night. The plants were placed on fertilized and limed peat moss slabs (30 c m × 13 c m X 110 cm) with an overall population density of 1.7 plants m -2. In each greenhouse, the treatments were replicated twice, so final layout in each house consisted of 50 plots of five plants surrounded overall by guard plants. The EC level of the nutrient solution was increased to 1.8 mS c m - ] after transplanting. Buds of side shoots were removed at appearance after transplanting. The main stem was decapitated 2.75 m above the slabs, just above the suspending wire. Flower buds were removed from the lowest axil up to Axil 5. The experiment,; were terminated 8 weeks after transplanting.
194
S.O. GRIMSTAD AND E. FRIMANSLUND
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Fig. 1. Effect of average daily temperature (A) on dry weight production; (B) root development (based on a 0-9 scale, where 9 was the highest score); (C) number of flower buds per node (Nodes 6-20); ( D ) rate of progress to harvest; (E) yield; (F) fruit quality of greenhouse cucumber 'Farbiola'. The regression analyses for dry weight, root development and flower buds are based on individual plant data. The markers indicate the means of the different day and night temperature regimes ( + D T > NT; *DT = NT; [] D T < N T ).
Five plants were collected from each temperature regime at the end o f propagation and plant height, leaf number, fresh and dry weight, and root d e v e l o p m e n t were recorded. R o o t d e v e l o p m e n t was visually e x a m i n e d and recorded based on a 0 - 9 scale where the following rating was used: ( 0 ) Poor root development. N o roots visible on the media surface. ( 9 ) Very vigorous
195
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root development. Approximately 100% of media surface covered with roots. The average internode length was calculated on basis of plant height and number of leaves larger than 1 cm. The number of flower buds at Node 6 and above were counted at 1-3 day interwals after transplanting. Cucumber fruits were harvested three times a week and the numbers, weights and market grades were recorded for each treatment at each harvest. Analysis of variance (ANOVA) and regression analysis of data were carried out according to standard procedures of Statistical Analysis System (SAS) programs at the Agricultural University of Norway. The analyses have nor-
196
S.O. GRIMSTADAND E. FRIMANSLUND 16 Y = - 19.6 + 2.74x - 0 . 0 5 5 x 2
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mally been based on individual plants as replicates with exception of such parameters as time to harvest, yield, and quality, which have been based on mean values from the different replicates. RESULTS Growth. - Plant dry weight increased as A D T increased (Fig. 1 ( A ) ) . In the temperature range of 15-27 ° C the data suggest a sigmoid response curve with inflections at about 17 and 24°C. In the range between these temperatures,
EFFECT OF DAY AND NIGHT TEMPERATUREON CUCUMBER
197
TABLE 1 Effect of different day and night temperature regimes on growth, flowering and early yield on greenhouse cucumber. The mean temperatures for the different regimes are 21 °C Temperature regime
Significance
DT>NT
DT=NT
DT
L
Q
22.1 7.1 22.6 1.5
23.1 7.3 24.1 1.6
10.9 6.8 15.9 1.1
*** Ns * **
* Ns Ns Ns
7.1
6.4
5.8
***
Ns
3.1 2.9 2.5
3.2 2.8 2.5
2.4 2.3 2.2
** * Ns
Ns Ns Ns
25.9 13.5 92
27.9 12.4 85
27.4 13.2 93
* Ns Ns
Ns ** ***
Growth Height qcm) No. leaves> 1 cm Fresh w~ight (g) Dry weight (g) Root development (Scale 0-9, where 9 is best)
Flowering Number of flowers Node 6-10 Node 11-15 Node 16-20
Yield Days to harvest Total yi,dd (kg m -2) First grade (%)
*, **, ***,Ns Linear (L) or quadratic (Q) models significant at P-< 0.05, 0.01, or 0.001 or non-significant, respectively, based on F values. 60
~ °E
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24 °C
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20
,180c 15°C
°14 1'~ 1'6 1'7 1'8 1'9 2'o 2'1 = 2'3 2'4 i5 2'e 2'7 2'8 29 Night temperature (°C) Fig. 2. Effect o f various D T and N T c o m b i n a t i o n s on plant height in greenhouse cucumber 'Farbiola'. The DT and NT were o f equal duration o f 12 h. Vertical bars indicate the 95% confidence interval o f the means.
198
S.O. GRIMSTADAND E. FRIMANSLUND 5
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Fig. 3. Relationship between (A) average node length at the end of the propagation period; ( B ) n u m b e r o f flower b u d s per node and D I F in g r e e n h o u s e c u c u m b e r ' F a r b i o l a ' . T h e m a r k e r s indicate the means of the different day and night temperature regimes ( D T / N T ) .
the plant dry weight increased 0.32 g per 1 ° C. In the same temperature range, root mass increased 124% (Fig. 1 ( B ) ) . Plant fresh and dry weight, root development and plant height were significantly lower when the DT was lower than NT compared with CT or DT > NT (Table 1 ). Averaged over all temperature treatments, plant height decreased approximately 50% when the DT < NT compared with DT = NT or DT > NT. Final plant height for the different temperature regimes is shown in Fig. 2.
199
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
0.6 Y = - 0.49 + 0.039x (R2 = 0.94)
21/27mj / 424/21 27/2~ 27/24 21/21 2 1 / 2 ~ /
~, 0.4-
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A v e r a g e daily t e m p e r a t u r e (°C)
Fig. 4. ',Effect of A D T during propagation on number of leaves unfolded per day in greenhouse cucumber 'Farbiola'. The regression analyses are based on individual plant data. The markers indicate the means of the different day and night temperature regimes ( + D T > NT; *DT = NT; [] D T < N T ) .
The different temperature regimes significantly affected the internode length ( P < 0.01 ). Both an increase in ADT and positive DIF enhanced internode elongation. However, increasing the ADT by only raising DT (increased positive l)IF) affects the elongation to a larger extent than a similar rise in DT and NT (Fig. 3 ( A ) ) . The effect of negative DIF on internode length was small, especially when plants were grown at low DT (Fig. 3 (A) ). Different values of DIF did not affect the number of leaves (Table 1 ). Leaf unfolding rate was a function of ADT where (Fig. 4 ) leaves per d a y = - 0 . 4 9 + (0.039 x A D T ) , (r2=0.94)
Flowering. - The number of flower buds per node (Nodes 6-20) was negatively correlated with the ADT ( r 2 = 0.93 ). Increasing ADT from 15 to 27 ° C decreased flower bud number from 5.8 to 1.2 (Fig. 1 (C) ). NT affected flower initiation to a greater extent than DT. An increase in NT from 15 to 27°C reduced the number of flower buds from 4.3 to 1.1 (74%) while a similar increase in DT reduced the number from 3.8 to 1.7 (55%). The number of flower buds for Nodes 6-15 was significantly higher with D T > N T or CT than N T > D T , but not for Nodes 16-20 (Table 1 ). A high value of DIF, positive or negative, led to a decrease in the number of flower buds. However, the effect of negative DIF was greater than positive DIF (Fig. 3(B)).
200
S.O. GRIMSTAD AND E. FRIMANSLUND
Yield. - Increasing ADT during the propagation period had a positive effect on earliness, yield and quality (Figs. 1 (D), 1 (E) and 1 ( F ) ) . The difference in time from planting to first harvest between the warmest (24-27 °C) and the coolest (CT 15 °C) temperature regime was 16 days. The data show that earliness of cropping, measured as rate of progress to harvest (1/days), could be described as a function of ADT where earliness = - 0 . 0 5 1 +0.0071 × A D T - 0 . 0 0 0 1 3 × A D T 2 (r2=0.94). The calculated regression equation for yield ( Y = - 1 9 . 6 + 2 . 7 4 x A D T - 0 . 0 5 5 x A D T 2 ; r2--0.95) indicated an o p t i m u m propagation temperature of about 25 °C. Raising ADT during the propagation by 1 °C in the range of 15 to 25°C reduced the time to harvest by 1.6 days per 1 °C and increased the average total yield by 0.54 kg m -2 according to the regression equations. The ADT during propagation also affected the fruit quality (r2--0.88), judged by the percentage of fruits graded in Class 1 (Fig. 1 ( F ) ) . The amount of first grade was about 40% higher on plants propagated at 27°C than at 15°C. Fluctuation between day and night temperature during propagation did not affect earliness or the yield (Table 1 ). However, temperature fluctuation, positive or negative, enhanced the fruit quality (Table 1 ). It should be kept in m i n d that all formulae in Figs. 1 ( A ) - I (F) show the results of a regression analysis. This means that extrapolation outside the temperature range used in our experiments is not valid. Furthermore, these regression formulae are based on only 25 DT and NT combinations and not a complete set of temperatures. DISCUSSION
For many species, DT > NT (positive DIF) promotes elongation more than CT or D T < N T (Verkerk, 1955; Heuvelink, 1989; Erwin et al., 1989; Moe, 1990). This was also true for greenhouse cucumber. When the influences of temperature regime on number of leaves and plant height are compared, differences obtained in plant length were a result of differences in internode elongation rather than a result of the number of nodes produced. The same is reported for soybean (Thomas and Raper, 1978 ) and for tomato (Heuvelink, 1989). In Campanula isophylla (Moe, 1990), Lilium longiflorum (Erwin et al., 1989 ) and Euphorbia puleherrima (Berghage and Heins, 1991 ) a quantitative response to DIF has been reported, i.e. the more positive the value of DIF, the greater the stem elongation and final plant height. The differences in cucumber internode length and final plant height in this study only partially followed the pattern of Campanula, Euphorbia, and Lilium. Internode length decreased as DIF decreased from + 12 °C to zero. A further decrease in DIF had only small effects on internode length. This was particularly true when the plants were grown at day temperatures below 21 ° C, which indicate
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
201
that for cucumber an interaction between DT and NT may exist ( P < 0.01 ). The different response of various plant species may be caused by temperatures exceeding the range in which the relationship between the rate of length growth and temperature is linear. Thi,; study indicates an ADT o p t i m u m for growth of young cucumber plants (based on dry weight) of 28-29 ° C. According to Friend and Helson ( 1976 ) and Karlsen (1978 ) the m a x i m u m rate of dry matter production in greenhouse cucumber is achieved at a constant air temperature of 30-35 °C whereas the o p t i m u m temperature for rapid expansion of individual leaves is 25 °C (Milthorpe, 1959 ). The disagreement can probably be explained by different light levels during the experimental period. This experiment was carried out in ear]iy spring without use of supplemental light. Frie, nd and Helson (1976) found that alternating the day and night temperature with an amplitude of 10 or 20 °C had no influence on the dry weight gain of young vegetative plants grown at an ADT of 30°C. However, most commercial cucumber crops grown under glass in northern Europe are grown at sub-optimal temperatures (below 30 oC). Under these conditions, in this study the dry weight increased, when both day and night temperature was increased. Increased day temperature enhanced dry weight more than a similar increase in night temperature. Slack and Hand (1983) found that cucumbe~r plants at the transplanting stage, propagated at 24 ° day/17 °C night, were 17% taller, 17% heavier (on dry weight basis) and had 6% more leaf area than plants grown at 21 ° C day and 19 ° C night. ADT for the two regimes was similar when allowance was made for the differences between the length of the day and night periods. Similar results have been reported for tomatoes (Calvert, 1964; Hussey, 1965; Klapwijk and Wubben, 1978; Heuvelink, 1989). Also Friend and Helson (1976), who investigated growth of seven crops, concluded that a high DT and lower NT results in greater plant dry weight than a low DT and higher NT for the same ADT which is also in accordance with the results obtained in this study. However, the findings also showed that an interaction exists between DT and NT ( P < 0.001 ). Raising the DT above NT was considerably more efficient when the plants have been grown at low than high NT (data not shown). This may indicate that ADT is more important for growth rate than DT or NT individually. Leaf number in this study was not affected by temperature fluctuation but was affected by ADT (Table 1 ). An additional 0.04 leaves per day is predicted for each 1 °C increase in ADT according to the calculated regression equation, compared with 0.022 leaves for sunflower (Rawson and Hindmarsh, 1982), 0.067 for maize (Tollenaar et al., 1979), 0.020 for pea (Balvoll and Bremer, 1965 ) and 0.094 for Easter lily (Karlsson et al., 1988 ). Heuvelink (1989) found that the number of tomato leaves was similar at zero and positive DIF at the same temperature integral. For negative DIF the leaf formation was slow. However, the reduction in leaf number was much
202
S.O. GRIMSTAD AND E. FRIMANSLUND
less than the reduction in plant dry weight. Heuvelink ( 1989 ) explained that the decrease in relative growth rate ( R G R ) under negative DIF was caused by changes in leaf area ratio (LAR) and not by changes in net assimilation rate (NAR). This corresponds with Challa and Brouwer (1985) who concluded for young cucumber plants that the influence of different NT regimes was mainly through LAR. Also Nilwik ( 1981 ) concluded that the influence of temperature on R G R of sweet pepper was largely mediated through changes in LAR. Changing the temperature condition from zero or positive DIF to negative DIF reduced the stem length 51% while plant fresh weight was reduced 32% only. The results also showed a negative effect of negative DIF on root development. Although no measurements of root temperature were made it is likely that the root temperature in the small pots followed the changes in air temperature. But of course the temperature changes are not likely to be as rapid in the growth medium. Flower abortion in cucumber is controlled by an endogenous reaction which is little, if at all, modified by the temperature (De Lint and Heij, 1980; Van der Vlugt, 1983 ). The results obtained in this experiment clearly demonstrate the negative correlation which exists between temperature and flower initiation in cucumber (Van der Vlugt, 1983). Night temperature affects flower induction in the 15-27°C range more than day temperature. The reason is not known. Temperature treatments had an after-effect on the number of flower buds in the first nodes developing after transplanting. Van der Vlugt ( 1983 ) found that the leaf primordia of Nodes 25-28 were visible at the time of transplanting, but no flower bud formation could be detected. This suggests that the number of flowers up to at least Node 25-28 may be affected by the temperature during propagation. In this study, only visible flower buds were inspected and a significant effect of propagation temperature could be recorded up to Node 18. Differences in plant size at the stage of transplanting may be the reason why only 18 nodes were affected. Particularly earliness and yield, and to some extent also fruit quality showed a very close relationship to ADT during propagation. The plant size at the end of the propagation period generally increased as the ADT increased (Fig. 1 (A)) while the number of flower buds per node decreased as the mean temperature increased (Fig. 1 (C) ). A reduction in fruit numbers per node is the likely reason for enhanced fruit quality as ADT during the propagation period increased. While higher ADT enhanced earliness, the rate of fruit production is largely independent of the preharvest temperature regime (Van de Vooren et al., 1978; Slack and Hand, 1980). Earliness of cropping is therefore very important and strongly influences the final yield. Plants grown at negative DIF during propagation showed a significant reduction in final plant height when compared with plants grown at constant temperature or positive DIF. In spite of reduced plant weight, root develop-
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
203
ment, and number of flowers, no effects on earliness of cropping and final yield were recorded. The conclusion from this study is that use of negative DIF during propagatio~ of cucumber is an effective way to reduce stem elongation in young plants without negative after-effects on earliness, final yield, and fruit quality. ACKNOWLEDGEMENTS
The, authors wish to thank Prof. Royal D. Heins and Prof. Zhangling Luo at the Department of Horticulture, Michigan State University, USA, for helpful comments on this paper. Financial support from the National Agricultural Research Council of Norway is recognized.
REFERENCES Anonymous, 1980. Grower Guide No. 15. Cucumber. Grower Books, London, 72 pp. Balvoll, G. and Bremer, A.H., 1965. The heat-unit system and plant production in connection with growth and development of different vegetables. Meldinger fra Norges landbruksh/Sgskole, 44: 1-18. (In Norwegian. ) Berghage, R.D. and Heins, R.D., 1991. Quantification of temperature effects on stem elongation in Poinsettia. J. Am. Soc. Hortic. Sci., 116( 1 ): 14-18. Calverl:, A., 1964. The effects of air temperature on growth of young tomato plants in natural light conditions. J. Hortic. Sci., 39:194-211. Challa, H. and Brouwer, P., 1985. Growth of young cucumber plants under different diurnal temperature patterns. Acta Hortic., 174:211-217. De Koaing, A.N.M., 1988. The effect of different day/night temperature regimes on growth, dev,elopment and yield of glasshouse tomatoes. J. Hortic. Sci., 63 (3): 465-471. De Lint, P.J.A.L., and Heij, G., 1980. Glasshouse cucumber, effects of planting date and night temperature on flowering and fruit development. Acta Hortic., 188:123-134. Erwin, J.E., Heins, R.D. and Karlsson, M.G., 1989. Thermomorphogenesis in Lilium long# flori~m. Am. J. Bot., 76: 47-52. Friend, D.H.C. and Helson, V.A., 1976. Thermoperiodic effects on the growth and photosynthesis of wheat and other crop plants. Bot. Gaz., 137: 75-84. Hand, D.W. and Hannah, M.A., 1978. Sweet pepper: Application of lower-than-normal temperatures for varying durations within the night periods. Annu. Rep. Glasshouse Crops Res. Inst., Littlehampton, 1977, pp. 53-55. Heuvelink, E., 1989. Influence of day and night temperature on the growth of young tomato plants. Scientia Hortic., 38:11-22. Hurd, R.G. and Graves C.J., 1984. The influence of different temperature patterns having the same integral on the earliness and yield of tomatoes. Acta Hortic., 148: 547-554. Hussey, G., 1965. Growth and development in the young tomato. III. The effect of night and day temperatures on vegetative growth. J. Exp. Bot., 16: 373-385. Karlse~l, P., 1978. Growth of young cucumber plants in relation to root and air temperature. The Royal Veterinary and Agricultural University, Copenhagen, Denmark. Yearbook 1978, pp. 45-52.
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Karlsson, M.G., Heins, R.D. and Erwin J.E., 1988. Quantifying temperature-controlled leaf unfolding rates in 'Nellie White' Easter Lily. J. Am. Soc. Hortic. Sci., 113 ( 1 ): 70-74. Klapwijk, D. and Wubben, C.F.M., 1978. Effect of various temperature regimes at a same temperature sum on plant growth. Annual Report 1977, Glasshouse Crops Research and Experiment Station Naaldwijk, pp. 27-28. Kristoffersen, T., 1963. Interactions of photoperiod and temperature in growth and fruiting of young tomato plants (Lycopersicon esculentum Mill. ). Physiol. Plant., Suppl., 1: 1-98. Leatherland, M., 1986. Dotting the i's in the Efford regime. Grower Suppl., 105(4) 83-87. Milthorpe, F.L., 1959. Studies on the expansion on the leaf surface. I. The influence of temperature. J. Exp. Bot., 10: 233-249. Moe, R., 1990. Effect of day and night temperature alternations and of plant growth regulators on stem elongation and flowering of the long-day plant Campanula isophylla Moretti. Scientia Hortic., 43: 291-305. Moe, R. and Heide O.M., 1985. Campanula isophylla. In: A.H. Halevy (Editor), Handbook of Flowering. Vol. III. CRC Press, Boca Raton, FL, pp. 117-122. Nilwik, H.J.M., 1981. Growth analysis of sweet pepper (Capsicum annuum L.). 2. Interacting effects ofirradiance, temperature and plant age in controlled conditions. Ann. Bot., 48:137145. Rawson, H.M. and Hindmarsh, J.H., 1982. Effects of temperature on leaf expansion in sunflower. Aust. J. Plant Physiol., 9:209-219. Slack, G. and Hand, D.W., 1980. Control of air temperature for cucumber production. Acta Hortic., 118: 174-186. Slack, G. and Hand, D.W., 1983. The effect of day and night temperatures on the growth, development and yield of glasshouse cucumbers. J. Hortic. Sci., 58: 567-573. Thomas, J.F. and Raper, C.D., 1978. Effect of day and night temperatures during floral induction on morphology of soybeans. Agron. J., 70: 893-898. Tollenaar, M., Daynard, T.B. and Hunter, R.B., 1979. Effect of temperature on rate of leaf appearance and flowering date in maize. Crop Sci., 19: 363-366. Van de Vooren, J., De Lint, P.J.A.L. and Challa, H., 1978. Influence of varying night temperatures on a cucumber crop. Acta Hortic., 87: 249-255. Van der Vlugt, J.L.F., 1983. The effect of temperature on formation and abortion of flower buds in gynoecious cucumber plants. Scientia Hortic., 20: 323-328. Verkerk, K., 1955. Temperature, light and the tomato. Meded. Landbouw Hogesch. Wageningen, Netherlands, 44:175-224. Went, F.W., 1944. Plant growth under controlled conditions. II. Thermoperiodicity in growth and fruiting of the tomato. Am. J. Bot., 31:135-150. Went, F.W., 1953. The effect of temperature on plant growth. Annu. Rev. Plant. Physiol., 4: 347-362.
191
Elsevier Science Publishers B.V., A m s t e r d a m
Effect of different day and night temperature regimes on greenhouse cucumber young plant production, flower bud formation and early yield Svein O. G r i m s t a d and Endre Frimanslund ~ Agricu,'tural University of Norway, Department of Horticulture, PO Box 22, N-1432•s-NLH, Norway (Accepted 22 July 1992)
ABSTRACT Grimstad, S.O. and Frimanslund, E., 1993. Effect of different day and night temperature regimes on greenhouse cucumber young plant production, flower bud formation and early yield. Scientia Hottic., 53: 191-204. The effects of different day temperature (DT) and night temperature (NT) combinations on the growth and development of young cucumber plants during propagation were investigated. The temperature combinations gave differences (DIF) between DT and NT ranging from - 12 to + 12°C. After propagation the plants were transferred to a greenhouse and grown at a constant DT and NT set point of 21 °C for 8 weeks after propagation. The experiment was carried out in early spring under natural light conditions. The rate of development was dependent on the temperature regime. DT lower than NT inhibited growth .and development of the young plants, except for leaf unfolding rate which depended on the average daily temperature (ADT) only. Both an increase in ADT and DIF enhanced internode length. However, increasing the ADT by only raising DT affected the elongation to a larger extent than an equal raise of DT and NT. For flower bud development an increase in negative values of DIF reduced the nurrtber of flower buds to a greater extent than an increase in positive values. The c ptimum propagation temperature, based on earliness (first harvest) and early yield was approximately 25 ° C. Raising the ADT from 15 to 25 °C reduced the time to first harvest by 1.6 days per 1 °C and increased the average total yield during the first 8 weeks after transplanting by 0.54 kg m -2 ° C - 1. DIF had no effect on earliness nor the yield, but improved fruit quality. Keywords: Cucumis sativus; flowering; fruit quality; temperature; yield. Abbreviations: ADT = average daily temperature; CT = constant temperature; DIF = difference between I3,T and NT; DT = day temperature; NT = night temperature.
Correspondence to: S.O. G r i m s t a d , Saerheim Research Station, N-4062, Klepp St., Norway. ~Present address: Ullensvang Research Station, N-5774 Lofthus, Norway.
© 1993 Elsevier Science Publishers B.V. All rights reserved 0 3 0 4 - 4 2 3 8 / 9 3 / $ 0 6 . 0 0
192
S.O. GR1MSTAD AND E. FRIMANSLUND
INTRODUCTION
Propagation of cucumber has considerable commercial importance for both the seedling producer and the greenhouse cucumber grower. At the time of transplanting, the cucumber plant should be vigorous, close to flowering, and not too tall. Cucumber growers want plants with only one or two fruits per node so only two flower buds should be formed (Anonymous, 1980). Maturity of a plant is often correlated with a large plant. However, if plants grow too tall, they become troublesome to maintain during propagation, transport and planting. One way to grow shorter plants is to apply a growth retardant (S.O. Grimstad, unpublished results, 1990). This is not legal on vegetables in many countries today because of the potential danger of chemical residues. Traditionally, young plants are cultivated at a day temperature (DT) somewhat higher than the night temperature (NT). It has been known for many years that stem elongation is thermoperiodic in many species (Went, 1944, 1953; Kristoffersen, 1963 ). Mature plant height increases when plants are grown with day temperature higher than night temperature in both short day and long day plants (Moe and Heide, 1985; Erwin et al., 1989). Erwin et al. (1989) found that the relationship between DT and NT influenced final plant height to a greater extent than DT, NT or ADT in Lilium longiflorum. Calvert (1964), Hussey (1965) and Klapwijk and Wubben (1978) reported a reduction in growth of young tomato plants with an inverse temperature regime. Hurd and Graves (1984) and De Koning (1988), however, found that total tomato yield was not influenced by the temperature regime, but mainly by the temperature integral. The same was found for the yield of sweet pepper (Hand and Hannah, 1978 ) and cucumber ( Slack and Hand, 1983 ). Negative DIF temperature regimes may be a strategy for the seedling producer to substitute growth retardants. Negative DIF temperature regimes may also result in profitable energy savings when thermal screens are used in winter (Leatherland, 1986). The objective of the present research was to quantify thermomorphogenic responses in young plants of greenhouse cucumbers. This is being done in a general attempt to find a way to control plant height without increase in flower buds, delay in time to first harvest, decrease in yield, or reduction in fruit quality. MATERIAL AND METHODS
Cucumber seeds (Cucumis sativus cultivar 'Farbiola') were sown separately in 12 cm plastic pots (0.671 ) filled with fertilized and limed Norwegian peat moss (Horalux) on 22 February and germinated at 25 + 1 °C. The seedlings received supplemental light (85/zmol m - 2 s - l ) for 20 h per day with
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
193
cool white fluorescent lamps (Philips TLD 33 ) after germination. Ten days after ,,;owing (when the cotyledons were horizontal), seedlings were selected for uniformity, whereafter they were placed in a daylight phytotron with temperature setpoints of 15, 18, 21, 24, or 27 °C and grown under ambient light conditions for 3 weeks (latitude 5 9 ° 5 6 ' N ) . Plants were placed on trolleys with a plant density of approximately 20 plants m -2 and moved among the five temperature sections twice a day to provide factorial DT and NT treatments; with 25 plants per treatment. To obtain the same transportation stress, plants held at constant temperature were moved from one location to another within the compartment. The 12-h DT period was defined from 07:00 to 19:00 h when the plants received the high irradiance of sunlight and the NT in the low light period from 19:00 to 07:00 h. Sunrise and sunset at the beginning of the experiment was algproximately 07:00 and 18:00 h, and at the end of the experiment approximately 06:00 and 18:45 h, respectively. The average daily global radiation during the propagation period was 19.2 mol m -2 day -1. Temperature variations in the phytotron compartments were < 0.5 °C deviation from the set point. The vapour pressure difference (zlX) was kept at 4.7 g m -3. No CO2 enrichment was given during propagation. The plants were watered regularly after emergence with a complete nutrient solution at an EC level of 1.0 mS cm -1 . The solution was composed of a complete fertilizer (Red Superba), calcium nitrate and potassium sulphate and contained: 158 N, 40 P, 244 K, 38 Mg, 116 Ca, 54 S, 2.0 Fe, 1.1 Mn, 0.20 Cu, 0.30 Zn, 0.33 B and 0.03 Mo mg 1Twenty plants were removed from the phytotron and planted in two 12 m X 1"7m greenhouses to observe subsequent development at the end of propagation, i.e. 21 days after cotyledon expansion. All plants were grown at constant 21 °C DT and NT set point under natural light conditions (average 36.8 mol m - 2 day- 1). The ventilation temperature was set at 25 ° C. Root temperature was maintained at 20-22 °C with substrate heating. Pure supplementary CO2 ( 1000 [tl 1-1 ) was given daily from planting, except when the ventilator opening exceeded 5 cm. Vapour pressure difference (•X) in the greenhouse was at m a x i m u m 5.6 g m -3 in the day and not less than 1.9 g m -3 during; the night. The plants were placed on fertilized and limed peat moss slabs (30 c m × 13 c m X 110 cm) with an overall population density of 1.7 plants m -2. In each greenhouse, the treatments were replicated twice, so final layout in each house consisted of 50 plots of five plants surrounded overall by guard plants. The EC level of the nutrient solution was increased to 1.8 mS c m - ] after transplanting. Buds of side shoots were removed at appearance after transplanting. The main stem was decapitated 2.75 m above the slabs, just above the suspending wire. Flower buds were removed from the lowest axil up to Axil 5. The experiment,; were terminated 8 weeks after transplanting.
194
S.O. GRIMSTAD AND E. FRIMANSLUND
4 ] Y = 28.5 - 4.396x + 0.216x & 0.00325x 3 " (R 2 = 0.94) 27/21+
2~7/24
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Fig. 1. Effect of average daily temperature (A) on dry weight production; (B) root development (based on a 0-9 scale, where 9 was the highest score); (C) number of flower buds per node (Nodes 6-20); ( D ) rate of progress to harvest; (E) yield; (F) fruit quality of greenhouse cucumber 'Farbiola'. The regression analyses for dry weight, root development and flower buds are based on individual plant data. The markers indicate the means of the different day and night temperature regimes ( + D T > NT; *DT = NT; [] D T < N T ).
Five plants were collected from each temperature regime at the end o f propagation and plant height, leaf number, fresh and dry weight, and root d e v e l o p m e n t were recorded. R o o t d e v e l o p m e n t was visually e x a m i n e d and recorded based on a 0 - 9 scale where the following rating was used: ( 0 ) Poor root development. N o roots visible on the media surface. ( 9 ) Very vigorous
195
EFFECT O F DAY AND NIGHT TEMPERATURE ON CUCUMBER
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Fig. 1. Continued.
root development. Approximately 100% of media surface covered with roots. The average internode length was calculated on basis of plant height and number of leaves larger than 1 cm. The number of flower buds at Node 6 and above were counted at 1-3 day interwals after transplanting. Cucumber fruits were harvested three times a week and the numbers, weights and market grades were recorded for each treatment at each harvest. Analysis of variance (ANOVA) and regression analysis of data were carried out according to standard procedures of Statistical Analysis System (SAS) programs at the Agricultural University of Norway. The analyses have nor-
196
S.O. GRIMSTADAND E. FRIMANSLUND 16 Y = - 19.6 + 2.74x - 0 . 0 5 5 x 2
24/21+ 27/21+ 21/27n
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(°C)
Fig. 1. Continued.
mally been based on individual plants as replicates with exception of such parameters as time to harvest, yield, and quality, which have been based on mean values from the different replicates. RESULTS Growth. - Plant dry weight increased as A D T increased (Fig. 1 ( A ) ) . In the temperature range of 15-27 ° C the data suggest a sigmoid response curve with inflections at about 17 and 24°C. In the range between these temperatures,
EFFECT OF DAY AND NIGHT TEMPERATUREON CUCUMBER
197
TABLE 1 Effect of different day and night temperature regimes on growth, flowering and early yield on greenhouse cucumber. The mean temperatures for the different regimes are 21 °C Temperature regime
Significance
DT>NT
DT=NT
DT
L
Q
22.1 7.1 22.6 1.5
23.1 7.3 24.1 1.6
10.9 6.8 15.9 1.1
*** Ns * **
* Ns Ns Ns
7.1
6.4
5.8
***
Ns
3.1 2.9 2.5
3.2 2.8 2.5
2.4 2.3 2.2
** * Ns
Ns Ns Ns
25.9 13.5 92
27.9 12.4 85
27.4 13.2 93
* Ns Ns
Ns ** ***
Growth Height qcm) No. leaves> 1 cm Fresh w~ight (g) Dry weight (g) Root development (Scale 0-9, where 9 is best)
Flowering Number of flowers Node 6-10 Node 11-15 Node 16-20
Yield Days to harvest Total yi,dd (kg m -2) First grade (%)
*, **, ***,Ns Linear (L) or quadratic (Q) models significant at P-< 0.05, 0.01, or 0.001 or non-significant, respectively, based on F values. 60
~ °E
I
~ / i 27 °C
24 °C
..E ._~ ¢-
E
20
,180c 15°C
°14 1'~ 1'6 1'7 1'8 1'9 2'o 2'1 = 2'3 2'4 i5 2'e 2'7 2'8 29 Night temperature (°C) Fig. 2. Effect o f various D T and N T c o m b i n a t i o n s on plant height in greenhouse cucumber 'Farbiola'. The DT and NT were o f equal duration o f 12 h. Vertical bars indicate the 95% confidence interval o f the means.
198
S.O. GRIMSTADAND E. FRIMANSLUND 5
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Fig. 3. Relationship between (A) average node length at the end of the propagation period; ( B ) n u m b e r o f flower b u d s per node and D I F in g r e e n h o u s e c u c u m b e r ' F a r b i o l a ' . T h e m a r k e r s indicate the means of the different day and night temperature regimes ( D T / N T ) .
the plant dry weight increased 0.32 g per 1 ° C. In the same temperature range, root mass increased 124% (Fig. 1 ( B ) ) . Plant fresh and dry weight, root development and plant height were significantly lower when the DT was lower than NT compared with CT or DT > NT (Table 1 ). Averaged over all temperature treatments, plant height decreased approximately 50% when the DT < NT compared with DT = NT or DT > NT. Final plant height for the different temperature regimes is shown in Fig. 2.
199
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
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Fig. 4. ',Effect of A D T during propagation on number of leaves unfolded per day in greenhouse cucumber 'Farbiola'. The regression analyses are based on individual plant data. The markers indicate the means of the different day and night temperature regimes ( + D T > NT; *DT = NT; [] D T < N T ) .
The different temperature regimes significantly affected the internode length ( P < 0.01 ). Both an increase in ADT and positive DIF enhanced internode elongation. However, increasing the ADT by only raising DT (increased positive l)IF) affects the elongation to a larger extent than a similar rise in DT and NT (Fig. 3 ( A ) ) . The effect of negative DIF on internode length was small, especially when plants were grown at low DT (Fig. 3 (A) ). Different values of DIF did not affect the number of leaves (Table 1 ). Leaf unfolding rate was a function of ADT where (Fig. 4 ) leaves per d a y = - 0 . 4 9 + (0.039 x A D T ) , (r2=0.94)
Flowering. - The number of flower buds per node (Nodes 6-20) was negatively correlated with the ADT ( r 2 = 0.93 ). Increasing ADT from 15 to 27 ° C decreased flower bud number from 5.8 to 1.2 (Fig. 1 (C) ). NT affected flower initiation to a greater extent than DT. An increase in NT from 15 to 27°C reduced the number of flower buds from 4.3 to 1.1 (74%) while a similar increase in DT reduced the number from 3.8 to 1.7 (55%). The number of flower buds for Nodes 6-15 was significantly higher with D T > N T or CT than N T > D T , but not for Nodes 16-20 (Table 1 ). A high value of DIF, positive or negative, led to a decrease in the number of flower buds. However, the effect of negative DIF was greater than positive DIF (Fig. 3(B)).
200
S.O. GRIMSTAD AND E. FRIMANSLUND
Yield. - Increasing ADT during the propagation period had a positive effect on earliness, yield and quality (Figs. 1 (D), 1 (E) and 1 ( F ) ) . The difference in time from planting to first harvest between the warmest (24-27 °C) and the coolest (CT 15 °C) temperature regime was 16 days. The data show that earliness of cropping, measured as rate of progress to harvest (1/days), could be described as a function of ADT where earliness = - 0 . 0 5 1 +0.0071 × A D T - 0 . 0 0 0 1 3 × A D T 2 (r2=0.94). The calculated regression equation for yield ( Y = - 1 9 . 6 + 2 . 7 4 x A D T - 0 . 0 5 5 x A D T 2 ; r2--0.95) indicated an o p t i m u m propagation temperature of about 25 °C. Raising ADT during the propagation by 1 °C in the range of 15 to 25°C reduced the time to harvest by 1.6 days per 1 °C and increased the average total yield by 0.54 kg m -2 according to the regression equations. The ADT during propagation also affected the fruit quality (r2--0.88), judged by the percentage of fruits graded in Class 1 (Fig. 1 ( F ) ) . The amount of first grade was about 40% higher on plants propagated at 27°C than at 15°C. Fluctuation between day and night temperature during propagation did not affect earliness or the yield (Table 1 ). However, temperature fluctuation, positive or negative, enhanced the fruit quality (Table 1 ). It should be kept in m i n d that all formulae in Figs. 1 ( A ) - I (F) show the results of a regression analysis. This means that extrapolation outside the temperature range used in our experiments is not valid. Furthermore, these regression formulae are based on only 25 DT and NT combinations and not a complete set of temperatures. DISCUSSION
For many species, DT > NT (positive DIF) promotes elongation more than CT or D T < N T (Verkerk, 1955; Heuvelink, 1989; Erwin et al., 1989; Moe, 1990). This was also true for greenhouse cucumber. When the influences of temperature regime on number of leaves and plant height are compared, differences obtained in plant length were a result of differences in internode elongation rather than a result of the number of nodes produced. The same is reported for soybean (Thomas and Raper, 1978 ) and for tomato (Heuvelink, 1989). In Campanula isophylla (Moe, 1990), Lilium longiflorum (Erwin et al., 1989 ) and Euphorbia puleherrima (Berghage and Heins, 1991 ) a quantitative response to DIF has been reported, i.e. the more positive the value of DIF, the greater the stem elongation and final plant height. The differences in cucumber internode length and final plant height in this study only partially followed the pattern of Campanula, Euphorbia, and Lilium. Internode length decreased as DIF decreased from + 12 °C to zero. A further decrease in DIF had only small effects on internode length. This was particularly true when the plants were grown at day temperatures below 21 ° C, which indicate
EFFECT OF DAY AND NIGHT TEMPERATURE ON CUCUMBER
201
that for cucumber an interaction between DT and NT may exist ( P < 0.01 ). The different response of various plant species may be caused by temperatures exceeding the range in which the relationship between the rate of length growth and temperature is linear. Thi,; study indicates an ADT o p t i m u m for growth of young cucumber plants (based on dry weight) of 28-29 ° C. According to Friend and Helson ( 1976 ) and Karlsen (1978 ) the m a x i m u m rate of dry matter production in greenhouse cucumber is achieved at a constant air temperature of 30-35 °C whereas the o p t i m u m temperature for rapid expansion of individual leaves is 25 °C (Milthorpe, 1959 ). The disagreement can probably be explained by different light levels during the experimental period. This experiment was carried out in ear]iy spring without use of supplemental light. Frie, nd and Helson (1976) found that alternating the day and night temperature with an amplitude of 10 or 20 °C had no influence on the dry weight gain of young vegetative plants grown at an ADT of 30°C. However, most commercial cucumber crops grown under glass in northern Europe are grown at sub-optimal temperatures (below 30 oC). Under these conditions, in this study the dry weight increased, when both day and night temperature was increased. Increased day temperature enhanced dry weight more than a similar increase in night temperature. Slack and Hand (1983) found that cucumbe~r plants at the transplanting stage, propagated at 24 ° day/17 °C night, were 17% taller, 17% heavier (on dry weight basis) and had 6% more leaf area than plants grown at 21 ° C day and 19 ° C night. ADT for the two regimes was similar when allowance was made for the differences between the length of the day and night periods. Similar results have been reported for tomatoes (Calvert, 1964; Hussey, 1965; Klapwijk and Wubben, 1978; Heuvelink, 1989). Also Friend and Helson (1976), who investigated growth of seven crops, concluded that a high DT and lower NT results in greater plant dry weight than a low DT and higher NT for the same ADT which is also in accordance with the results obtained in this study. However, the findings also showed that an interaction exists between DT and NT ( P < 0.001 ). Raising the DT above NT was considerably more efficient when the plants have been grown at low than high NT (data not shown). This may indicate that ADT is more important for growth rate than DT or NT individually. Leaf number in this study was not affected by temperature fluctuation but was affected by ADT (Table 1 ). An additional 0.04 leaves per day is predicted for each 1 °C increase in ADT according to the calculated regression equation, compared with 0.022 leaves for sunflower (Rawson and Hindmarsh, 1982), 0.067 for maize (Tollenaar et al., 1979), 0.020 for pea (Balvoll and Bremer, 1965 ) and 0.094 for Easter lily (Karlsson et al., 1988 ). Heuvelink (1989) found that the number of tomato leaves was similar at zero and positive DIF at the same temperature integral. For negative DIF the leaf formation was slow. However, the reduction in leaf number was much
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less than the reduction in plant dry weight. Heuvelink ( 1989 ) explained that the decrease in relative growth rate ( R G R ) under negative DIF was caused by changes in leaf area ratio (LAR) and not by changes in net assimilation rate (NAR). This corresponds with Challa and Brouwer (1985) who concluded for young cucumber plants that the influence of different NT regimes was mainly through LAR. Also Nilwik ( 1981 ) concluded that the influence of temperature on R G R of sweet pepper was largely mediated through changes in LAR. Changing the temperature condition from zero or positive DIF to negative DIF reduced the stem length 51% while plant fresh weight was reduced 32% only. The results also showed a negative effect of negative DIF on root development. Although no measurements of root temperature were made it is likely that the root temperature in the small pots followed the changes in air temperature. But of course the temperature changes are not likely to be as rapid in the growth medium. Flower abortion in cucumber is controlled by an endogenous reaction which is little, if at all, modified by the temperature (De Lint and Heij, 1980; Van der Vlugt, 1983 ). The results obtained in this experiment clearly demonstrate the negative correlation which exists between temperature and flower initiation in cucumber (Van der Vlugt, 1983). Night temperature affects flower induction in the 15-27°C range more than day temperature. The reason is not known. Temperature treatments had an after-effect on the number of flower buds in the first nodes developing after transplanting. Van der Vlugt ( 1983 ) found that the leaf primordia of Nodes 25-28 were visible at the time of transplanting, but no flower bud formation could be detected. This suggests that the number of flowers up to at least Node 25-28 may be affected by the temperature during propagation. In this study, only visible flower buds were inspected and a significant effect of propagation temperature could be recorded up to Node 18. Differences in plant size at the stage of transplanting may be the reason why only 18 nodes were affected. Particularly earliness and yield, and to some extent also fruit quality showed a very close relationship to ADT during propagation. The plant size at the end of the propagation period generally increased as the ADT increased (Fig. 1 (A)) while the number of flower buds per node decreased as the mean temperature increased (Fig. 1 (C) ). A reduction in fruit numbers per node is the likely reason for enhanced fruit quality as ADT during the propagation period increased. While higher ADT enhanced earliness, the rate of fruit production is largely independent of the preharvest temperature regime (Van de Vooren et al., 1978; Slack and Hand, 1980). Earliness of cropping is therefore very important and strongly influences the final yield. Plants grown at negative DIF during propagation showed a significant reduction in final plant height when compared with plants grown at constant temperature or positive DIF. In spite of reduced plant weight, root develop-
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ment, and number of flowers, no effects on earliness of cropping and final yield were recorded. The conclusion from this study is that use of negative DIF during propagatio~ of cucumber is an effective way to reduce stem elongation in young plants without negative after-effects on earliness, final yield, and fruit quality. ACKNOWLEDGEMENTS
The, authors wish to thank Prof. Royal D. Heins and Prof. Zhangling Luo at the Department of Horticulture, Michigan State University, USA, for helpful comments on this paper. Financial support from the National Agricultural Research Council of Norway is recognized.
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