Effect of frequency of axillary bud pruning
on vegetative growth and fruit yield in
greenhouse tomato crops
M. Navarrete
a,*, B. Jeannequin
b aInstitut National de la Recherche Agronomique, Station d'EcodeÂveloppement, Site Agroparc, 84-914 Avignon Cedex 9, France
b
Institut National de la Recherche Agronomique, Domaine horticole du Mas Blanc, 66-200 AleÂnya, France
Accepted 12 February 2000
Abstract
In greenhouse tomato crops, several manual operations are performed each week to keep the plants in optimal growth conditions. But growers are trying to reduce labour costs by spacing out the manual operations. An experimental study was conducted on one particular operation, axillary bud deshooting. The aim is to determine the effect of the deshooting frequency on vegetative growth and fruit yield, in order to help growers to determine the optimal frequency. The trials were conducted in an experimental station in AleÂnya (south France). Four deshooting frequencies were compared on two cultivars: every 7 (control), 10, 14 and 21 days. Deshooting frequency affected both vegetative growth and yield: when deshooting was performed seldom (every 21 days), the stem diameter and the vigour scored by experts were decreased; the number of fruits per m2was also reduced, leading to a signi®cantly lower yield. Moreover, the harvest started later than on the control. When the axillary buds were eliminated frequently (7 days), even those located near the apex, it reduced vegetative growth, but not yield. Therefore, from a biological point of view, the optimal deshooting frequency lies between 7 and 14 days, probably depending on climate, season and cultivar vigour.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Tomato; Fruit yield; Vegetative growth; Deshooting;Lycopersicon esculentumMill.
*
Corresponding author. Tel.:33-4-32-72-25-86; fax:33-4-32-72-25-62.
E-mail address: navarret@avignon.inra.fr (M. Navarrete).
1. Introduction
Greenhouse tomato production is very intensive and requires a high amount of input, particularly energy and manpower. For example, labour costs amount to about 30% of production costs. Several manual operations are performed very steadily: removing the axillary shoots and training the main stem keeps the crop in optimal conditions as regards light interception; delea®ng consists in removing the oldest leaves, which are no longer photosynthetically active, in order to avoid plant diseases and facilitate harvesting; truss pruning aims at adapting the fruit load to assimilate production, in order to improve fruit grade and quality. For a long time, growers and technical advisors thought that manual operations had to be carried out about once a week to maximise the yield of tomato crops. But drastic changes have occurred in the last 10 years. As competition among production areas throughout Europe and the Mediterranean area is increasing and tomato prices are decreasing, growers are trying to reduce production costs, in particular labour costs. Some operations have been suppressed or replaced by alternative less time-consuming methods (e.g. pollination, previously carried out manually, is now done by bumblebees). For most of the other operations, growers are trying to reduce the frequency at which they are carried out. For example, delea®ng and training are sometimes performed only twice a month. At the same time, several experiments have indicated that manual operations on tomato plants stress them because they bring about frequent movements of the leaves and stems. Buitelaar (1988) even found a reduction in the yield of 9% when tomato plants were shaken every day and 17% when they were shaken twice a day. On sweet pepper plants, a decrease in height, leaf area and yield was also observed when plants were submitted to frequent mechanical measurements on leaves and fruits (length, diameter) in comparison with plants which were never measured (KlaÈring, 1999). This phenomenon, known by scientists as mechanically induced stress (Biddington, 1986), also incites to reduce the frequency of manual operations.
Nevertheless, the precise agronomic consequences of the various manual operations are not yet known. They depend on several phenomena: when reducing the frequency, plants are stressed less often, but each operation may be more stressful (e.g. more leaves or axillary shoots are removed each time). Moreover, plants should be in worse conditions between two successive operations, as regards light interception, air circulation or disease risks.
No studies were found in the literature on the effect of deshooting frequency, but one can say that it determines the age of the axillary shoots remaining on the plant and therefore the carbon demand of these organs. A young axillary shoot, as any young organ, is a sink and uses the assimilates produced by the leaves of the main stem; when reducing the frequency, each axillary shoot grows longer and becomes a stronger competitor for assimilates for the main stem, the roots and fruits. Finally, the axillary shoot becomes a carbon source. But the relation between the carbon status of the axillary shoot and the deshooting frequency is not known. In a greenhouse trial, Hartmann (1977) compared 12 cultivars varying in vigour. Plant vigour is a qualitative characterisation of vegetative growth, in close correlation with leaf area, leaf dry weight and stem diameter (Hall, 1983; Navarrete et al., 1997). The various cultivars produced varying amounts of axillary shoots over the same period. Hartmann (1977) found a negative correlation between the weight of axillary shoots collected and yield, which con®rms that the axillary shoots are in competition with fruits. According to this trial, reducing the frequency of deshooting may affect the yield.
In order to determine the effects on plants of reducing the frequency of deshooting, two experiments were conducted in 1996 and 1998, which made it possible to test four deshooting frequencies.
2. Materials and methods
2.1. The plants and cropping conditions
Both experiments were conducted in a high greenhouse located at the INRA station of AleÂnya (south France). The double-rows were aligned SE±NW. There was no carbon dioxide enrichment. The mean temperatures were 168C at night and 198C at day in winter, and 188C at night and 238C at day in spring and summer. The temperature was adapted weekly to the external climate and to the vigour of the plants. The climatic conditions are summarised in Table 1. Plant nutrition followed commercial practices.
Experiment 1 (1996). Seeds of Lycopersicon esculentum Mill. (cv. Synergie) were sown in a nursery on 15 November 1995, and transferred onto a rockwool substrate in the greenhouse on 21 December. They were planted on 12 January 1996 at 2.4 plants/m2. The trusses were pruned to ®ve fruits. The plants were grown in the greenhouse until 23 August 1996, but the harvest stopped on 15 July because at that time most of the fruits were affected by blossom-end-rot.
The major difference between the two cultivars was that Egeris is more vigorous than Synergie.
2.2. The treatments and experimental design
As space is limited in experimental greenhouses, only a small number of treatments could be compared in the same place. In this case, three out of four deshooting frequencies were compared each year.
In Experiment 1, deshooting was carried out every 7, 10 or 14 days (the treatments were called 7D, 10D and 14D, respectively). The 7D treatment was considered as a control, since it is the frequency observed most often in commercial holdings in France. In Experiment 2, the 7D and 14D treatments were repeated. The third treatment (21D) consisted in deshooting every 14 days till 31 March, and from then on every 21 days; this treatment was supposed to better suit the variations of vegetative growth during the cropping cycle: as plants are getting older, their growth rate is reduced (de Koning, 1994), and we hypothesised that deshooting could be done less often in the last part of the cycle. Moreover, the 21D treatment was also justi®ed by labour organisation trends in greenhouse holdings: when harvest begins, the other manual operations are performed less frequently through lack of time.
All the treatments consisted in removing all the axillary shoots of the plant by hand, as is done in greenhouse holdings. For the 7D treatment of the ®rst experi-ment, even the smaller axillary shoots located near the apex were removed. As this appeared to be too stressful for the plants, only the axillary shoots over 2 cm were removed in Experiment 2. The 14D treatments were identical in both experiments. The greenhouse was divided into six blocks in Experiment 1 and four blocks in Experiment 2, to take into account the climatic heterogeneity of the greenhouse. In each block, the three treatments were applied randomly. The plots in Experiments 1 and 2 contained nine and eight plants, respectively, in one row.
Table 1
Climatic conditions of the two trials
December January February March April May June July August
Global radiationa(MJ/m2)
Experiment 1 140 159 259 416 450 628 622 699 589
Experiment 2 149 269 356 471 655 695 669 504
Mean temperatureb(8C)
Experiment 1 18.0 17.3 18.2 19.2 19.2 21.1 22.9 24.9 Experiment 2 18.2 17.4 17.9 19.0 19.4 20.9 23.8 23.0
a
The global radiation was measured outside the greenhouse.
b
2.3. Scores and measurements
The main aim of the trials was to determine the effects of deshooting on yield. But several other variables were measured to characterise the deshooting treatments and analyse their consequences on vegetative growth.
Each time the axillary shoots were removed from the plants, they were dried in an oven at 908C for 2 days and weighed. The total weight per plot was recorded. This made it possible to quantify and compare the treatments.
The vegetative growth of the main shoot was estimated visually by vigour scores and by stem diameter measurements. Both notations are useful: the vigour score is an accurate estimation of the vegetative growth of plants and is more comprehensive than stem diameter; but stem diameter is an objective indicator of the vegetative dry weight and leaf area of a plant (Hall, 1983; Navarrete et al., 1997). The vigour was scored plant by plant by two experts on a scoring scale ranging from 1 to 3, according to the protocol de®ned by Navarrete et al. (1997). The stem diameter was measured using a calliper. As it evolved along the plant, it was measured at several levels along the stem, between trusses 2 and 14 in Experiment 1 and trusses 10 and 28 in Experiment 2, every two trusses. At one particular level, the measurement was made 1 cm above the truss, when the stem had reached its maximum width, i.e. when the third truss above had ¯owered.
The fruits were harvested once to twice a week, depending on the season and internal climate of the greenhouse. We recorded the number and weight of mature fruits on each plot, separating the marketable and unmarketable fruits. The harvest stopped on 15 July in Experiment 1, and 7 September in Experiment 2. In Experiment 1, the number of set fruits was also recorded on each truss, in order to detect possible defaults of fruit setting.
The ¯owering stage was estimated every 2 weeks by the number of the last truss in bloom and the number of open ¯owers on it, and the duration of development was expressed in degree-days from the sum of mean daily temperature (with a base temperature equal to 08C).
3. Results
3.1. Characterisation of the experimental treatments
the axillary shoots on a weekly basis, every 10 days being suf®cient. After mid-April, light and temperature in the greenhouse increased, and deshooting only every 10 days led to increasing losses of dry matter. The curves were similar in Experiment 2: the 14D treatment clearly differed from that of 7D from the ®rst measurements; the dry weight of shoots produced in the 21D treatment was greater than in the 14D treatment as soon as the deshooting frequency increased from 14 to 21 days, i.e. from 31 March (160 days after sowing). At the end of the experiments, the dry weight of axillary shoots produced in the 14D treatment was 3.8 and 2.5 times as much as in the 7D treatment, in Experiments 1 and 2, respectively.
3.2. Effect of the treatments on development and vegetative growth
The number of trusses appeared per day varied from 0.10 in winter to 0.16 in summer (Experiment 1), depending on the temperature in the greenhouse. In south France, the temperature under the greenhouse in winter mainly depends on the heating system, whereas in summer, it increases because of the outside temperature (Table 1). On the whole season, the mean values were 0.121 and 0.123 trusses per day in Experiments 1 and 2, respectively, i.e. 5.9910ÿ3 and
6.0410ÿ3truss per degree-day. Therefore, there was no signi®cant difference on
development rate between the two cultivars. The deshooting frequency had no signi®cant effect on development rate (not shown).
Vegetative growth was estimated by two kinds of measurements: vigour scores and stem diameter measurements (Table 2). The diameter and vigour evolved along the stem, on a similar way for all the treatments (Fig. 2). In Experiment 1 (Fig. 2a), the higher values were observed on the lower part of the plant (i.e. during the ®rst part of the cropping cycle, when fruit load was small). Then, stem diameter decreased until harvest began, i.e. at the level of truss 8. Stem diameter tended to increase later on (Fig. 2b), depending on the reproductive/vegetative balance on the plant.
Only the mean values per plant are indicated in Table 2. In Experiment 1, the 7D treatment had a lower stem diameter (measured between trusses 2 and 14) than the other two treatments, which were not signi®cantly different. There was no difference in vigour scores, probably because it was recorded much later than
Table 2
Effects of deshooting frequency on vegetative growtha
Treatments Vigour scores Mean stem diameter (mm)
Experiment 1
7D 2.8 3.4a 2.7 13.15a 14.10 14.50a
10D 2.7 ± ± 13.50b ± ±
14D 2.8 3.0b 2.7 13.45b 13.87 13.99b
21D ± 2.8b 2.4 ± 13.55 13.47c
P 0.7740 0.0083 0.6925 0.0227 0.1085 0.0005
ns ** ns * ns ***
aFor each experiment, the level at which measurements were made is indicated in brackets. The trusses
indicated for vigour scores are those in ¯ower when vigour was recorded. Figures followed by the same letter are not signi®cantly different at 5% level.
stem diameter, at the level of truss 21. On the contrary, in Experiment 2, the 7D treatment had a larger stem diameter than that of 14D, which was in turn higher than that of 21D. The differences were very signi®cant on the higher part of the plants (at the level of trusses 22±28) and nearly signi®cant in the middle part of the plants (trusses 10±16). As regards vigour scores, the 7D treatment was more vigorous than the other two at the ¯owering of truss 14, and the differences were signi®cant. The vigour of the 14D and 21D treatments was similar, which is consistent with the fact that, when plant vigour was noted (on 5 April), the plants of the 21D treatment were still deshooted every 14 days, as in the 14D treatment. On 20 July, at the ¯owering of truss 27, the vigours of the three treatments were
no longer signi®cantly different, although plants had been subjected to the different treatments for a long time. This was probably due to increased heterogeneity among plants of a same plot (diseases, broken plants), which was all the higher as the size of the samples was rather small.
The results observed on vigour and stem diameter are consistent: the treatments which had the most vigorous plants (higher values of vigour) also had the plants with the largest stem diameter, when it was measured at the level of trusses which had ¯owered at the time of vigour scoring. This result had also been observed in previous trials (Navarrete et al., 1997). But the stem diameter data from the two experiments seem contradictory at ®rst sight. In fact, it is likely that the small diameters measured on the 7D treatment of Experiment 1 is due to the deshooting practice: the ®rst year of experimentation, the axillary shoots were removed whatever their size, even if they were very young and near the apex, which may have stressed the plant and decreased stem diameter of the 7D treatment. Except for this treatment, it seems that vegetative growth decreases when the period between two successive deshooting increases a lot (i.e. over more than 14 days). The stem diameters measured at the same level on the plant (i.e. at the level of trusses 10±14 for Experiment 1, and 10±16 for Experiment 2) on the 14D treatment were 13.0 and 13.9 mm, respectively, which con®rms that Experiment 2 cultivar (Egeris) was more vigorous than Experiment 1 cultivar (Synergie).
3.3. Effect of the treatments on yield
In Experiment 1, harvesting began 130 days after sowing (2305 degree-days), i.e. a bit later than in Experiment 2 (120 days, 2146 degree-days). As no difference was observed on the duration of the vegetative phase, it means that the period of fruit development was longer on Synergie cultivar than on Egeris. After a few weeks of harvesting, the total yield (marketable and unmarketable yield) in Experiment 1 was higher than in Experiment 2 and this phenomenon was observed throughout the cropping cycle, though the Experiment 1 cultivar was weaker (Fig. 3). The most probable explanation is that the crop of Experiment 2 was sowed 3 weeks earlier and therefore, grew in worse light conditions (the cumulated radiation on the ®rst 8 months was 3078 MJ/m2in Experiment 1 and 2730 MJ/m2in Experiment 2, i.e. 11% less). Unmarketable yield in Experiment 1 amounted to about 13% of the total yield and was due to blossom-end-rot. In Experiment 2, it amounted only to 4%.
The total yield of the 21D treatment in Experiment 2 was about 7% lower than that of the 7D treatment (Table 3). The difference was not signi®cant, but the probability was rather low (P0.12). Therefore, deshooting rarely tended to
whereas mean fruit weight was not affected. As we have no information in Experiment 2 on the number of fruits per truss, it is impossible to determine whether deshooting frequency affected fruit setting or the duration of the growing period. The yields in the 7D and 14D treatments were nearly equal as regards
Fig. 3. The comparison of yield in the two experiments. The total (&) and marketable (&) yields are compared 8 months (exactly 242 days) after sowing, i.e. at the end of the crop for Experiment 1 and on 21 July for Experiment 2. The vertical bars represent the con®dence intervals atP0.05.
Table 3
Effects of deshooting frequency on yield componentsa
Treatments Yield components on 3 August (Experiment 2) Earliness of harvesting
Total yield (kg/m2)
Marketable yield Experiment 1 Experiment 2
No. fruits per m2
Mean fruit weight (g)
No. fruits per m2on
9 April 5 March
7D 32.2 222a 141 15.2a 9.5
10D 12.9b ±
14D 31.6 224a 138 13.1b 8.1
21D 30.1 211b 140 ± 7.4
P 0.1193 0.0142 0.6007 0.0436 0.1041
ns * ns * ns
a
The earliness of harvesting is estimated by the amount of fruits collected within the ®rst 15 days of harvest. Figures followed by the same letter are not signi®cantly different at 5% level.
nsnon-signi®cant.
total or marketable yield. In Experiment 1, the yield of the 14D treatment was slightly lower than that of the 7D one, but the difference was not signi®cant.
These results show the effects of deshooting frequency on the ®nal yield. But growers are also interested in the time course of production, particularly in the ®rst months of production, because the prices are higher in winter than in spring. Therefore, the number of fruits harvested during the 15 ®rst days was also analysed. In both experiments, the 7D treatments produced a greater number of fruits 15 days after the beginning of harvesting (Table 3) and was therefore early-fruiting. The difference was signi®cant in Experiment 1 and nearly signi®cant in Experiment 2.
4. Discussion
Four deshooting frequencies were compared in a 2-year trial, and their effects on development, vegetative growth and yield were compared.
The axillary shoots left on the plants until the next deshooting operation consisted of a few young leaves (each one being shorter than 15 cm) and were removed before the ®rst truss ¯owered on them. Their dry weight was all the higher as the deshooting was performed less often (from 0.03 g per plant per day for the 7D treatment to 0.19 for the 21D one). This result is consistent with the dynamics of dry weight accumulation in shoots, which is exponential: the longer the shoots are left on the plants, the higher their growth rate. In Experiment 2, on 3 August, the weight of the axillary shoots was compared to the total dry weight of the plants, which was estimated. Assuming that23of the assimilates are diverted to fruits and that fruit water content is 94% (Ho and Hewitt, 1986), and considering the total yield of each treatment on 3 August, the axillary shoots represented at that date 0.6, 1.7 and 3.3% of the estimated plant's dry weight production for the 7D, 14D and 21D treatments, respectively. The data from Experiment 1 lead to the same conclusion. These fractions are rather low and nevertheless, the frequency at which axillary shoots are removed appear to have several consequences on vegetative growth and yield.
stem is diverted to them, at the expense of vegetative and fruit growth on the main stem. The same phenomenon may explain the present results on the 21D treatment although no study was found in the literature on dry matter partitioning between the main stem and the axillary shoots. Yet, experimental measurements of dry matter partitioning on tomato plants have been always made on plants whose axillary shoots have been removed, as in commercial crops (e.g. Khan and Sagar, 1967; Heuvelink and Marcelis, 1989). When axillary shoots are left on plants to increase crop density (to ®t better the increase in radiation in spring), dry matter production and partitioning are observed only when plants already bear fruits (Cockshull and Ho, 1995; Delambre, 1998), and not in the transitional phase.
In our experiment, the harvest started later on plants of the 21D treatment than on those of the 7D treatment. Kazanovich (cited by Aung and Kelly, 1966) compared deshooted plants and plants which were never deshooted; he also found a positive effect of deshooting on early ripening of tomato fruits. In the present trials, when plants were deshooted every 14 days, the effects were qualitatively similar to the case of deshooting every 21 days, but rather limited.
In the 7D treatment of Experiment 1, the plants were deshooted weekly and even the smaller shoots located near the apex were removed. Therefore, the weight of axillary shoot collected each time was low. In that case, the stem diameter was smaller than that in the 14D treatment, but yield was not affected. This phenomenon could be due to the mechanical stress caused by the high frequency of deshooting operations performed during the season. Tomato is known to be a species which is rather sensitive to mechanical stress (Heuchert and Mitchell, 1983). Biddington (1986) and Mitchell and Myers (1995) reviewed several studies on mechanical stress, on several species. On tomato, most of the mechanical stresses tested reduce leaf area and leaf dry weight, stem diameter and even sometimes yield. But usually, the experimental treatments tested differ greatly to the reality of greenhouse production, and the effects of involuntary plant movements have not been tested. The studies which most closely resemble greenhouse tomato crop conditions are those of Buitelaar (1988): the experimental treatments consisted in taking the head of each plant between the thumb and fore®nger, and lifting the plant vigorously up and down three or four times. This movement reduced the mean weight of the fruits and provoked a magnesium de®ciency, which also had a negative effect on fruit yield. The effects were all the greater as the plants were moved frequently. This could explain why, in Experiment 1, the stem diameter of the 7D treatment was the lowest of all the treatments experimented.
should not be passed to prevent a yield decrease, which is about 2 weeks. Deshooting every 3 weeks must be avoided, since vegetative growth and yield are affected. Within 1±2 weeks, no signi®cant trend on yield or vegetative growth was observed, and the choice of the deshooting frequency will depend on labour organisation in the horticultural holding. In particular, a deshooting operation performed after 14 days takes more time than that after 7 days because a larger number of shoots must be removed, but the labour cost calculated over the whole cropping cycle is reduced. Yet, in some holdings, growers prefer to go on deshooting weekly for practical reasons, because this operation is performed at the same time as truss pruning, which is necessarily done weekly. Moreover, it appears that deshooting every week enables earlier harvest, which is rather interesting from an economical point of view.
Acknowledgements
The authors are grateful to L. Pares (INRA AleÂnya) for technical assistance.
References
Aung, L.H., Kelly, W.C., 1966. In¯uence of defoliation on vegetative, ¯oral and fruit development in tomatoes (Lycopersicon esculentumMill.). Am. Soc. Hort. Sci. 89, 563±570.
Biddington, N., 1986. The effect of mechanically induced stress in plants Ð a review. Plant growth regulation 4, 103±123.
Buitelaar, K., 1988. Are your tomatoes a little shaky? Grower, February 4, 1988, pp. 32±33. Cockshull, K.E., Ho, L.C., 1995. Regulation of tomato fruit size by plant density and truss thinning.
J. Hort. Sci 70, 395±407.
de Koning, A.N.M., 1994. Development and dry matter distribution in glasshouse tomato: a quantitative approach. Thesis. Agricultural University, Wageningen, the Netherland, 240 pp. Delambre, M., 1998. Equilibre veÂgeÂtatif et geÂneÂratif de plants de tomate cultiveÂs en serre sous
l'effet de la charge en fruits et de la densite de peuplement. ConseÂquences pour le modeÁle Tompousse. Student Report. ENITA Clermont-Ferrand, France, 41 pp.
Hall, D., 1983. The in¯uence of nitrogen concentration and salinity of recirculating solutions on the early season vigour and productivity of glasshouse tomatoes. J. Hort. Sci. 58, 411±415. Hartmann, H.D., 1977. In¯uence of axillary shoots on growth and yield of tomato varieties.
Gartenbauwissenschaft 42, 178±184.
Heuchert, J.C., Mitchell, C.A., 1983. Inhibition of shoot growth in greenhouse tomato by periodic gyratory shaking. J. Am. Soc. Hort. Sci. 108, 801±805.
Heuvelink, E., Marcelis, L.F.M., 1989. Dry matter distribution in tomato and cucumber. Acta Hort. 260, 149±150.
Ho, L.C., Hewitt, J.D., 1986. Fruit development. In: Atherton, J.G., Rudish, J. (Eds.), The Tomato Crop. A Scienti®c Basis for Improvement. Chapman & Hall, London, pp. 201±239.
KlaÈring, H.P., 1999. Effects of non-destructive mechanical measurements on plant growth: a study with sweet pepper (Capsicum annuumL.). Sci. Hort. 81, 369±375.
Mitchell, C., Myers, P., 1995. Mechanical stress regulation of plant growth and development. Hort. Rev. 17, 1±41.
