Fruit drop was also observed in winter crop, and yield in the most ... As regards the effect of water stress on different lemon crops, the aim of this report is to ...
Proceedings of the Sixth International Citrus Congress
717
Tel Aviv, Israel, March 6-11, 1988, R. Goren and K. Mendel (Eds) © 1988 Balaban Publishers, Philadelphia/Rehovot Margraf Scientific Books, 0 -6992 Weikersheim ISBN 3-8236-1136-4
Effects of Different Levels of Water Stress on Yield and Quality of Lemon Trees G. BARBERA AND F. CARIMI
Istituto di Coltivazioni Arboree dell'Universita' degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy F. lEONE
Associazione Quadrato Verde, Via Roma 97, 90100 Palermo, Italy Additional Index Words: Summer crop, winter crop, water potential
Abstract Three levels of water stress were caused by using the 'forzatura' technique to determine its effects on summer and winter crops. In summer fruits about to be harvested when the stress occurred, high percentage of drop was noticed after the resumption of irrigation in the most stressed treatment. Fruit drop was also observed in winter crop, and yield in the most severely stressed treatment was lower. In the new summer crop obtained under water stress, the number of flowers/m3 was clearly higher in the most stressed treatment, indicating a relationship between the severity of stress and the flowering response. Fruit set, on the other hand, was very low, so that no difference in yield was observed.
Introduction The recourse to water stress for stimulating flowering is a practice ('forzatura') adopted for over a century now in the Sicilian lemon industry. By withholding irrigation during the summer until trees are wilted, summer bloom is accentuated and it is possible to obtain in the following summer a crop of economic importance: the so-called 'verdelli' (1). A problem of great interest for the correct application of the technique is to find out the optimum stress level at which irrigation should be resumed after the dry period (i.e. the interval between irrigations). Too moderate levels of stress can result in an inadequate flowering response. On the other hand, too severe water stress
718 involves negative effects, such as excessive leaf drop, damage to the root system (3) and high percentages of flower abortion (6). Furthermore, the development of fruits from the spring blooms, and fruits from the summer bloom of the previous year still on trees, can suffer from a too prolonged dry period. The percentage of winter fruit shedding reaches very high values and their quality can be adversely affected (2). As regards the effect of water stress on different lemon crops, the aim of this report is to describe the relationship between the length of the dry period and the quantitative and qualitative response of the trees.
Materials and Methods The experiment was carried out in 1985 and 1986 in a lemon orchard near Palermo, but some data concern also an interrupted experiment conducted in 1983. Thirty five years old 'Femminello comune' lemon trees (Citrus limon Burm.), grafted onto sour orange, were selected. Three treatments (A,B,C)~ involving various lengths of the dry period, were compared: A- seven weeks interval between the last irrigation (May 28) and rewatering; B - nine weeks interval; C - ten weeks interval. Water stress was measured as leaf xylem pressure potential- MPa - by the pressure bomb technique (4). Three old leaves per tree (9 leaves/treatment) were measured at predawn. The experimental design was a completely randomized block with three replicates. Observations were made on winter and summer fruits ('verdelli') from the spring 1985 and summer 1984 bloom which were on the trees during the dry period, and on the 'verdelli' from the 1985 summer bloom. Observations concerning: yield per tree, fruit growth, fruit drop at the end of the dry period and after the resumption of irrigation, and some qualitative parameters (percent of juice, acidity, and tss, fruit diameter, peel thickness) which were recorded at harvest time. Concerning the summer 1985 bloom, investigation involved the interval between the resumption of irrigation and the new generative flush, its duration, the number of flowers/m3, the percentage of fruit set, and the final yield.
Results The different lengths of the interval between the last irrigation and rewatering caused different levels of water stress in the three treatments (Table 1). With regard to the 'Verdelli' from summer 1984 bloom, which were about to be harvested when water stress occurred, Table 2 shows the high percentage of fruit drop in the most stressed treatment (C). The drop was particularly enhanced after the resumption of irrigation, both in 1985 and in 1983, when it followed a stress level of - 2.9 MPa. In the most severe stress treatment yield results were statistically the lowest (Table 3).
719 Table 1: Effect of the Length of the Dry Period on Leaf Xylem Pressure Potential in the Different Treatments Treatment
Leaf xylem pressure (MPa) Potential
Length of dry period (weeks)
A B
-2.17* -2.70 -3.08
7 9 10
c
± 1.95 ±1.88 ±0.58
* Values represent the means of 9 leaf replicates/treaunent ±SO.
Table 2: Summer Fruits (1985) Drop Percentage Treatment
A B
c
At the end of dry period
After the resumption of irrigation 14.5 b 17.3 b 37.3 a
9.9 a* 16.0 a 13.8 a
At harvest time 23.7 B 19.7 B 43.1 A
* Mean separation by Duncan's multiple range test: 5% level (lower case letters) and 1% level (capital letters).
Table 3: Yields (kg/tree) under Different Treatments Treatment
A B
c
Summer 1985 crop (kg/tree)
Summer 1986 crop (kg/tree)
Winter 1986 crop (kg/tree)
16 .5 a* 16.4 a 12.0 b
30.3 a 39.3 a 35.0 a
24.5 b 29.8 a 19.1 c
* Mean separation by Duncan's multiple range test: 5% level.
Moreover, the content of sst and the acidity of the juice reached statistically lower values in the less stressed treatments, while fruit diameter was lower in the most severe treatment (Table 4).
720 Table 4: Effect of the Different Treatments on some Qualitative Characters of Summer Fruits (1985) Treatment
Fruit Diameter (em)
Peel Thickness (mm)
A
B
c
53.6 a* 59.2 a 50.8 a
53.3 a 52.9 a 48.7 b
TSS (%)
Acidity (%of citric acid)
6.6 b 7.6 ab 8.9 a
6.3 b 7.5 a 7.3 a
Juice (%)
28.7 a 30.4 a 27.8 a
* Mean separation by Duncan's multiple range test: 5% level.
As far as the winter crop is concerned, the rate of fruit growth was higher during the stress period. Fruit diameter was highest (Table 6) in the less stressed treatment (A), but at harvest time no difference in fruit diameter (Table 6) was found among the treatments. Fruit drop, measured both during the dry period and at the resumption of irrigation, was statistically different; yield was lower in the most severely stressed treatment (Tables 3 and 5). However, no differences were found among qualitative parameters of winter fruits in the three treatments (Table 6).
Table 5: Rate of Drop(%) of Winter Fruits (1986) Treatment
A B
c
At the end of dry period 26.2 cB* 34.2 bB 50.2 aA
After the resumption of irrigation 29.8 cB 39.2 bB 56.5 aA
At harvest time 53.9 b 57.2 b 65.2 a
*Mean separation by Duncan's multiple range test: 5% level (lower case letters) and 1% level (capital letters).
Bloom occurred 20-22 days after the resumption of irrigation, and was slightly concentrated in the most severely stressed treatment. However, the most important difference concerns the number of flowers/m3 which is clearly higher in treatment C (Table 7). The rate of fruit set, on the other hand, was very low, so that at harvest time no differences in yield could be observed (Table 3).
721 Table 6: Effect of the Different Treatments on Some Qualitative Characters of Winter Fruits (1986) Treatment
Peel Thickness
Fruit Diameter (em)
(mm) 59.0 a* 61.0 a 53 .0 a
A B
c
53.70 a 49.80 a 48.90 a
Acidity (%of citric acid)
TSS
(%)
6.0 a 6.3 a 6.3 a
6.18 a 6.42 a 5.79 a
Juice (%)
35.8 a 33.2 a 32.8 a
* Mean separation by Duncan's multiple range test: 5% level.
Table 7: Effects of the Different Treatments on Summer Blooming, Number of Flowers/m3 and Fruit Set Treatment Blooming Blooming starts ends set (days after (days after (%) irrigation) irrigation) A
B
c
20 22 21
32 34 31
68.2 52.3 13.4
Fruit
Flowers/m 3
116 B* 202 B 379 A
* Mean separation by Duncan's multiple range test, 1% level.
Discussion This report points out the importance in the 'forzatura' technique in gaining an adequate stress level before the resumption of irrigation. This is in agreement with the presence, at the same time, of two crops and the flowering process that will produce a new 'verdelli' crop. According to Southwick (5), the relationship between stress severity and the flowering response is conftrmed. However, when water stress is severe, flower disorders induce a heavy drop that balances the ftnal yield. Moderate levels of stress are recommended in view of the high drop rate of fruits from the summer bloom of the previous year which are about to ripen. Drop is probably caused by tissue senescence and vegetative disorders caused by competition among different sinks, but this hypothesis deserves further investigations. Moreover, water loss from small winter fruits can cause their drop when stress level is high, resulting in lower yield. Overall, the response of lemon trees seems to be better when water potential does not exceed -2,7 MPa ca.
722
Literature Cited 1 . Barbera, G., Fatta Del Bosco, G. and Lo Cascio, B. 1985. Effects of water stress on lemon summer bloom: the 'forzatura' technique in the Sicilian citrus industry. Acta Horticulturae 171:391-397. 2. Calabrese, F. 1986. Osservazioni sugli effetti della forzatura in piante di limone. Frutticoltura 11- 12:833-842. 3. Crescimanno, F.G. 1959. Ricerche sulla forzatura dellimone. Tecnica Agricola 45:1-11.
4. Scholander, S.M., Hammel, H.T., Bradstreet, E.D. and Hemmingsen, E.A. 1965. Sap pressure in vascular plants. Science 148:339-346. 5 . Southwick, S.M., and Davenport, T.L. 1986. Characterization of water stress and low temperature effects on flower induction in citrus. Plan.J Physiol. 81:26-29. 6. Torrisi, M. 1952. lndagini fisiologiche sugli agrumi: Ia differenziazione delle gemme a fiore del limone e dati preliminari sulla forzatura. Ann. Sperimen.Jazione Agraria 881-894.
