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Growth and fruiting responses of strawberry plants grown on rockwool to shading and salinity
SCIENTIA HORlCULTllM Scientia Horticulturae62 ( 1995) 25-31
Growth and fruiting responses of strawberry plants grown on rockwool to shading and salinity Yahya B. Awang”, J.G. Atherton Department of Agriculture and Horticulture, University of Nottingham. Sutton Bonington Campus, Loughborough LEI2 5RD, UK
Accepted29 December 1994
Abstract Strawberry cultivar ‘Rapella’ grown in a glasshouse responded to shade with reductions in leaf area, number of leaves, crowns and inflorescences and shoot dry weight. There was no apparent interaction of shading and salinity on vegetative growth. Shading depressed the fruit dry weight but not fresh weight, resulting in fruits with a higher moisture content. Fruit number was reduced under shaded conditions. Salinity did not affect fruit number but both fresh and dry weights of fruit were lower at high salinity. The negative effects of high salinity on fruit yield were more marked under shaded conditions. The percentage of dry matter was highest in unshaded fruits produced at high salinity. Increased concentration of reducing sugars per unit fruit fresh weight at high salinity was only apparent in unshaded plants. Acidity in fresh fruit was promoted by increased salinity both under shaded and unshaded conditions. Shading increased acid concentration per unit dry weight but not on a fresh weight or a per fruit basis. Unshaded plants allocated more dry matter to fruits at the expense of leaf growth. Keywords: Fruit quality; Fruit yield; Shading; Salinity; Strawberry
1. Introduction One of the key difficulties encountered in heated greenhouse crop production in winter is that low irradiauce tends to direct the plants towards ‘excessive’ vegetative growth and this may retard reproductive development (Cockshull, 1988). Increased irradiance reduces this problem, as reported for several crop species such as tomatoes (Logendra and Janes, 1992) but supplementary lighting is expensive. In strawberry, vegetative and reproductive * Corresponding author at: Horticulture Division, Malaysian Agricultural Research and Development Institute, P.O. Box 12301, GPO, 50774 Kuala Lumpur, Malaysia. 0304-4238/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SsDIO304-4238(95)00770-9
26
Y.B. Awang, J. G. Atherton / Scientia Horticulturae 62 (1995) 25-31
growth are competing sinks and both are reduced under UK summer and autumn light conditions by the relatively inexpensive method of raising irrigation salinity (Awang et al., 1993a). Retardant effects of salinity on vegetative growth could be more marked under winter low irradiance, with consequent potential benefits for fruiting. Possible interactions between irradiance and salinity are therefore examined with a view towards determining the potential of salinisation as a tool for favouring fruit development in strawberry under glass in winter.
2. Materials and methods 2.1. Experimental treatments Strawberry plants cv. ‘Rapella’ established in rockwool cubes were transferred onto rockwool growing slabs on 22 July 1992. Two plants were grown on each slab and spaced 45 cm apart. Each plot comprised a bench holding eight plants in four rockwool slabs. A total of 12 benches (plots) containing 96 plants were used in the study. During the study period, all runners generated by plants were removed. Plants were subjected to three levels of salinity in their root zones (electrical conductivities of 2.6, 5.9 and 8.6 mS cm-‘) and two daily solar radiation integrals (total irradiance) : 2.1 (shaded) and 4.9 (unshaded) MJ m-* day- ‘. The two higher electrical conductivities (ECs) were achieved by injecting concentrated NaCl solution into the basic liquid nutrient solution through an automatic drip irrigation system. Irrigation was scheduled to deliver the solutions six times per day at 06:00,09:00, 12:00, 15:00, 18:00 and 21:00 h supplying 60 ml per plant on each occasion. The composition of the basic solution was described previously (Awang et al., 1993a). Differences in the mean daily solar radiation integrals intercepted by the plants were obtained by suspending shade green polypropylene netting approximately 1 m above the uppermost leaves of the plants. This had been shown to have no effect on light quality (Meiri et al., 1982). Radiation integrals beneath the shades and above the plant canopy were continuously measured using tube solarimeters (Green and Deuchar, 1985). Treatments were randomised in a split plot design with salinity as main plot and shading as subplots, each treatments being replicated four times. 2.2. Data collection
At each harvest, all ripe fruits produced by plants (eight plants per plot) were counted and weighed and their accumulated weights were calculated. Fruit dry matter content, reducing sugars and titratable acidity from a sample of 12 fruits per plot were determined on 11 September 1992 using techniques described by Awang et al. ( 1993b). At the end of the study ( 16 September 1992)) the shoots of two plants per plot were harvested by cutting at the rockwool cube level and separated into their principal components. Leaves, crowns, inflorescences and unripe fruits (fruits over 0.5 cm in diameter) were counted and leaf areas were determined. Their dry weights were determined after drying for 48 h at 80°C in a forced draught oven.
Y.B. Awang, J.G. Ah-ton
/Scientia Horticulturae 62 (1995) 25-31
21
3. Results 3. I. Leaf and crown development
Shading reduced total leaf growth. Unshaded plants produced 26% more leaves than those grown under shaded conditions (P < 0.001) (Table 1). Similar effects of high radiation were also detected on total leaf area (P < 0.01) and leaf dry weight per plant (P < 0.001). As with leaves, initiation of crowns was also enhanced at higher radiation and this was reflected as more crowns produced under unshaded conditions (P < 0.01). Negative effects of salinity on vegetative development are also shown in Table 1. Depressed leaf initiation was seen as a significant reduction in leaf number at higher salinity (P < 0.05). Because salinised plants produced fewer and smaller leaves, total leaf area per plant was reduced. Increasing salinity significantly reduced (P < 0.05) the number of crowns per plant (Table 1). No significant interactions between shading and salinity were detected. 3.2. Floral development
Shading had a strong inhibitory effect on floral development. The number of inflorescences per plant, number of flowers per inflorescence and number of fruits per inflorescence were reduced by 32%, 15% and 20% respectively (Table 2). Increasing salinity significantly reduced inflorescence number (P < 0.05) but not the number of flowers per inflorescence or the number of fruits per inflorescence. Table 1 Effects of shading and salinity on leaf area, leaf number, crown number and principal shoot components. Each value is a mean of four replicates Parameter
Leafarea (cm*)
Leaf no.
Crown no.
LeafDW (g)
Shading
Unshaded Shaded Mean
EC (mS cm-‘)
Mean
2.6
5.9
8.6
2595 1896 2245
1889 1497 1693
1610 1254 1432
Shading
EC
Interaction
***
*
NS
***
*
NS
***
*
NS
***
*
NS
2031 1549
Unshaded Shaded Mean
52.4 39.9 46.2
39.0 36.6 37.8
37.6 29.1 33.6
43.0 35.4
Unshaded Shaded Mean
9.2 6.6 7.9
1.4 7.0 7.2
6.5 5.1 5.8
7.7 6.2
Unshaded Shaded Mean
16.3 10.7 13.5
12.5 8.5 10.5
11.1 7.4 9.2
12.1 8.9
*P
F-test
28
Y.B.Awang. J.G. Atherton /Scientia Horticulhrrae62 (1995) 25-31
Table 2 Effects of shading and salinity on flower and fruit number. Each value is a mean of four replicates Parameter
Inflorescences per plant Plowers per inflorescence Fruits per inflorescence
Shading
EC (mS cm-‘)
Mean
2.6
5.9
8.6
Unshaded Shaded Mean
19.6 13.1 16.3
15.6 10.5 13.1
13.6 9.5 11.5
16.3 11.0
Unshaded Shaded Mean
12.3 9.7 11.0
12.4 9.4 10.9
9.6 10.1 9.8
11.4 9.7
Unshaded Shaded Mean
7.3 5.6 6.4
7.4 5.0 6.2
5.6 5.6 5.6
6.8 5.4
F-test Shading
EC
Interaction
***
*
NS
***
NS
NS
***
NS
NS
*P
3.3. Fruit yield Plants grown under unshaded conditions produced significantly more fruits than those grown under shaded conditions (P < 0.001) but salinity had no significant effect. The greater number of fruits produced by unshaded plants was due to a higher number of inflorescences and more flowers setting fruits (Table 2). Total fresh weight of fruits from the unsalinised plants was significantly (P < 0.05) heavier than from the salinised plants (Table 3)) with the negative effect of salinity less pronounced at higher radiation integrals. 3.4. Fruit dry matter content, reducing sugars and titratable acidity Shading and salinity significantly affected the composition of individual fruits. Reducing radiation integrals reduced fruit dry matter (P < 0.05) whilst raising salinity increased the fruit dry matter content (P < 0.05) (Table 4). Table 3 Effects of shading and salinity on accumulated fruit number and total fresh fruit yield. Each value is a mean of four replicates Parameter
Shading
EC (mS cm-‘) 2.6
Fruits per plant
Total fresh fruit wt. per plant
5.9
Mean 8.6
Unshaded Shaded Mean
30 18 24
28 17 22
26 15 20
28 17
Unshaded Shaded Mean
228 129 178
163 105 148
144 77 111
178 104
*P
F-test Shading
EC
Interaction
***
NS
NS
**
*
*
Y.B. Awang, LG. Atherton/Scientia
Horticulturae 62 (1995) 25-31
29
Table 4 Effects of shading and salinity on dry matter content, reducing sugars and titratable acidity of strawberry fruits. Each value is a mean of four replicates Parameter
Dry matter (46)
Shading
Unshaded Shaded Mean
EC (mS cm-‘) 2.6
5.9
8.6
9.39 9.11 9.25
10.86 9.92 10.39
11.48 10.03 10.75
10.58 9.69
6.68 4.38 5.53
6.96 4.65 5.80
6.18 4.61
0.521 0.525 0.523
0.615 0.440 0.527
0.604 0.459 0.531
0.580 0.475
0.442 0.429 0.435
0.449 0.303 0.376
0.447 0.262 0.354
0.446 0.331
1.172 1.154 1.163
1.259 1.174 1.216
1.177 1.151
Reducing sugars (glucose equivalent) Unshaded 4.91 gpet 1OOg fresh wt. Shaded 4.80 Mean 4.86 g g-’ dry wt.
g per fruit
Unshaded Shaded
Unshaded Shaded
Titratable acidity (citric acid equivalent) Unshaded 1.101 Shaded 1.25 Mean 1.113
gper 1OOg fresh wt.
gg-IdrY wt.
g per fruit
Mean
Unshaded Shaded Mean
0.118 0.123 0.120
0.108 0.117 0.112
0.109 0.117 0.113
0.112 0.119
Unshaded Shaded
0.099 0.100 0.099
0.080 0.080 0.080
0.081 0.067 0.074
0.087 0.082
F-test Shading
EC
Interaction
*
*
NS
**
NS
*
***
NS
NS
***
NS
NS
NS
*
NS
*
NS
NS
NS
**
NS
*P
The effects of shading on concentration of reducing sugars were significantly (P < 0.05) dependent on salinity (Table 4). The concentration of reducing sugars per unit weight of fresh fruit increased as salinity increased in unshaded plants but not in shaded plants. The beneficial effect of salinity in the enhancement of sugar concentration therefore can only be expected under high radiation conditions. When expressed on a dry weight basis, the sugar concentration in unshaded fruits was unaffected by salinity treatment. At higher salinity and under shade, sugar levels in the fruit dry matter were significantly lower than that produced by unshaded plants. The same was true for the weight of sugar per fruit. A lower weight of sugar per fruit at higher salinity (57 DAT) was associated with the smaller fruits ( r = 0.75). Titratable acidity was also changed by the treatments. Whilst shading did not alter the acid concentration per unit fresh weight or per fruit, acidity was increased in unshaded fruits on a per unit dry weight basis (P < 0.05). This reflected again that fruits from shaded plants contain more moisture than unshaded fruits. This is confirmed by a high negative coefficient of correlation between dry matter percentage and acid per gram dry weight (r= - 0.79).
30
Y.B. Awang, J.G. Atherton /Scientia Horticulturae 62 (1995) 25-31
No similar relationship existed for sugars. Salinity increased acid concentration on a fresh weight basis but not on a dry weight basis. Owing to the reduction in the fruit size by salinity, the weight of acid per fruit was significantly reduced at higher salinities (P < 0.01) . As the effect of salinity on reducing sugars was less marked for shaded fruits, the increase in the sugar:acid ratio at higher salinity was only displayed in unshaded plants. The absence of any significant effect of shading on the acid content of fresh fruit significantly (P < 0.001) decreased the sugar:acid ratio (data not shown). 3.5, Dry matter distribution Shading significantly altered shoot dry weight and dry matter distribution but there was no interaction with salinity. Lowering the radiation level from 4.9 to 2.1 MJ m-* day-’ significantly reduced total shoot dry weight from 45 to 27 g. The relative distribution of dry matter to various shoot components was significantly affected by shading treatment. Within unshaded plants, the dry matter allocation to leaves, crowns + petioles + flower stalk, and fruits were 29%, 19% and 52% compared with allocations of 33%, 20% and 48%, respectively in the shaded plants. This clearly indicates that increased radiation promoted the accumulation of the current assimilates into the fruits at the expense of leaf dry weight. The relative distribution of dry matter into leaves and fruits was negatively correlated (r = - 0.95).
4. Discussion Overall this study showed that under low irradiance conditions, increasing salinity did not stimulate fruiting in glasshouse strawberry. Depressive effects of salinity on growth and reproductive development were consistent with earlier findings (Awang et al., 1993a). At high irradiance (4.9 MJ m-* day-‘), the negative effects of salinity on fruit yield were reduced, suggesting an increase in plant salt tolerance. Similar results were reported for faba bean plants by Helal and Mengel ( 1981) and melons by Meiri et al. ( 1982). Better attraction for the current assimilates towards fruit development as well as faster CO2 assimilation (Awang and Atherton, 1994) could be responsible for the higher yield recorded in the unshaded plants (Table 3). Fruit yield was only recorded in the present study for a relatively short time (20 days after the first harvest) and yields were reduced through salinity affecting fruit weight rather than fruit number. Fruit number would probably have been reduced as well if the study had been extended as there were fewer inflorescences in salinised plants, reinforcing the view that salinisation under shaded conditions is not a commercial proposition. Long-term depressive effects of salinity on fruit yield were reported by Adams and Ho ( 1989) in tomatoes. They noticed that significant reduction in the fruit number was absent for the short-term crop (4 weeks) when salinity was increased to 8 mS cm-’ with NaCl, but a decrease of about 12% was found in a longer season crop (26 weeks). At lower radiation level, fruit dry weight was reduced at higher salinity but fresh weight was no different to that produced under unshaded conditions (Table 3). This implies that at equivalent salinity, fruits from shaded plants would have a lower concentration of dry
Y.B. Awang, J.G. AthertonIScientia Horticulturae 62 (1995) 25-31
31
matter and consequently poorer quality. This was reflected in the reduction in the concentration of reducing sugars on the fresh weight basis (Table 4). Lower concentrations of reducing sugars in the shaded fruits produced by salinised plants were also detected. This was consistent with radiation effects reported for tomatoes (Winsor and Adams, 1976) and apples (Campbell and Marini, 1992). Besides reducing assimilation, shading would have caused less carbon to be partitioned into sucrose and less to be translocated out of the leaves to the fruits (Ho, 1976; Logendra and Janes, 1992). Unlike reducing sugars, acid concentration was higher in the shaded than the unshaded treatments, providing one of the few benefits of salinisation under shaded conditions. This was consistent with the report by Robinson et al. (1983) for apple and could be associated with the higher leaf K concentration of shaded plants (Awang and Atherton, 1994). The combination of the lower reducing sugars and higher titratable acidity produced by salinised plants under shade suggests a delay to ripening, which could have commercial benefits for fruit shelf life. However, fruits produced under this environment had lower sugar:acid ratio and therefore inferior consumer quality (Awang et al., 1993b). This study clearly showed that radiation level played a important role in moderating the plant’s response to salinity. Generally, the beneficial effect of salinity in the improvement of fruit quality was only operational under high radiation level. Besides increasing EC of the nutrient solution, it is therefore important for the growers to use supplementary lighting or other means of enhancing net carbon assimilation to produce high quality fruits for the winter strawberry production. References Adams, P. and Ho, L.C., 1989. Effect of constant and fluctuating salinity on the yield, quality and calcium status of tomatoes. J. Hortic. Sci., 64: 752-732. Awang, Y.B. and Atherton, J.G., 1994. Salinity and shading effects on leaf water relations and ionic composition of strawberry plants grown on rockwool. J. Hortic. Sci., 69: 377-383. Awang, Y.B., Atherton, J.G. and Taylor, A.J., 1993a. Salinity effects on strawberry plants grown in rockwool. I. Growth and leaf water relations. J. Hortic. Sci., 68: 783-790. Awang, Y.B., Atherton, J.G. and Taylor, A.J., 1993b. Salinity effects on strawberry plants grown in rockwool. II. Fruit quality. J. Hortic. Sci., 68: 791-795. Campbell, R.J. and Marini, R.P., 1992. Light environment and time of harvest affect ‘Delicious’ apple fruit quality characteristics. J. Am. Sot. Hortic. Sci., 117: 551-557. Cockshull, K.E., 1988. The integration of plant physiology with physical changes in the greenhouse climate. Acta Hortic., 229: 113-123. Green, C.F. and Deuchar, C.N., 1985. On improved tube solarimeter construction. J. Exp. Bot., 36: 690-693. Helal, H.M. and Mengel, K., 1981. Interaction between light intensity and NaCl salinity and their effects on growth, CO2 assimilation and photosynthate conversion in young broadbeans. Plant Physiol., 67: 999-1002. Ho, L.C., 1976. The relationship between the rates of carbon transport and of photosynthesis in tomato leaves. J. Exp. Bot., 27: 87-97. Logendra, S. and Janes, H.W., 1992. Light duration effects on carbon partitioning and translocation in tomato. Sci. Hortic., 52: 19-25. Meiri, A., Hoffman, G.J., Shannon, M.C. and Poss, J.A., 1982. Salt tolerance of two muskmelon cultivars under two irradiance levels. J. Am. Sot. Hortic. Sci., 107: 1168-I 172. Robinson, T.L., Seeley, E.J. and Barritt, B.H., 1983. Effect of light environment and spurage on Delicious apple fruit size and quality. J. Am. Sot. Hortic. Sci., 108: 855-861. Winsor, W.G. and Adams, P., 1976. Changes in the composition and quality oftomato fruit throughout the season. Annu. Rep. Glasshouse Crops Res. Inst., 1975: 134-142.
Growth and fruiting responses of strawberry plants grown on rockwool to shading and salinity Yahya B. Awang”, J.G. Atherton Department of Agriculture and Horticulture, University of Nottingham. Sutton Bonington Campus, Loughborough LEI2 5RD, UK
Accepted29 December 1994
Abstract Strawberry cultivar ‘Rapella’ grown in a glasshouse responded to shade with reductions in leaf area, number of leaves, crowns and inflorescences and shoot dry weight. There was no apparent interaction of shading and salinity on vegetative growth. Shading depressed the fruit dry weight but not fresh weight, resulting in fruits with a higher moisture content. Fruit number was reduced under shaded conditions. Salinity did not affect fruit number but both fresh and dry weights of fruit were lower at high salinity. The negative effects of high salinity on fruit yield were more marked under shaded conditions. The percentage of dry matter was highest in unshaded fruits produced at high salinity. Increased concentration of reducing sugars per unit fruit fresh weight at high salinity was only apparent in unshaded plants. Acidity in fresh fruit was promoted by increased salinity both under shaded and unshaded conditions. Shading increased acid concentration per unit dry weight but not on a fresh weight or a per fruit basis. Unshaded plants allocated more dry matter to fruits at the expense of leaf growth. Keywords: Fruit quality; Fruit yield; Shading; Salinity; Strawberry
1. Introduction One of the key difficulties encountered in heated greenhouse crop production in winter is that low irradiauce tends to direct the plants towards ‘excessive’ vegetative growth and this may retard reproductive development (Cockshull, 1988). Increased irradiance reduces this problem, as reported for several crop species such as tomatoes (Logendra and Janes, 1992) but supplementary lighting is expensive. In strawberry, vegetative and reproductive * Corresponding author at: Horticulture Division, Malaysian Agricultural Research and Development Institute, P.O. Box 12301, GPO, 50774 Kuala Lumpur, Malaysia. 0304-4238/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SsDIO304-4238(95)00770-9
26
Y.B. Awang, J. G. Atherton / Scientia Horticulturae 62 (1995) 25-31
growth are competing sinks and both are reduced under UK summer and autumn light conditions by the relatively inexpensive method of raising irrigation salinity (Awang et al., 1993a). Retardant effects of salinity on vegetative growth could be more marked under winter low irradiance, with consequent potential benefits for fruiting. Possible interactions between irradiance and salinity are therefore examined with a view towards determining the potential of salinisation as a tool for favouring fruit development in strawberry under glass in winter.
2. Materials and methods 2.1. Experimental treatments Strawberry plants cv. ‘Rapella’ established in rockwool cubes were transferred onto rockwool growing slabs on 22 July 1992. Two plants were grown on each slab and spaced 45 cm apart. Each plot comprised a bench holding eight plants in four rockwool slabs. A total of 12 benches (plots) containing 96 plants were used in the study. During the study period, all runners generated by plants were removed. Plants were subjected to three levels of salinity in their root zones (electrical conductivities of 2.6, 5.9 and 8.6 mS cm-‘) and two daily solar radiation integrals (total irradiance) : 2.1 (shaded) and 4.9 (unshaded) MJ m-* day- ‘. The two higher electrical conductivities (ECs) were achieved by injecting concentrated NaCl solution into the basic liquid nutrient solution through an automatic drip irrigation system. Irrigation was scheduled to deliver the solutions six times per day at 06:00,09:00, 12:00, 15:00, 18:00 and 21:00 h supplying 60 ml per plant on each occasion. The composition of the basic solution was described previously (Awang et al., 1993a). Differences in the mean daily solar radiation integrals intercepted by the plants were obtained by suspending shade green polypropylene netting approximately 1 m above the uppermost leaves of the plants. This had been shown to have no effect on light quality (Meiri et al., 1982). Radiation integrals beneath the shades and above the plant canopy were continuously measured using tube solarimeters (Green and Deuchar, 1985). Treatments were randomised in a split plot design with salinity as main plot and shading as subplots, each treatments being replicated four times. 2.2. Data collection
At each harvest, all ripe fruits produced by plants (eight plants per plot) were counted and weighed and their accumulated weights were calculated. Fruit dry matter content, reducing sugars and titratable acidity from a sample of 12 fruits per plot were determined on 11 September 1992 using techniques described by Awang et al. ( 1993b). At the end of the study ( 16 September 1992)) the shoots of two plants per plot were harvested by cutting at the rockwool cube level and separated into their principal components. Leaves, crowns, inflorescences and unripe fruits (fruits over 0.5 cm in diameter) were counted and leaf areas were determined. Their dry weights were determined after drying for 48 h at 80°C in a forced draught oven.
Y.B. Awang, J.G. Ah-ton
/Scientia Horticulturae 62 (1995) 25-31
21
3. Results 3. I. Leaf and crown development
Shading reduced total leaf growth. Unshaded plants produced 26% more leaves than those grown under shaded conditions (P < 0.001) (Table 1). Similar effects of high radiation were also detected on total leaf area (P < 0.01) and leaf dry weight per plant (P < 0.001). As with leaves, initiation of crowns was also enhanced at higher radiation and this was reflected as more crowns produced under unshaded conditions (P < 0.01). Negative effects of salinity on vegetative development are also shown in Table 1. Depressed leaf initiation was seen as a significant reduction in leaf number at higher salinity (P < 0.05). Because salinised plants produced fewer and smaller leaves, total leaf area per plant was reduced. Increasing salinity significantly reduced (P < 0.05) the number of crowns per plant (Table 1). No significant interactions between shading and salinity were detected. 3.2. Floral development
Shading had a strong inhibitory effect on floral development. The number of inflorescences per plant, number of flowers per inflorescence and number of fruits per inflorescence were reduced by 32%, 15% and 20% respectively (Table 2). Increasing salinity significantly reduced inflorescence number (P < 0.05) but not the number of flowers per inflorescence or the number of fruits per inflorescence. Table 1 Effects of shading and salinity on leaf area, leaf number, crown number and principal shoot components. Each value is a mean of four replicates Parameter
Leafarea (cm*)
Leaf no.
Crown no.
LeafDW (g)
Shading
Unshaded Shaded Mean
EC (mS cm-‘)
Mean
2.6
5.9
8.6
2595 1896 2245
1889 1497 1693
1610 1254 1432
Shading
EC
Interaction
***
*
NS
***
*
NS
***
*
NS
***
*
NS
2031 1549
Unshaded Shaded Mean
52.4 39.9 46.2
39.0 36.6 37.8
37.6 29.1 33.6
43.0 35.4
Unshaded Shaded Mean
9.2 6.6 7.9
1.4 7.0 7.2
6.5 5.1 5.8
7.7 6.2
Unshaded Shaded Mean
16.3 10.7 13.5
12.5 8.5 10.5
11.1 7.4 9.2
12.1 8.9
*P
F-test
28
Y.B.Awang. J.G. Atherton /Scientia Horticulhrrae62 (1995) 25-31
Table 2 Effects of shading and salinity on flower and fruit number. Each value is a mean of four replicates Parameter
Inflorescences per plant Plowers per inflorescence Fruits per inflorescence
Shading
EC (mS cm-‘)
Mean
2.6
5.9
8.6
Unshaded Shaded Mean
19.6 13.1 16.3
15.6 10.5 13.1
13.6 9.5 11.5
16.3 11.0
Unshaded Shaded Mean
12.3 9.7 11.0
12.4 9.4 10.9
9.6 10.1 9.8
11.4 9.7
Unshaded Shaded Mean
7.3 5.6 6.4
7.4 5.0 6.2
5.6 5.6 5.6
6.8 5.4
F-test Shading
EC
Interaction
***
*
NS
***
NS
NS
***
NS
NS
*P
3.3. Fruit yield Plants grown under unshaded conditions produced significantly more fruits than those grown under shaded conditions (P < 0.001) but salinity had no significant effect. The greater number of fruits produced by unshaded plants was due to a higher number of inflorescences and more flowers setting fruits (Table 2). Total fresh weight of fruits from the unsalinised plants was significantly (P < 0.05) heavier than from the salinised plants (Table 3)) with the negative effect of salinity less pronounced at higher radiation integrals. 3.4. Fruit dry matter content, reducing sugars and titratable acidity Shading and salinity significantly affected the composition of individual fruits. Reducing radiation integrals reduced fruit dry matter (P < 0.05) whilst raising salinity increased the fruit dry matter content (P < 0.05) (Table 4). Table 3 Effects of shading and salinity on accumulated fruit number and total fresh fruit yield. Each value is a mean of four replicates Parameter
Shading
EC (mS cm-‘) 2.6
Fruits per plant
Total fresh fruit wt. per plant
5.9
Mean 8.6
Unshaded Shaded Mean
30 18 24
28 17 22
26 15 20
28 17
Unshaded Shaded Mean
228 129 178
163 105 148
144 77 111
178 104
*P
F-test Shading
EC
Interaction
***
NS
NS
**
*
*
Y.B. Awang, LG. Atherton/Scientia
Horticulturae 62 (1995) 25-31
29
Table 4 Effects of shading and salinity on dry matter content, reducing sugars and titratable acidity of strawberry fruits. Each value is a mean of four replicates Parameter
Dry matter (46)
Shading
Unshaded Shaded Mean
EC (mS cm-‘) 2.6
5.9
8.6
9.39 9.11 9.25
10.86 9.92 10.39
11.48 10.03 10.75
10.58 9.69
6.68 4.38 5.53
6.96 4.65 5.80
6.18 4.61
0.521 0.525 0.523
0.615 0.440 0.527
0.604 0.459 0.531
0.580 0.475
0.442 0.429 0.435
0.449 0.303 0.376
0.447 0.262 0.354
0.446 0.331
1.172 1.154 1.163
1.259 1.174 1.216
1.177 1.151
Reducing sugars (glucose equivalent) Unshaded 4.91 gpet 1OOg fresh wt. Shaded 4.80 Mean 4.86 g g-’ dry wt.
g per fruit
Unshaded Shaded
Unshaded Shaded
Titratable acidity (citric acid equivalent) Unshaded 1.101 Shaded 1.25 Mean 1.113
gper 1OOg fresh wt.
gg-IdrY wt.
g per fruit
Mean
Unshaded Shaded Mean
0.118 0.123 0.120
0.108 0.117 0.112
0.109 0.117 0.113
0.112 0.119
Unshaded Shaded
0.099 0.100 0.099
0.080 0.080 0.080
0.081 0.067 0.074
0.087 0.082
F-test Shading
EC
Interaction
*
*
NS
**
NS
*
***
NS
NS
***
NS
NS
NS
*
NS
*
NS
NS
NS
**
NS
*P
The effects of shading on concentration of reducing sugars were significantly (P < 0.05) dependent on salinity (Table 4). The concentration of reducing sugars per unit weight of fresh fruit increased as salinity increased in unshaded plants but not in shaded plants. The beneficial effect of salinity in the enhancement of sugar concentration therefore can only be expected under high radiation conditions. When expressed on a dry weight basis, the sugar concentration in unshaded fruits was unaffected by salinity treatment. At higher salinity and under shade, sugar levels in the fruit dry matter were significantly lower than that produced by unshaded plants. The same was true for the weight of sugar per fruit. A lower weight of sugar per fruit at higher salinity (57 DAT) was associated with the smaller fruits ( r = 0.75). Titratable acidity was also changed by the treatments. Whilst shading did not alter the acid concentration per unit fresh weight or per fruit, acidity was increased in unshaded fruits on a per unit dry weight basis (P < 0.05). This reflected again that fruits from shaded plants contain more moisture than unshaded fruits. This is confirmed by a high negative coefficient of correlation between dry matter percentage and acid per gram dry weight (r= - 0.79).
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Y.B. Awang, J.G. Atherton /Scientia Horticulturae 62 (1995) 25-31
No similar relationship existed for sugars. Salinity increased acid concentration on a fresh weight basis but not on a dry weight basis. Owing to the reduction in the fruit size by salinity, the weight of acid per fruit was significantly reduced at higher salinities (P < 0.01) . As the effect of salinity on reducing sugars was less marked for shaded fruits, the increase in the sugar:acid ratio at higher salinity was only displayed in unshaded plants. The absence of any significant effect of shading on the acid content of fresh fruit significantly (P < 0.001) decreased the sugar:acid ratio (data not shown). 3.5, Dry matter distribution Shading significantly altered shoot dry weight and dry matter distribution but there was no interaction with salinity. Lowering the radiation level from 4.9 to 2.1 MJ m-* day-’ significantly reduced total shoot dry weight from 45 to 27 g. The relative distribution of dry matter to various shoot components was significantly affected by shading treatment. Within unshaded plants, the dry matter allocation to leaves, crowns + petioles + flower stalk, and fruits were 29%, 19% and 52% compared with allocations of 33%, 20% and 48%, respectively in the shaded plants. This clearly indicates that increased radiation promoted the accumulation of the current assimilates into the fruits at the expense of leaf dry weight. The relative distribution of dry matter into leaves and fruits was negatively correlated (r = - 0.95).
4. Discussion Overall this study showed that under low irradiance conditions, increasing salinity did not stimulate fruiting in glasshouse strawberry. Depressive effects of salinity on growth and reproductive development were consistent with earlier findings (Awang et al., 1993a). At high irradiance (4.9 MJ m-* day-‘), the negative effects of salinity on fruit yield were reduced, suggesting an increase in plant salt tolerance. Similar results were reported for faba bean plants by Helal and Mengel ( 1981) and melons by Meiri et al. ( 1982). Better attraction for the current assimilates towards fruit development as well as faster CO2 assimilation (Awang and Atherton, 1994) could be responsible for the higher yield recorded in the unshaded plants (Table 3). Fruit yield was only recorded in the present study for a relatively short time (20 days after the first harvest) and yields were reduced through salinity affecting fruit weight rather than fruit number. Fruit number would probably have been reduced as well if the study had been extended as there were fewer inflorescences in salinised plants, reinforcing the view that salinisation under shaded conditions is not a commercial proposition. Long-term depressive effects of salinity on fruit yield were reported by Adams and Ho ( 1989) in tomatoes. They noticed that significant reduction in the fruit number was absent for the short-term crop (4 weeks) when salinity was increased to 8 mS cm-’ with NaCl, but a decrease of about 12% was found in a longer season crop (26 weeks). At lower radiation level, fruit dry weight was reduced at higher salinity but fresh weight was no different to that produced under unshaded conditions (Table 3). This implies that at equivalent salinity, fruits from shaded plants would have a lower concentration of dry
Y.B. Awang, J.G. AthertonIScientia Horticulturae 62 (1995) 25-31
31
matter and consequently poorer quality. This was reflected in the reduction in the concentration of reducing sugars on the fresh weight basis (Table 4). Lower concentrations of reducing sugars in the shaded fruits produced by salinised plants were also detected. This was consistent with radiation effects reported for tomatoes (Winsor and Adams, 1976) and apples (Campbell and Marini, 1992). Besides reducing assimilation, shading would have caused less carbon to be partitioned into sucrose and less to be translocated out of the leaves to the fruits (Ho, 1976; Logendra and Janes, 1992). Unlike reducing sugars, acid concentration was higher in the shaded than the unshaded treatments, providing one of the few benefits of salinisation under shaded conditions. This was consistent with the report by Robinson et al. (1983) for apple and could be associated with the higher leaf K concentration of shaded plants (Awang and Atherton, 1994). The combination of the lower reducing sugars and higher titratable acidity produced by salinised plants under shade suggests a delay to ripening, which could have commercial benefits for fruit shelf life. However, fruits produced under this environment had lower sugar:acid ratio and therefore inferior consumer quality (Awang et al., 1993b). This study clearly showed that radiation level played a important role in moderating the plant’s response to salinity. Generally, the beneficial effect of salinity in the improvement of fruit quality was only operational under high radiation level. Besides increasing EC of the nutrient solution, it is therefore important for the growers to use supplementary lighting or other means of enhancing net carbon assimilation to produce high quality fruits for the winter strawberry production. References Adams, P. and Ho, L.C., 1989. Effect of constant and fluctuating salinity on the yield, quality and calcium status of tomatoes. J. Hortic. Sci., 64: 752-732. Awang, Y.B. and Atherton, J.G., 1994. Salinity and shading effects on leaf water relations and ionic composition of strawberry plants grown on rockwool. J. Hortic. Sci., 69: 377-383. Awang, Y.B., Atherton, J.G. and Taylor, A.J., 1993a. Salinity effects on strawberry plants grown in rockwool. I. Growth and leaf water relations. J. Hortic. Sci., 68: 783-790. Awang, Y.B., Atherton, J.G. and Taylor, A.J., 1993b. Salinity effects on strawberry plants grown in rockwool. II. Fruit quality. J. Hortic. Sci., 68: 791-795. Campbell, R.J. and Marini, R.P., 1992. Light environment and time of harvest affect ‘Delicious’ apple fruit quality characteristics. J. Am. Sot. Hortic. Sci., 117: 551-557. Cockshull, K.E., 1988. The integration of plant physiology with physical changes in the greenhouse climate. Acta Hortic., 229: 113-123. Green, C.F. and Deuchar, C.N., 1985. On improved tube solarimeter construction. J. Exp. Bot., 36: 690-693. Helal, H.M. and Mengel, K., 1981. Interaction between light intensity and NaCl salinity and their effects on growth, CO2 assimilation and photosynthate conversion in young broadbeans. Plant Physiol., 67: 999-1002. Ho, L.C., 1976. The relationship between the rates of carbon transport and of photosynthesis in tomato leaves. J. Exp. Bot., 27: 87-97. Logendra, S. and Janes, H.W., 1992. Light duration effects on carbon partitioning and translocation in tomato. Sci. Hortic., 52: 19-25. Meiri, A., Hoffman, G.J., Shannon, M.C. and Poss, J.A., 1982. Salt tolerance of two muskmelon cultivars under two irradiance levels. J. Am. Sot. Hortic. Sci., 107: 1168-I 172. Robinson, T.L., Seeley, E.J. and Barritt, B.H., 1983. Effect of light environment and spurage on Delicious apple fruit size and quality. J. Am. Sot. Hortic. Sci., 108: 855-861. Winsor, W.G. and Adams, P., 1976. Changes in the composition and quality oftomato fruit throughout the season. Annu. Rep. Glasshouse Crops Res. Inst., 1975: 134-142.