MATERIALS AND METHODS

Five short-day strawberry (Fragaria x ananassa Duch) cultivars (Sweet Charlie, Redlans Hope, Kabarla, Festival and Camarosa) were grown in a raised bed on clay-loam soil in 2004-2005 growing season in the research field of The Black Sea Agricultural Research Institute, Samsun, Turkey (41° 21’ N Latitude, 36° 15’ E Longitude, 4 m elevation). Chemical and physical properties of the experimental soils are given in Table 1. Experimental design was randomized complete block with four replications. All plots received 40 t ha^-1 farmyard manure prior to the planting.

In organic system, based on its chemical composition, wood ash was incorporated into the soil at a rate of equal to 150 kg P ha^-1. Green manure (Vicia sativa L.) sown in 2002 autumn was incorporated into the soil at the flowering stage in the spring of 2003. Straw was used as mulch material. After planting, aqueous solution of poultry manure was applied with irrigation water via trickle irrigation at once per week, terminating at the end of September in 2004 and 2005 growing seasons. Chemical composition of poultry manure and wood ash are given in Table 2.

In conventional system, prior to the planting 150 kg P ha^-1 as triple superphosphate (42% P₂O₅) and 200 kg K ha^-1 as potassium sulphate (50% K₂O) were applied as basal dressings. After planting, total of 139.1 kg N ha^-1 (200 mg L^-1) as ammonium nitrate (33% N) was supplied with irrigation water through drip irrigation system once in a week from April to the end of September in 2004 and 2005 growing seasons. Black plastic mulch was used in this growing system.

Daughter plants were sown on a raised bed on July 18, 2003. Each plot had 20 plants at 30x32 cm inter row and plant spacing. Leaf samples were taken for determination of total N and CHL reading on June 01, 2004. The leaf chlorophyll was measured using Minolta SPAD-502 chlorophyll-meter, total of 30 chlorophyll meter readings were recorded on ten leaves of each plot and 3 readings on each leaf. Readings were taken from the same point/position of the leaves as much as possible. The same leaves were dried at 70°C for 48 h and prepared for chemical analysis. Leaf N was determined by micro-kjeldahl method (Chapman and Pratt, 1961).

RESULTS AND DISCUSSION

Flowering and harvest dates: In conventional system, strawberry cultivars flowered a few days earlier than those of organic system (Table 3). First harvest was two to seven days earlier depending on cultivars in conventional system in both years. Harvest duration was one (Kabarla) or two weeks (Camarosa) longer in conventional than organic system in 2004. Whereas in 2005 the last harvest was made on the same day (30 June) for all cultivars except for Redlans Hope which ended 10 days before (20 June). Early flowering resulted in early fruit set in conventional system which was attributed to higher solubility and easy availability of inorganic fertilizer used, especially nitrogen, in this system. Organic fertilizers used in organic system need time to be available to the plants. This is one of the most important challenges for organic agriculture (Muramoto et al., 2004; Finckh et al., 2006). Another reason for early flowering and early fruit setting may be utilization of plastic mulch in conventional system. Findings of Kaikas and Luik (2002) and Sing et al. (2007) support this assumption. There have been many studies reporting positive effects of plastic mulch on the nutrients availability and nutrient uptake of the plant through increasing soil water and temperature (Birkeland et al., 2002; Sharma and Sharma, 2003; Sonsteby et al., 2004; Singh et al., 2005; Berglund et al., 2006).

Fruit weight, total soluble solid and titretable acidity: There were significant differences among cultivars on average fruit weight in organic and conventional system in both years (Table 4). Redlans Hope had the highest mean fruit weight under conventional and organic system in both years, followed by Camarosa and Kabarla. When taken into account the mean values of two years, Redlans Hope was the cultivar having the highest mean fruit weight under both growing conditions, followed by Camarosa and Kabarla.

Table 3:	First flowering, first and last harvest dates of cultivars in 2004 and 2005

Table 4:	Mean fruit weight, total soluble solid and titretable acidity of strawberry cultivars grown organically and conventionally
*: p<0.05, **: p<0.01, NS: Non significant, + and - in difference column indicate high and low values of the means, respectively, in organic plot when compared to conventional, Values with the same letter(s) are not significantly different at the 5% level

According to t-test, the difference between growing systems in terms of mean fruit weight of each cultivar was not significant.

There were significant differences in TSS content of fruit among cultivars grown organically or conventionally (Table 4). Sweet Charlie and Festival cultivars had the highest TSS content under conventional system in both years. Under organic system, TSS content of Sweet Charlie and Festival cultivars showed variability, namely the values were lower in the second year when compared to the first year. This might be resulted from susceptibility of these cultivars to nutritional differences in the soil. According to mean values, although there were significant differences in TSS content among cultivars under two growing conditions, differences between systems in relation to this trait of each cultivar was not significant. TSS content is a cultivar-specific character. While some cultivars had higher TSS, the others had the lower TSS under the same growing conditions.

Titretable acidity of cultivars was significantly influenced by growing system (Table 4). It can be concluded from that TA was significantly affected by fertilizer management. TA of the cultivars under organic system was lower when compared to the conventional system. This was attributed to differences in nutritional power of the soil under organic and conventional system. Although the same amounts of fertilizers were supplied with different fertilizer sources for organic and conventional systems, differences in fertilizer solubility resulted in different concentrations in soil. Of all the fertilizer, potassium has the most important effect on fruit quality, especially TA of fruit (Locascio et al., 1990; Güler, 1997). In present study, potassium was supplied by farm yard manure (0.30% K), poultry manure (4.55% K) and wood ash (7.22%) in organic system (Table 2), however lower availability of K derived from these sources might lead to lower TA of strawberry fruit. Plastic mulch might be another factor influencing TA through positive effect on soil humidity and temperature which have crucial influence on many soil factors and plant nutrient uptake mechanism (Singh et al., 2007).

Table 5:	Yield of strawberry cultivars grown organic and conventional system in 2004 and 2005 (g plant-1)
**: p<0.01, NS: Non significant, - in the last column indicates difference of means between organic and conventional system according to t-test, Values with the same letter(s) are not significantly different at the 5% level

Table 6:	Monthly yield of strawberry cultivars grown under organic and conventional system in 2004 and 2005 (g plant-1)
Values with the same letter(s) are not significantly different at the 5% level

According to the mean values, TA of Sweet Charlie, Camarosa and Redlans Hope grown organically were significantly different than those of grown conventionally.

Yield: In both years, strawberry yield was higher in conventional than organic system (Table 5). Most of the fruits from both growing systems were picked in May during the experiment (Table 6). Differences between the growing systems in terms of yield were significant for Kabarla, Sweet Charlie and Camarosa. In the first year, of all the cultivars Kabarla and Camarosa gave the highest yield, while differences were significant for only Festival in the second year. In the first year yield, differences varied from 3% (Festival) to 39% (Sweet Charlie). Whereas, in the second year differences in yield varied from 22% (Sweet Charlie and Camarosa) to 50% (Festival). Small differences in yield in the first year might be explained by differences in soil nutrient level of each year (Table 1). According to total yield of two years, there were significant differences between two growing systems, ranging from 21% (Camarosa) to 29% (Sweet Charlie) (Table 7). This finding is in agreement with the result of Gliessman et al. (1996) who reported that organic system yields of strawberry California were depressed relative to conventional system yields by 39% in the first, 30% in the second and 28% in the third year. Although each cultivar showed different performance in present study, Kabarla and Camarosa were the less depressed cultivars. Working with seven strawberry cultivars, Daugaard (2001) reported that nutrient level is a cultivar-specific property and this must be taken into account in evaluating the nutrient demands of strawberries.

Table 7:	Total yield of strawberry cultivars (g plant-1)
**: p<0.01, in the last column indicates difference, *: p<0.05, **: p<0.01, in the yield of the same cultivar grown organically and conventionally according to t-test, Values with the same letter(s) are not significantly different at the 5% level

Bull et al. (2001) tested twelve commercial strawberry cultivars in California and found that Aromas, Pacific and Seascape were the best performed cultivars under organic growing conditions. Black plastic mulch might be another factor leading to higher yield in conventional system. This result is compatible with the results of Kaikas and Luik (2002) and Singh et al. (2007).

Leaf nitrogen and chlorophyll: There were significant differences in leaf CHL reading values and leaf N among cultivars tested (Table 8). Kabarla and Redlans Hope had the highest CHL reading values and leaf N under both growing systems. Significant growing system by cultivar interaction for leaf CHL reading value indicated that cultivars had different response under different growing systems. Muramoto et al. (2003) tested five strawberry cultivars under organic system and found that there were significant differences in leaf N and petiole NO₃-N among cultivars.

Table 8:	Leaf CHL reading value and leaf N (%) of five strawberry cultivars under organic and conventional system conditions
Cultivar x growing system interaction for chlorophyll p<0.01, for leaf nitrogen p>0.05, **: p<0.01, Values with the same letter(s) are not significantly different at the 5% level

Among the cultivars Aromas and Seascape had the highest leaf N and petiole NO₃-N and had significant correlation between total leaf N and total fruit yield (p<0.001) and stated that total nitrogen in leaf blade may provide a better indicator for N status in organic strawberries. We found significant correlation between leaf CHL and leaf N (r = 0.551, p<0.001). Leaf N level has significant effect on yield and fruit quality. Therefore, leaf N should be monitored frequently in order to maintain it above or near to the sufficiency (2.8% N) level (Ulrich et al., 1980). Leaf N can be estimated by using leaf CHL reading values measured by SPAD chlorophyll meter (Leaf N = -0.32 + 0.079 CHL).

Diseases and pests: During the experiment plants were observed in relation to diseases and pests especially for Tetranychus sp. and grey mould (Botrytis cinerea pers). Tetranychus sp. population was under the economic threshold value (15 numbers per leaf) in both of growing systems, so no chemicals were used. We encountered some predators during observation. The low level of Tetranychus sp. population might be attributed to these predators.

In the first year, intensive grey mould was seen in both growing systems due to heavy rain continuing nine days without interval. No chemical was used against grey mould (Botrytis cinerea pers), it was controlled by cultural method, ie removing infected plants from the experimental area. In the second year, grey mould was not a serious problem under both growing systems.

CONCLUSIONS

This study revealed that Kabarla and Camarosa performed well not only under conventional but also organic conditions. These cultivars can successfully be grown organically under the same conditions of this study. Slow release of the nutrients from organic fertilizer is the main cause of lower yield of organically grown strawberry. Titretable acidity of fruit was strongly affected by fertilizer management and it was lower under organic growing conditions when compared to the conventional system. High correlation between leaf N and CHL indicated that leaf N can be monitored by measuring leaf CHL throughout the growing season.

ACKNOWLEDGMENT

The authors are grateful to Hayriye Ibrikci (Cukurova University, Agricultural Faculty, Adana, Turkey) for her contribution to the manuscript.