**9. Source-sink relations**

The term source-sink relations points to the balance between supplying plant organs and receiving ones in respect to any kind of an essential resource, such as water, nutrients, carbohydrates, or other secondary metabolites. Strawberry plants are simple and small, lacking organs or tissues specialized in reserve accumulation, such as tuber, trunk or others. Therefore, source-sink relations in the strawberry plant are fragile; when demands exceed supply, a competition between sink organs would immediately occur, with crucial consequences on further plant development, including fruit yield and quality [49–51].

The case of out-of-season, early yielding strawberries, is particularly interesting in respect of the carbohydrate balance. Naturally, under their original temperate climate, young strawberry plants develop a considerable vegetative biomass during the early spring, which would adequately support the carbohydrate requirements of the emerging reproductive phase. Nevertheless, when strawberry seedlings are manipulated to early fruit bearing, the carbohydrate balance might be extremely brittle. While possibly adequate to successfully support initial reproductive development (flower bud differentiation, initiation, and even fruit growth and development), the foliar biomass existing at that stage might be too small to reinforce further plant development. The typically declining temperature and the descending light availability during autumn both limit carbon assimilation, and hence, further restrict plant growth. In extreme cases, crop development and production might be restrained for a long period. Under less extreme situations, the fruit yield might considerably fluctuate over time with the alternating vegetative and reproductive flushes, with negative consequences on the marketing.

The strawberry growing season of 2020–2021 at Ramat Negev provides an example of the interactions between the temperature regime and the current fruit yield, fruit size, and TSS during the season (**Figure 8**). Compared to the temperature means of the recent decade (**Figure 6**), November 2020 was considerably cold, particularly at night (**Figure 8A**). However, the rest of the winter (December–January) was consistently warmer than the average of the former decade, with average maximum and minimum temperatures above 20°C and 7°C, respectively (**Figures 6** and **8A**). In contrast, February and March 2021 were quite normal. The relatively cold November repressed fruit growth and ripening, giving rise to lower early yielding in the five cultivars examined. Significant differences between cultivars occurred during December, with 'Aya' and 'Daniel' displaying two peaks while the other three cultivars exhibited moderate to low yields. In January, 'Rocky' and '415' emerged to yield, while 'Aya', 'Daniel' and '6050' produced low fruit yield. From February and later on, all five cultivars showed increasing yield levels that followed the rising but fluctuating temperature (**Figure 8A** and **B**).

*Winter Strawberries Production in Greenhouse Soilless Culture under an Arid Climate… DOI: http://dx.doi.org/10.5772/intechopen.104390*

#### **Figure 8.**

*Yield parameters of five early-yielding strawberry cultivars during the 2020–2021 season at Ramat Negev, Israel. Daily maximum and minimum temperatures from 31 October 2020 until 27 March 2021 (A); weekly fruit yield (B); mean fruit size (C); the average concentration of total soluble solids in the fruit (D).*

Fruit size (**Figure 8C**) was relatively small until late December, ranging from 15 to 30 g per fruit. It steadily increased during January, and then gradually declined until the end of the season. Beyond being a varietal trait, fruit size negatively corresponds

with the number of fruit per plant. Thus, fruit size is suppressed at the beginning of the fruiting season by the large number of fruit relative to the current plant canopy size. During January, fruit size peaked with the depletion in the number of fruit, however, it decreased again when plants' productivity was restored from February and later on.

Fruit TSS (**Figure 8d**) negatively reflected the changes in fruit yield (**Figure 8b**) during the season, but also expressed the current plant capacity. Thus, TSS was quite high until late December, as long as the fruit yield was low; it surged at the beginning of January as a result of a relatively long period of low yield *vs*. increased plant canopy and capacity, but then sharply declined in response to the rising fruit size and, later on, fruit yield. At the end of the season, TSS was partially restored with the warming weather and increasing plant capacity. Unfortunately, the frequency of TSS measurements did not allow a direct analysis of its relationships with other yield parameters (**Figure 8**).

There are several possible practices that might assist avoiding imbalanced sourcesink relations. The most critical one is planting well-established seedlings that already have significantly developed root system, and a considerable number of healthy leaves. Suitable cultivars should display a good balance between extensive vigor and adequate tendency to reproductive development; excessive vigor or ample reproductive potential should be avoided. Heating [52] and additional artificial light [53] can be considered when necessary and economically feasible. In addition, a temporary application of fortified nitrogen nutrition, especially in the beginning of the season before night temperatures drop, was found to support the vegetative growth, thus increasing the leaves/fruit ratio (data not shown).

Recent research efforts have opened deep insight into mechanisms governing sinksource relations and carbohydrate translocation in strawberry [54, 55]. Nonetheless, the effects of leaf canopy manipulations may vary considerably between cultivars, production systems, and with varying time and duration of application [51]. Consequently, recurrent efforts will be needed to fit an appropriate solution to each combination of cultivar, climate, and growing system.
