Introduction
Strawberries (Fragaria ananassa Duch) are important horticultural crops grown for their juicy and sweet fruits that are rich in vitamins, sugars, and amino acids. Carbohydrate metabolism in strawberries and other plants determines the fruit size, the number of fruits, and even the sweetness of the fruits. In essence, a deep understanding of the principles of carbohydrate metabolism in horticultural crops is of great importance to fruit farmers. Recent advances in science have found ways of modulating the carbohydrate metabolism in plants in order to produce more yields, higher quality yield, and yield with a longer shelf-life. This paper explores the principles of carbohydrate metabolism and partitioning in strawberry plants, the influence of various biotic and abiotic factors on these, and the principles of harvesting and storing strawberry yield.
Discussion
Production and Usage
Strawberries are common fruits in all parts of the world. Temperate regions and climates favor their production most (Salami et al., 2010 p. 653). Production of the fruits occurs on both on large and small scale. On a larger scale, especially in temperate regions, strawberry trees are grown in plantations using various planting methods (Salami et al., 2010 p. 653). The mature fruits assume various colors depending on the variety of the crop. Harvesting is done once they are mature. The harvested fruits could be already ripe or can ripen while in storage. Once ripe, the fruits can be eaten raw or made into juices and other products before being consumed. As earlier alluded to, the fruit has high nutritional value and is thought to have many health benefits. Moreover, that the cultivation and production of the fruit have contributed massively to the economies of the regions and countries where its cultivation is done.
Types of Carbohydrates
Among the most important nutritional components of strawberriess are carbohydrates. The fruit also has important vitamins and amino acids in addition to the carbohydrates. The strawberry tree itself also contains numerous other compounds. The strawberry fruit has all the three classes of carbohydrates. Disaccharides, however, predominate with sucrose being the predominant carbohydrate in the fruits. The fruits also have trace maltose. Fructose and glucose are the monosaccharides in the fruit while starch and cellulose are the polysaccharides in the fruit (Souleyre et al., 2004 p.369; Marcias-Rodriguez et al., 2002 p.1317). Cellulose is the most common carbohydrate in the rest of the strawberry plant. The fruits also contain large amounts of abscisic acid, an intermediary in carbohydrate metabolism (Jia et al., 2013 p. 456).
Carbohydrate transport: sources and sinks
Synthesis of carbohydrates and all other nutrients utilized and stored by green plants occurs in the leaves in the process of photosynthesis (Yamaki, 2010 p.1). These are then transported and stored in various storage sinks including roots, stems, and fruits. As Yamaki (2010 p.1) puts it, the steps from photosynthesis to storage of carbohydrates in fruits are the synthesis of translocation sugars, loading the translocation sugars, their translocation, their unloading, membrane transport, their metabolic conversion, and their compartmentalization into vacuoles. Sucrose is the most important translocation sugar; others include raffinose, sorbitol, mannitol, and stachyose. In the case of sucrose, H+ dependent co-transporters arerequired to load the sucrose manufactured in leaves into the phloem for transportation. The transport mechanism in the phloem is water-dependent. At the storage site, sucrose is unloaded through either the plasmodesmata or by way of cellular transporters (Yamaki, 210 p. 4). In the storage site, the sugars are converted into a preferable storage form, for example, starch or fructose. The amount of carbohydrates that stored in the various sinks influences the rate of production in the carbohydrates in the leaves (Fischer et al., 2012 p.244). An increased carbohydrate store leads to a lower rate of photosynthesis principally by a reflex reduction in the number of patent stomata (Blanke, 2007 p. 17).
Photosynthesis
Strawberries and other green plants use light energy from the sun to synthesize reduced carbon compounds from water and carbon (IV) oxide (Fischer et al., 2012 p.245). A pigment in the leaves and shoots of the green plants called chlorophyll taps light energy from the sun. Chloroplasts which are most abundant in the leaves of the strawberry plant contain chlorophyll. Chlorophyll loses one electron when it absorbs one photon of light. Liberation of the electron is by the splitting of a water molecule which liberates an oxygen molecule. The electron liberated enters an electron transport chain that eventually leads to the reduction of nicotinamide adenine dinucleotide phosphate (NADP) into NADPH and the formation of adenosine triphosphate (ATP). NADPH and ATP are the energy stores of the plant cells. NADPH is used together with CO2 captured from the atmosphere to form three-carbon sugars. Most of these are combined to form glucose. The glucose is then respired to provide the plant with energy. Carbohydrates, proteins, and lipids for storage and for varied plant functions are made from the remaining glucose.
Source Feedback
As earlier alluded to, many factors including the status of the sink, biotic, and abiotic factors influence the rate of photosynthesis of strawberry plants. Genard et al., (2008 p.278) found that a decreased need of sugar in the carbohydrate sink of a strawberry plant led to an accumulation of translocation sugars including sorbitol and sucrose in the leaves and a marked decrease in the rate of photosynthesis. The stomatal patency and cellular CO2 concentration also decreased thus indicating a decrease in photosynthetic rate. An increased need for carbohydrates in the sink and a reduction in the effective photosynthetic surface area, lead to an increased translocation of sugars from the remaining source leaves. In addition, the photosynthetic rate in the leaves increased as evidenced by a gradual increase in the sugar levels after a marked initial decrease. Genard et al., (2008 p.279) thus concluded that the rate of photosynthesis in the leaves is influenced greatly by the status of the sink and the demand of carbohydrates.
Competition and partitioning between types of growth
Depending on the growth patterns and the prevailing conditions, various plant parts compete with one another for the photosynthetic assimilates and the mechanisms of carbon partitioning differ greatly and are majorly dependent on the abilities of individual plant organs to compete for limited carbohydrates (Goyal and Bishnoi, 2017 p.483).The competitiveness of a sink is dependent on its distance from the source and its activity (Goyal and Bishnoi, 2017 p.482). On the other side, the partitioning of carbohydrates is dependent on the needs of the predominant sinks. As Nishizawa and Shishindo (1998 p. 54) found out, increased demand of carbohydrates in shoots tended to deprive the tubers of carbohydrate supply. Studies have found out that the levels of glucose kept oscillating in the growth period; fruits which were growing on their own had more fructose and thus were sweeter than those growing in dense orchids which had more sorbitol.
Competition between fruit
Fruits comprise the sink with the greatest strength (Fischer et al., 2012 p.248). Completion between similar sink organs is mainly dependent on their distance from the source and their level of maturity (Goyal and Bishnoi, 2017 p.483). The sink’s level of maturity determines its level of activity. Fruits which are younger have more activities and require more carbohydrate input than the fruits which are mature and ripen. Fruits which are closest to the source leaves are also more competitive from those which are further.
Rootstocks and root pruning
Strawberry rootstocks efficiently reduce the size of the tree and increase production efficiency (Ferree et al., 1991 p.158). Root pruning is also meant to have the same effect of reducing tree size and increasing yield and productivity (Chandler et al., 1988 pp.3). Both root stocking and root pruning reduce the effectiveness of the root system in absorbing water and minerals hence reducing the general plant size, cell growth, and metabolism. In mature plants, root pruning tends to reduce the requirements of the roots as a carbohydrate sink thus allowing the fruits to predominate hence the increased productivity and yield. Moreover, in strawberries, root pruning has been known to be beneficial in controlling pests and diseases. Although root stocks can have their own limitations as Ferree et al., (1991 p. 161) indicate, their use and root pruning is the way to go in strawberry farming.
Modulation and storage of carbohydrate
Sucrose is the main transport carbohydrate in strawberries; it is translocated from the source to the fruit in the phloem while suspended in sorbitol (Fischer et al., 2014 p.247). At the fruit, sieving occurs. Inside the fruit, most of the sucrose is converted to abscisic acid. Modulation of sucrose to various storage forms is dependent on the developmental stage and the environmental factors prevailing at the time. At most times, starch is the product of modulation. With the maturity and ripening of the fruit, the remaining sucrose is broken to fructose and glucose.
Environmental Factors
Apart from influencing the growth and general productivity of fruits, environmental factors also play an important role in modulating carbohydrate metabolism and partitioning. Lemoine et al., (2013 p. 275) assert that a relative deficit in water leads to decreased growth in a plant and thus less flow of carbon into sinks. Instead, there is starch hydrolysis in leaves hence increased sucrose levels in the leaves. A deficiency of nitrogen and phosphate generally limit the growth of crops but increase photo-assimilate allocation to the tubers, especially roots. Deficiency of magnesium or potassium or both causes accumulation of carbohydrates in the leaves at the expense of fruits and leaves (Lemoine et al., 2013 p. 275). Salt stress causes the same effect that water deprivation has on fruits. Deficiency of light increases the carbohydrate requirements and thus the competitiveness of the shaded shoots for the available carbohydrates hence limiting the amount that is available for fruits. Low temperatures decrease the rate of carbohydrate loading hence reducing the carbohydrate content of fruits (Lemoine et al., 2013 p. 275). Miyoshi et al., (2017 p.56) also expose the impact of light conditions on the rate of photosynthesis and the rate with which the photo-assimilate is up-taken by various plant sinks of the strawberry.
Biennial bearing
Biennial bearing can be a problem for strawberry farmers. Although some cultivars are naturally biennial, soil fertility and weather conditions also contribute to it. The problem starts with challenges in fruit bud formation after an on-year (Jonkers, 1979 p. 307). The tree usually has exhausted most of its resources hence the off-year. To prevent this, farmers need to control the growth of their strawberry trees. Practices such as pruning, rootstock, and use of growth regulators can aid in dealing with the problem (Jonkers, 1979 p. 313).
Fruit harvest and storage
The time of the year, the color of the fruits depending on the variety, the ease of plucking them, fall of some fruits, and an increase in softness and sweetness of the fruit indicate the time for harvesting. Strawberries are harvested by twisting the fruit upwards on the shoot. After harvesting, bruised fruits which are unfit for storage are segregated from the others and used rapidly. Clean and dry wooden boxes which have good ventilation are used to store the fruits. The storage is at 30-32 degrees Fahrenheit (Shin et al., 2008 p.204). The fruits should be stored in a high humidity area and away from strong-smelling items. Humidity greatly influences the rate at which the fruits ripen or go bad (Shin et al., 2008 p.204). Unripe fruits ripen during storage.
Conclusion
Understanding the carbohydrate metabolism of strawberries is important in order to enhance the production of the crop. Understanding the metabolism will aid in choosing the best practices meant to ensure maximum and regular fruiting. Moreover, this understanding can help to overcome the challenges paused by abiotic factors in the cultivation and even to take advantage of some situations for the good of the crop. Understanding metabolism will help farmers to choose the cultivars which are best suited to the prevailing weather and soil conditions. Furthermore, it is evident that choosing good rootstocks and root pruning can increase quantity and quality of yield and aid farmers in preventing biennial bearing. Root pruning can also aid in prevention of diseases and pest infestation of the crop. Pruning of excess shoots and proper spacing to ensure good access to light can also modulate carbohydrate metabolism in strawberries to an optimum status for high yield that is of good quality.
Bibliography
Anuradha, R.K. and Bishnoi, C., 2017. Assimilate partitioning and distribution in fruit crops: A review. Journal of Pharmacognosy and Phytochemistry, 6(3), pp.479-484.
Blanke, M.M., 2007, May. Regulatory mechanisms in source sink relationships in plants–A review. In International Symposium on Source-Sink Relationships in Plants 835 (pp. 13-20).
Chandler, C.K., Miller, D.D. and Ferree, D.C., 1988. Influence of leaf removal, root pruning, and soil addition on the growth of greenhouse-grown strawberry plants. Journal of the American Society for Horticultural Science (USA).
Ferree, D.C., Myers, S.C. and Schupp, J.R., 1991, July. Root pruning and root restriction of fruit trees-current review. In I International Symposium on Training and Pruning of Fruit Trees 322 (pp. 153-166).
Fischer, G., Almanza-Merchán, P.J. and Ramírez, F., 2012. Source-sink relationships in fruit species: A review. Revista Colombiana de Ciencias Hortícolas, 6(2), pp.238-253.
Génard, M., Dauzat, J., Franck, N., Lescourret, F., Moitrier, N., Vaast, P. and Vercambre, G., 2008. Carbon allocation in fruit trees: from theory to modelling. Trees, 22(3), pp.269-282.
Jia, H., Wang, Y., Sun, M., Li, B., Han, Y., Zhao, Y., Li, X., Ding, N., Li, C., Ji, W. and Jia, W., 2013. Sucrose functions as a signal involved in the regulation of strawberry fruit development and ripening. New Phytologist, 198(2), pp.453-465.
Jonkers, H., 1979. Biennial bearing in apple and pear: a literature survey. Scientia Horticulturae, 11(4), pp.303-317.
Lemoine, R., La Camera, S., Atanassova, R., Dédaldéchamp, F., Allario, T., Pourtau, N., Bonnemain, J.L., Laloi, M., Coutos-Thévenot, P., Maurousset, L. and Faucher, M., 2013. Source-to-sink transport of sugar and regulation by environmental factors. Frontiers in Plant Science, 4.
Macías-Rodríguez, L., Quero, E. and López, M.G., 2002. Carbohydrate differences in strawberry crowns and fruit (Fragaria× ananassa) during plant development. Journal of agricultural and food chemistry, 50(11), pp.3317-3321.
Miyoshi, Y., Hidaka, T., Hidaka, K., Okayasu, T., Yasutake, D. and Kitano, M., 2017. Dynamics of Photosynthate Loading in Strawberries Affected by Light Condition on Source Leaves. Environmental Control in Biology, 55(1), pp.53-58.
Nishizawa, T. and Shishido, Y., 1998. Changes in sugar and starch concentrations of forced June-bearing strawberry plants as influenced by fruiting. Journal of the American Society for Horticultural Science, 123(1), pp.52-55.
Salami, P., Ahmadi, H. and Keyhani, A., 2010. Energy use and economic analysis of strawberry production in Sanandaj zone of Iran. Biotechnologie, Agronomie, Société et Environnement, 14(4), p.653.
Shin, Y., Ryu, J.A., Liu, R.H., Nock, J.F. and Watkins, C.B., 2008. Harvest maturity, storage temperature and relative humidity affect fruit quality, antioxidant contents and activity, and inhibition of cell proliferation of strawberry fruit. Postharvest Biology and Technology, 49(2), pp.201-209.
Souleyre, E. J., Iannetta, P. P., Ross, H. A., Hancock, R. D., Shepherd, L. V., Viola, R., … & Davies, H. V. (2004). Starch metabolism in developing strawberry (Fragaria× ananassa) fruits. Physiologia Plantarum, 121(3), 369-376.
Yamaki, S., 2010. Metabolism and accumulation of sugars translocated to fruit and their regulation. Journal of the Japanese Society for Horticultural Science, 79(1), pp.1-15.
