Drought is the Most Significant Aspect Restraining the Development and Efficiency of Tomato and all other Vegetables.
Muhammad Munir1, Rashad Mukhtar Balal1, Muhammad Adnan Shahid1,3, Muhammad Zubair1 and Mujahid Ali2
- Department of Horticulture, University College of Agriculture, University of Sargodha
- Institute of Horticultural Sciences, University of Agriculture, Faisalabad
- Horticultural Sciences Department, Institute of Food & Agricultural Sciences, University of Florida, Florida, USA.
Among all the vegetables, Tomato (Solanum lycopersicum L.) is common cultivated crop all over the world. It belongs to Solanaceae family and is grown as an annual crop in most regions of the world. It was originated in the South American Andes and its use as a food originated in Mexico and spread throughout the world following the Spanish colonization of the Americas (Dias, 2014). Tomato was introduced in Indo-Pak subcontinent from Europe about 200 years ago (Ali, 1979). As compared to other countries of South-east Asia average tomato yield in Pakistan is quite low (11.05 tons per hectare), total area under cultivation is 44.46 thousand hectares with total production of 491.4 thousand tons (GOP, 2016).
Tomato has wide range of varieties grown as greenhouses and kitchen garden crop in cooler climatic regions of the world. The tomato is well known for its edible fruits, with thousands of cultivars having been selected with varying fruit types and for optimum growth in differing growing conditions. Cultivated tomatoes vary in size, from tom berries, about 5 mm in diameter, through cherry tomatoes, about the same 1-2 cm size as the wild tomato, up to beefsteak tomatoes 10 cm or more in diameter. The most widely grown commercial tomatoes tend to be in the 5-6 cm diameter range. Most cultivars produce red fruit, but a number of cultivars with yellow, orange, pink, purple, green, black, or white fruit are also available. Multicolored and striped fruit can also be quite striking. Tomatoes grown for canning and sauces are often elongated, 7-9 cm long and 4-5 cm diameter; they are known as plum tomatoes and have a lower water content. Roma-type tomatoes are important cultivars in the Sacramento Valley (AMS, 1991, 2007).
Tomatoes can be determinate or indeterminate type of growth habit. The determinate or bush, types bear a full crop all at once and top off at a specific height; they are often good choices for container growing. Determinate types are preferred by commercial growers who wish to harvest a whole field at one time or home growers interested in canning. Indeterminate varieties develop into vines that never top off and continue producing until killed by frost. They are preferred by home growers and local-market farmers who want ripe fruit throughout the season (Goncalveset al., 2009). Most modern tomato cultivars are smooth surfaced, but some older tomato cultivars and most modern beefsteaks often show pronounced ribbing, a feature that may have been common to virtually all pre-Columbian cultivars. While virtually all commercial tomato varieties are red, some cultivars, especially heirlooms which produce fruit in other colors, including green, yellow, orange, pink, black, brown, ivory, white and purple. Such fruits are not widely available in grocery stores, nor are their seedlings available in typical nurseries, but they can be bought as seed (Hillhouse, 2005).
Unfavorable environmental conditions or stress are the principal cause of crop losses worldwide with an average reduction in production of most vegetables by more than 50%. The brutality of environmental stress imposed on vegetable crops depends on climatic changes. Moreover, increasing temperatures, flooding, drought and salinity are the major restrictive factors in increasing and sustaining vegetable productivity (Boyer, 1982; Brayet al., 2000).The hazardous effect of environmental stresses on crop plants depends on the length and severity of the stress and plant developmental stage (Bray, 2002).
The response of plants to counteract the effect of stress is being done by regulating various morphological or biochemical mechanisms. Until now the scientific data depicts that tomato and chilies are most seriously affected by abiotic stresses among the vegetables. There is a huge need to do more scientific research on all the other vegetables which are affected by brutal effects of the environment extremes. Among the abiotic stresses drought and salinity are of paramount importance for crop growth and yield (Capiatiet al., 2006; Nono-womdim, 2001). The world food security is highly disturbed due to change in rain fall pattern and unpredictable drought which also serves as catalyst of the great famines of the past (CGIAR, 2003). Vegetables normally contain more than 90% water, hence are succulent in nature (AVRDC, 1990). Thus quality and production of vegetables is greatly affected by water supply. Water deficit conditions considerably decrease vegetable yield. The concentration of solute increases around the root zone under drought conditions leading to diffusion of water out of plant cells. This direct to an augment of the solute ratio in plant cells thereby mitigating the water potential, disturbing cells and membranes processes such as photosynthesis. A drop of soil water content, turgor loss and reduced leaf water potential, closing of stomata and lessen cell enlargement and growth rate are the typical symptoms induced by drought stress. Sever water stress may cause reduction of photosynthesis, disturbance of metabolism and consequently results in death of the plant. The magnitude of the drought stress highly depends on duration, intensity and timing of the drought (Jaimezet al., 2000).
The drought stress also leads to saline environment for the crops. The increased soil salinity greatly hampered vegetable production particularly in irrigated crop land belts which provides a share of about 40% of the world’s food. The production of many vegetables and agricultural crops is highly sensitive to excessive salinity. High evapo-transpiration due to dry and hot weather leads to considerable water loss thus leaving salt around the plant roots which obstruct with the plant’s capacity to uptake water. Physiological effects of salinity are the preliminary water deficient condition that occurs because of comparatively high solute ratios in the soil, which causes ion toxicity occurring from distorted K+/Na+ ratios and direct to enhance the Cl- and Na+ that are injurious to crop plants (Yamaguchi and Blumwald, 2005). Plant exposure salinity is depicted in turgor loss, reduction of growth, wilting, epinasty and curling of leaf, reduced photosynthesis, changes in respiration, loss of cellular integrity, tissue necrosis and finally the plant death (Jones, 1986; Cheeseman, 1988).Until now the scientific data depicts that tomato and chilies are most seriously affected by drought stress among the vegetables (Capiatiet al., 2006).
The frequency and brutality of drought will surely increase in the coming future due to global warming which will lead to a promising decline in total food production. Predicted increase of 1.5-5.8 °C in temperatures by 2100 will create severe problems for agriculture produce (Rosenzweiget al., 2001). At the same time, gradually growing human population which may reach to 9 billion by the end of 2050 demands an upsurge in food supplies. As the desertification further increases due to continuous loss of arable area the situation will be more devastating in the coming days (IPCC, 2007). Recent agro-breeding programs are being tested by the scientists to augment tolerance in plants against water deficit conditions by introducing the new transgenic plants with some novel genes against drought stress (Lu et al., 2013). An effective and promising way to enhance agricultural productivity and tolerance against water deficit and drought conditions is the introduction and development of new transgenic varieties through plants breeding and genetic engineering. However, the intricacy of abiotic stress tolerance mechanisms makes the job of announcing new tolerant varieties very problematic and also a very long and time consuming procedure (Wahid et al., 2007).
The scope of plant genetic improvement through the manipulation of available genetic variability under stress and non-stress condition is still equally believed by all plant scientists. For enhancement of drought tolerance potential in a crop either through selection or breeding basic important criteria is presence of genetic variability for stress resilience (Dias, 2010). The knowledge of genetic variability is useful tool in order to maintain, evaluate and utilize germplasm effectively under control and water stress conditions (Dias, 2014). The material from diverse geographical origin of the crop species can help to ensure conservation of co-adapted gene complexes, because genetically heterogeneous populations produce more and stable yield than genetically homogenous lines. The application of genetic variation can also be manipulated either selecting superior genotypes or to be utilized as parents for the development of future cultivars through hybridization (Goncalveset al., 2009).
