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After potato, tomato (Lycopersicon esculentum) is considered the most valuable vegetable crop grown globally. While the net land area that tomato is cultivated on has remained the same, production has starkly increased over the last decade. Currently, approximately 200 million tonnes of tomato are produced on sites that total 3.7 million hectares in size. China, USA, India and Turkey are the top tomato producing countries. While Belgium (4,996,000 Hg/ha) and the Netherlands (4,835,973 Hg/ha) have the highest fruit yield per hectare. Tomatoes normally grow in tropical, subtropical and warm temperate climates which facilitate longer growing seasons such as the climates present in China, India and North America. Climate change and an increasing world population has created a volatile agricultural sector and an impending strain on food security. The agricultural sector is fixated on increasing production and improving the health of crop varieties through the application of good husbandry and scientific solutions. Many researchers, such as Ainsworth and Ort (2010) have forecasted the deficit in yield as a result of climatic changes.
On top of being an economically and nutritionally important crop globally, tomato possesses many characteristics which make it an ideal model plant species for experiments. It has well documented genetic information and it has a short reproductive period. While Arabidopsis thaliana is the standard plant model, there are several developmental traits found in tomato that are not present in A. thaliana such as; photoperiod independent sympodial flowering, climate dependent fruit formation, compound leaves, mycorrhizal roots and glandular trichomes. Furthermore, numerous varieties of cultivars exist on the market that can cater for a vast range of research such as the dwarf cultivar Micro Tom (.
Reproductive development tends to be affected more by high temperatures than vegetative development in nearly all crop varieties. Even slight increases in temperature can impact on fruit yield, therefore, decreases are not just related to periods of extreme heat stress. It has been found that the female reproductive organs are more resilient to such periods than their male counterparts. Day / night temperatures of 26/ 20oC can alter tomato fruit development while higher temperatures of 38/27oC during the day/night can cause fruit abortion. Losses of up to 70% can be seen in areas affected by Summers with unusually high temperatures. High temperature induced reductions in fruit set can be related to damage during the development of reproductive tissues, an imbalance in plant hormones and reduced photosynthetic parameters. Photosynthetic mechanisms which can be damaged include the ratio between chlorophyll and carotenoid levels, hydraulic forces which transport nutrients and water around the plant architecture.
One of the main stress hormones and regulators that increase due to sub optimal conditions is abscisic acid (ABA). Stress response genes appear to be upregulated as a response of signalling by ABA in order to induce stress tolerance, while ABA also provides feedback information for sugar signalling pathways. Singh and Sawhney (1998) observed the correlation between abnormal developments in the anther and increased ABA levels within the stamen leading to plant sterility. It is now commonly accepted that ABA is a stress responsive signal which causes pollen to be aborted due to withholding sugars from being transported within the anther. This can be seen in the work of De Storme and Geelen (2014) who noted the decreases in the transport of sucrose and damaged sucrose metabolism led to high pollen abortion and large accumulations of ABA in wheat and rice anthers during stressful conditions.
Heat sensitive cultivars tend to have less viable pollen grains being created when exposed to higher temperatures than heat tolerant cultivars. The main factor that influences pollen development during stressful periods is that carbohydrate metabolism and transportation of proline is disrupted. This disruption causes a deficit in starch and sugar accumulation in mature pollen grains and thus an increase in pollen sterility. When heat stressed microspores of mature tomatoes were examined via transcriptome profiling, the relationship between heat stress and heat shock proteins was revealed.
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