2013年4月6日雅思大作文真题:人类在哪些领域取得了进步

文中词汇表达灵活多样请继续保持，学术词汇积累也做的很棒；作者句法基本功不错，可适当增加从句的使用；文章层次较为清晰。

Sucrose is one of the major products of photosynthesis in plants, and is the principal form in which carbon is translocated from source leaves to sink organs. Our study showed that the tendency for sucrose accumulation to increase has been found in flag leaf due to oversupply of photosynthate at early grain filling ,while sucrose content of flag leaf declined rapidly when the supply of sucrose was tight at late grain filling. However, sucrose content of ear organs declined sharply from early grain filling. Grains weight decreased significantly by removing ear bracts (Millet et al 1980). lemma and palea have phloem connections with the pericarp (O'Brien 1985), so sucrose of ear bracts was transported to grains participating the developmental process of the grain in early grain filling stage. Direct measurements of partitioning of newly fixed 14CO2 have two-thirds of all the carbon fixed during photosynthesis in wheat (Trevanion 2002). In our study, water deficit reduced photosynthetic rate of flag leaf at early grain filling, which is possibly caused by the leaf senescence (Martinez et al. 2003), and also by feedback inhibition of photosynthate (Russo et al. 2010; Sinha et al. 2011). Our results showed that photosynthetic rate of ear was enhanced by water deficit, indicating that ear possessed higher tolerance to drought compared with leaf. T.inoue (2004) The effect of water deficit on ear photosynthetic rate showed differences between different varieties. Ear photosynthetic rate of tolerant variety increased and that of sensitive variety decreased by water deficit. Our results showed that ear photosynthetic rate of both wheat varieties was enhanced by water deficit. Ear photosynthetic rate of Xinong1043 lasted till later grain filling, which supplied sufficient photosynthate for grain filling. By contrast, Shan253 showed a high peak of ear photosynthetic rate, but the decreasing rate was faster. Sucrose phosphate synthase (SPS) is known to be the rate-limiting enzyme in sucrose synthesis and to play a dominant role in sucrose metabolism (Winter and Huber 2000, Lunn and MacRae 2003). Our study showed that SPS activity of flag leaf increased at early grain filling and decreased at later grain filling under water deficit treatment, and sucrose content showed a similar changing pattern. In ears, however, SPS activity and sucrose content of sensitive variety Shan253 were enhanced significantly by water deficit at early grain filling, whereas SPS increased but sucrose content decreased under water deficit in tolerant variety Xinong1043. Results indicate ears of wheat had the complex sucrose biosynthesis and metabolic mechanism difference from flag leaf. It has also been suggested that SS play a role in sucrose synthase (Dejardi n et al. 1997; Niedzwiedz et al. 2004). Ratio SPS/SS presented relative activity of SPS and SS. The role of SPS was gradually decreased and SS played a main role in the synthesis of sucrose at later grain filling. The mechanism of SPS regulation depends on the phosphorylation/dephosphorylation state of the protein (Pathre et al., 2000), limited photosynthesis is probably one of reasons leading to decrease of SPS activity at late grain filling. However Ratio SPS/SS of ear bracts were little changed throughout grain filling in both wheat varieties, indicating that SPS and SS had same effects on sucrose synthase in ears. In addition, According to the correlation analysis, water deficit enhanced the role of SS in sucrose synthase in ears in Shan253. The final grain yield can be considered as the balance between the supply of carbohydrate (source) and the capacity of the grains to accumulate available carbohydrates (sink) (Zhang et al. 2012). In the present study, SPS activity, SS activity and sucrose content of flag leaf were reduced by water deficit especially in late grain filling, indicating that capability of manufacturing photosynthate of flag leaf was possibly affected by water stress. However, sucrose content of grains still maintained at a high level under water deficit. The reason might be that the decrease of photosynthate of flag leaf was not a cause of reduction of grain weight, and assimilate of ear bracts was deemed to be compensation for the decrease of photosynthate. Other studies also showed that beyond an optimal level of sucrose, there is no futher increase in starch synthesis or dry mass in the grain of wheat or rice (Barloe et al. 1983; Cervantes et al. 1989). Our study showed that sucrose content of ear bracts in Shan253 were significantly higher than that in Xinong1043, but Shan253 did not show higher starch content of grains. These results are in good accordance with those reported by Post-flowering source-strength is commonly in excess to meet the demands of growing grains in cereals (Richards, 1996 and Bingham et al., 2007). According to Sanchez (2013), the ear contribution to filling grains is not only dependent on sink strength but also on genotypic variation. In the present study, sink strength of Shan253 was decreased remarkably by water stress. In conclusion, export of sucrose of ear bracts made important contribution to the growing process of the grain in early grain filling, and compensated for limited sucrose synthase of leaves under water deficit. Furthermore, SS played a role in sucrose synthase in wheat flag leaf and ear bracts.