Storage and remobilization of photoassimilates in wheat: Agronomic and morpho- physiologic aspects

Wheat is the most important source of carbohydrates in a majority of countries. It provides more nourishment for people than any other food source. Limited rainfall and drought stress occur frequently during the grain filling stage of wheat in many areas. Accumulation of photoasimilates in the stem of wheat and remobilization of theses storages could be considered as important adaptive traits to environmental stresses and are important in breeding cultivars with improved grain yield. Previous published researches on carbohydrates accumulation and remobilization in wheat were studied. The results of these articles were summarized systematically and then critically analyzed. Approaches for future works in terms of carbon accumulation and remobilization were also presented. Water-soluble carbohydrates (glucose, fructose, sucrose, and fructans) accumulate in the stem of wheat plants. The accumulation of reserves in the different internodes started near the end of extension growth. That is to say, the accumulation of reserves in the lower internodes takes place over long periods of time compared to the upper internodes. Accumulation of stem reserves, depend on environmental conditions and cultivars, continues until 10-25 days after anthesis when maximal amounts are reached. Wheat cultivars with optimum stem length and stem specific weight (stem dry weight per unit stem length) have higher potential for stem storages. Under optimal conditions, where photosynthesis takes place over long periods of time, storage of assimilates is high. In contrast, stress conditions such as drought reduce the amount of accumulated carbohydrates. Typically, remobilizations of stem reserves are started at the second half of linear grain growth when the current photosynthesis is declined. Physiological bases for remobilization initiation have not been understood well. It is probable that sucrose level as well as abscisic acid (ABA) concentration in the stem are involved in this process. The amount of remobilization in each cultivar is determined by stem reserves and remobilization efficiency. The later factor is affected, in turn, by grain number and grain weight (sink strength). That is to say, cultivars with higher stem storage do not necessarily show higher carbohydrates remobilization. Abiotic stresses (such as drought, salinity and heat stress) have pronounced effects on the amount and initiation of carbohydrate remobilization. However, wheat cultivars respond to such conditions differently. Researches on 81 Iranian wheat cultivars showed that drought stress increased stem dry matter remobilization from 2 to 45% whereas this trait was decreased from 1 to 72 percent in the remaining cultivars. Interestingly, the response of each stem segment (internode) to imposed stress conditions may be different with respect to remobilization amount. The flow of carbon (carbohydrates) to the grain from stored stem materials has been classified to pre- (All the carbon in the grain which is derived from photosynthesis prior to anthesis) and post-anthesis (All the carbon in the grain which is derived from photosynthesis after anthesis) remobilization. The contribution of stem dry matter to grain growth is not consistent and varies depends on the cultivar, environmental conditions, and grain weight changes. Clear association is not found between grain yield and remobilization. No clear relation was found between stem reserve mobilization and year of cultivar release. Weather conditions have pronounced effects on carbohydrates accumulation and its subsequent remobilization in wheat stem. Therefore, when breeding for these traits are considered, special attention should be paid to the environmental conditions. Typically, the lower internodes of wheat stem have higher potential for carbohydrates accumulation and remobilization when compared with the upper internodes (peduncle and penultimate). Clear relations were not found between grain yield and remobilization. This suggests that manipulating of this trait (remobilization) in wheat breeding program is a challenging task.