The Source-sink Relationship In Potato Yield Formation And The Regulation Of Water And Nitrogen On It

The source-sink relationship determines crop yield. This has been widely investigated in cereals such as wheat and rice, but fewer studies have been conducted in root and tuber crops such as potato(Solanum tuberosum L.). As one of the major food crops in the world, potato is a typical kind of tuber crops. Its harvest organ tuber is a kind of modification of stems. It is a vegetative organ and not a reproductive organ like seed in cereal crops. Thus, understanding the source-sink relationship in potato is very important for improving potato potential yield and understanding the source-sink relationships in tuber crops. Water and nitrogen are two key factors affected crop production, and are also two main factors that can be regulated by farmers. There are lots of studies about the regulatory mechanisms of water and nitrogen on source-sink relationships in cereals, but in potato these studies are few. Therefore, using virus-free plantlets of the Atlantic potato cultivar, a pot experiment was conducted to study the influence of water and nitrogen on the tuber yield, source-sink relationship, carbon and nitrogen metabolism relationship, water and nitrogen use efficiency in potato. The results would help to determine the source-sink relationship in potato and understand the regulatory mechanism of water and nitrogen on it. The main results were as follows.1. The source-sink relationship in potato yield formation. By designing different water and nitrogen levels, the changes of tuber yield, whole plant biomass, harvest index, source capacity, sink capacity, water and nitrogen use efficiency in potato during the tuber bulking stage were studied. The results showed that, plant source capacities were small under all water and nitrogen levels at the end of the experiment, but at this time, sink tuber still had a large potential capacity to take in assimilates. It suggests that the source-supplied assimilates were not sufficient enough to meet the demands of sink growth. Thus, we concluded that, unlike cereals, potato yield is more likely to be source-limited than sink-limited during the tuber bulking stage. Different water and nitrogen levels affected tuber yield by influencing plant source capacity. Well-watered(90% of field capacity) and sufficient nitrogen(0.2 g N/kg soil) conditions increased yield mainly by enhancing the source capacity. In addition, among all treatments, tuber yield and total biomass were the highest under the well-watered and sufficient nitrogen conditions, but harvest index were not the highest under these conditions. It also suggests that well-watered and sufficient nitrogen conditions increased the amount of total biomass plant accumulated, but not increased the amount of biomass partitioned to tubers(harvest index) to increase tuber yield, and then increase water and nitrogen use efficiency. Therefore, these results suggest that plant source capacity is the main limiting factor in potato production. Increasing net photosynthetic rate, total leaf area and leaf life span to increase plant source capacity is more crucial to improve potato potential yield.2. The carbon-nitrogen metabolism coordination and source-sink relationship in potato. The coordination of carbon and nitrogen metabolism determines crop yield. By designing different water and nitrogen levels, the changes of carbon-nitrogen metabolism coordination, namely carbon/nitrogen ratio in potato during the tuber bulking stage were studied. The results showed that, carbon/nitrogen ratio in potato leaves were negatively correlated to the chlorophyll content. Water stress increased the carbon/nitrogen ratio, and sufficient nitrogen level maintained it at a low level for a long time. To further understand the effects of the coordination of carbon and nitrogen metabolism on the source-sink relationship in potato, we designed splitting application of nitrogen under the same nitrogen application rate to study the changes of carbon/nitrogen ratio. The results showed that, applied a small amount of nitrogen(50 mg N/kg soil) at transplanting and added a lot of nitrogen(200 mg N/kg soil) at tuber formation not only helped to decrease the carbon/nitrogen ratio before tuber bulking, but also helped to decrease it during the tuber bulking stage. These decreases coordinated the relationship between carbon and nitrogen metabolism during the whole growing period in potato, prolonged leaf life time, improved the photosynthetic rate, maintained plant source capacity a high level for a long time, and then increased the whole plant dry matter accumulation, ultimately increased tuber yield. It suggests that different water and nitrogen levels and split application of nitrogen affected the carbon-nitrogen metabolism coordination to affect plant source capacity, and then influenced tuber yield. Reasonable water and nitrogen management coordinated the relationship between carbon and nitrogen metabolism, maintained plant source capacity at a high level, thereby increased tuber yield, water and nitrogen use efficiency.