Photosynthesis Product Allocation and Yield in Sweet Potato in Response to Different Late-Season Irrigation Levels

Soil water deficit is an important factor affecting the source-sink balance of sweet potato during its late-season growth, but water regulation during this period has not been well studied. Therefore, the aim of this study was to determine the appropriate irrigation level in late-season sweet potato, and the effect of irrigation level on accumulation and allocation of photosynthetic products. In this study, two yield-based field trials (2021-2022) were conducted in which five late-season irrigation levels set according to the crop evapotranspiration rate were tested (T-0: non-irrigation, T-1: 33% ETc, T-2: 75% ETc, T-3: 100% ETc, T-4: 125% ETc). The effects of the different irrigation levels on photosynthetic physiological indexes, C-13 transfer allocation, water use efficiency (WUE), water productivity (WP), and the yield and economic benefit of sweet potato were studied. The results showed that late-season irrigation significantly increased the total chlorophyll content and net photosynthetic rate of functional leaves, in addition to promoting the accumulation of above-ground-source organic biomass (p < 0.05). The rate of C-13 allocation, maximum accumulation rate (V-max), and average accumulation rate (V-mean) of dry matter in storage root were significantly higher under T-2 irrigation than under the other treatments (p < 0.05). This suggests that both non-irrigation (T-0) and over-irrigation (T-4) were not conducive to the transfer and allocation of photosynthetic products to storage roots in late-season sweet potato. However, moderate irrigation (T-2) effectively promoted the source-sink balance, enhanced the source photosynthetic rate and stimulated the sink activity, such that more photosynthate was allocated to the storage sink. The results also showed that T-2 irrigation treatments significantly increased yield, WUE and WP compared to T-0 and T-4 (p < 0.05), suggesting that moderate irrigation (T-2) can significantly promote the potential of storage root production and field productivity. There was a close relationship between economic benefit and marketable sweet potato yield, and both were highest under T-2 (p < 0.05), increasing by 36.1% and 59.9% compared with T-0 over the two-year study period. In conclusion, irrigation of late-season sweet potato with 75% evapotranspiration (T-2) can improve both the yield and production potential. Together, these results support the use of late-season water management in the production of sweet potato.