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a College of Agric., Yangzhou Univ., Yangzhou, Jiangsu, People's Republic of China
b Dep. of Biology, Hong Kong Baptist Univ., Kowloon Tong, Hong Kong, People's Republic of China
c Agric. Bureau of Liangyungang, Jiangsu, People's Republic of China
jzhang{at}net1.hkbu.edu.hk
Heavy use of N fertilizers delays plant senescence in wheat (Triticum aestivum L.) and results in slow grain filling and a low harvest index. This study investigated whether senescence is enhanced by a controlled soil drying during the late grain-filling stage and whether such an enhancement can lead to better remobilization of reserved C to the grains. Two wheat cultivars were raised in pots and one was grown in the field. Two levels of nitrogen, either normal (NN) or high (HN) amounts, were applied at heading. Controlled soil drying was imposed 9 d after anthesis until maturity. Leaf water potential and conductance for the soil-drying treatments were lower during the day but completely recovered by the early morning. Photosynthetic rate and chlorophyll content in the flag leaves declined more quickly in the soil-drying treatments than in the well-watered ones, indicating that soil drying enhanced senescence. Nonstructural carbohydrate in the stem and sheath at maturity was greatly reduced, and the partitioning of fixed C from flag leaves into the grains and the final harvest index were significantly increased by soil drying under both nitrogen treatments for the three cultivars. Soil drying shortened the grain-filling period but grain-filling rate was substantially increased by all the soil drying treatments except one NN treatment with severe soil drying in the pot experiment. Soil drying actually increased the grain yield at HN in both experiments. We conclude that senescence induced by controlled soil drying during grain filling can promote the remobilization of prestored assimilates to the grains, accelerate grain filling, and improve yield in cases where senescence is unfavorably delayed by heavy use of nitrogen.
Abbreviations: DAA, days after anthesis • DAS, days after sowing • gleaf, leaf conductance • HN, high nitrogen treatment • MD, mild soil drying • NN, normal nitrogen treatment • NSC, nonstructural carbohydrate • 
leaf, leaf water potential • soil, soil water potential • PAR, photosynthetically active radiation • Pr, photosynthetic rate • SD, severe soil drying • SE, standard error • WS, water stressed • WW, well watered
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