Postponed nitrogen application enhances root morphology recovery and photosynthetic characteristics of summer maize waterlogging stressed at seedling stage

Maize is one of the main summer crops in Anhui Province.However, maize production in this region is often subjected to waterlogging stress at seedling stage.Nitrogen (N) is a key nutrient that influencing growth, yield and quality of maize, but what role it plays in relieving waterlogging stress on summer maize remains unclear.Thus field experiments were carried out to determine the effect of N application on the recovery of root morphology and photosynthetic characteristics of summer maize cultivar'Longping 206'after waterlogging stress at seedling stage.Under both waterlogging and control (normal water) conditions, four treatments of different N application rates at land preparation, jointing stage, and big flare stage (N1:10:0:0;N2:7:3:0;N3:5:5:0 and N4:3:5:2) were set with a total N amount of 240 kg·hm^-2.The results showed that waterlogging stress at seedling stage significantly inhibited maize root growth.Root weight, length, surface area and diameter significantly decreased compared with those of the control.The degree of inhibiting effect on root morphology was in the order of total root length > root surface area > root diameter.Delayed N application had a compensation effect on root growth after waterlogging.Eighteen days after waterlogging, total root length of N1 to N4 treatments increased by 1.9-5.1 folds that before waterlogging.Root surface area and diameter increased by 6.3-10.3 folds and 0.7-1.0 folds, respectively.The degree of compensation effect of delayed N application on root morphology was in the order of root surface area > total root length > root diameter.Waterlogging stress at seedling stage significantly decreased leaf area index (LAI) at spinning stage by a range of 9.3%-22.5%.Compared with N1 treatment, N2, N3 and N4 treatments increased LAI within 3.2%-20.7% under waterlogging treatment.Waterlogging induced a noticeable decline in P_n, respectively by 16.1% and 28.9% compared with the control at mid grain-filling and late grain-filling stages.At late grain-filling stage, P_n decreased faster than that of control.Under waterlogging stress, non-stomatal restriction was the main factor driving the decline of photosynthetic capacity.Moreover, delayed N application improved photosynthetic capacity of ear leaf, which photosynthetic capacity was better than basal N application.In conclusion, the morphological characteristics of root and leaf along with the photosynthetic characteristics of maize under waterlogging responded positively to delayed N supplication.Therefore to improve the growth of maize in the study area (which could be affected by waterlogging at seedling stage), it was recommended to apply N at jointing and big-flare stages.