| Field experiments were conducted using split plot design with different nitrogen application levels,i.e.,0 kg/ha(U0),150 kg/ha(U150),and 300 kg/ha(U300)]urea for the main plots,and three sugarcane varieties,i.e.,ROC22,GT11 and GXB9 for the subplots.The plant cane was grown in 2015,and ratoon crops were kept in 2016 and 2017 continuously.The purpose of this study was to investigate the effects of different nitrogen application levels on the ratoon crop growth of different sugarcane varieties through comparing the agronomic traits,physiological and biochemical indexes,and cane yield and quality,in order to provide a reference for rational application of nitrogen fertilizer in sugarcane ratoon crops.The results were as follows.(1)Agronomic traits:The ratoon shooting rate was higher in GXB9 than ROC22 and GT11 under no N application condition;higher in GXB9 and ROC22 than GT11 under N application condition,and higher in the U150 and U300 treatments than the UO treatment while that in GT11 showed U300>U150>UO significantly each other,it also showed the same trend for tillering rate.The tillering rate is poor in ROC22 even under U300 condition.The incidence of smut was the lowest in GXB9,and highest in ROC22.GT11 showed low incidence of smut disease under high nitrogen condition.For ROC22,however,the incidence of smut was getting higher with the increase of N fertilizer application rate.The SPAD in leaves increased with increasing nitrogen levels for all the varieties,and the dead heart rate did not show significant difference from different nitrogen levels.(2)Cane yield and its components:The millable stalks/plot and cane yield showed U300>U150>UO in the ratoon crops for all the varieties.Under UO condition,GXB9 showed the highest stalk length,ROC22 showed the highest in U150 treatment,and lowest in UO treatment.Stalk diameter did not show significant difference in the treatments with different N application rates.(3)Cane quality:In most cases,there were no apparent differences in cane quality traits between U300 and U150 treatments,but higher than UO treatment.ROC22 showed the highest sucrose content in cane,and the highest sucrose content in cane showed the trend U150>U300>UO in different N level treatment.(4)Accumulation and distribution of dry matter:At tillering stage,there was no significant difference between U150 and U300 treatments in stem dry matter of GT11 while significantly increased with the increasing N levels in GXB9 and ROC22.At elongation stage,dry matter mainly accumulated in stalk gradually,and also accumulated in leaf but slowly.The accumulation of dry matter gradually showed significant differences in different N level treatment,which was U300>U150>U0.(5)Nitrogen metabolism:On different nitrogen levels,the GPT activity in leaf +1 of ratoon cane was overall higher in GT11 and GXB9 than ROC22,higher in U150 treatment than U300 and UO treatments for both GXB9 and ROC22.The GOT activity in leaf + 1 was higher in U150 treatment than U300 treatment for GT11 and GXB9,while the highest in U300 and then UO treatment for ROC22.The NR activity in leaf +1 showed differences in different sugarcane varieties,the highest in GXB9,and that in ROC22 increased with N levels at late stage,while that in GT11 showed relatively smaller differences on different N levels.The GS activity in leaf+1 showed no differences on different N levels when it showed maximum values in ROC22 and GT11,and that in GXB9 was similar on different N levels but higher than that in the other varieties.The nitrate nitrogen content in leaves was higher in U300 treatment for GXB9 and ROPC22 while higher in U150 treatment for GT11.The soluble protein content in leaves was higher in GXB9 than ROC22 and GT11,and that in GXB9 and GT11 showed significantly higher in U150 treatment while that in ROC22 differed at different stages.The total amino acid content in leaves showed higher in U150 and U300 treatments than UO treatment for GXB9 and ROC22,while that was higher in U150 and UO treatments for GT11. |
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