| In order to study the effect of nitrogen on midrib characteristics and curing characteristics of flue-cured tobacco leaves,and the relationship between midrib characteristics and curing characteristics of flue-cured tobacco leaves,three nitrogen application treatments were set in Enshi,Hubei Province in 2019,using flue-cured tobacco variety Yunyan 87 as the experimental material:75 kg/hm2(low nitrogen),105 kg/hm2(normal)and 135 kg/hm2(high nitrogen).We investigated the water transport rate of midrib of fresh tobacco.The changes of moisture,chemical composition and physical properties of the midrib of tobacco leaves,and the characteristics of water loss and yellowing of tobacco leaves and their physiological changes;And the morphological changes of tobacco leaves during curing.The main research results are as follows:(1)Water transport rate of vascular bundle of midrib of fresh tobacco with different nitrogen application rates:with the increase of nitrogen application rate,the time for dye solution to reach the tip of midrib decreased.Among them,the tobacco leaves with normal nitrogen application rate decreased about 9.73 s compared with the tobacco leaves with low nitrogen application rate,and the tobacco leaves with high nitrogen application rate decreased about 9.15 s compared with that with normal nitrogen application rate.The water transport rate of midrib vascular bundle in low nitrogen tobacco leaves,with the value of 0.174 cm/s,was the lowest.The water transport rate of midrib vascular bundle of tobacco leaves with normal nitrogen application rate,with the value of 0.185 cm/s,was in the middle.The water transport rate of midrib vascular bundle in high-nitrogen tobacco leaves,with the value of 0.209cm/s,was the highest.(2)Changes of midrib of tobacco leaf with different nitrogen application rates in the dark box test:the hardness and lignin content of midrib of tobacco leaf with different nitrogen application rates showed a decreasing trend.The water content of the midrib decreased slowly,from 24 h on,the water loss was relatively faster,and the water loss rate was high nitrogen>normal>low nitrogen.The ratio of bound water to free water in midrib showed the trend of low nitrogen>normal>high nitrogen,with the increasing post-harvest time,the ratio of bound water to free water in midrib of tobacco showed an increasing trend.(3)Midrib characteristics of tobacco leaves with different nitrogen application rates during curing:the midrib hardness of tobacco leaves with different nitrogen application rates increased firstly and then decreased.The midrib lignin content of tobacco leaves decreased rapidly with different nitrogen application rates.Change of midrib water with different nitrogen application rates showed:the water loss of midrib was slow in the yellowing stage,and water loss mainly occurred in the leaf-drying stage.The ratio of bound water to free water increased from fresh tobacco leaves to leaf-drying stage.Tobacco leaves with low nitrogen application rate:the water loss of the midrib was less,and the water loss rate was 0.07%/h at the fixed color stage.The ratio of bound water to free water was higher,the hardness of midrib was smaller and changed slowly,and the lignin content was lower.Tobacco leaves with normal nitrogen application rate:the water loss of the midrib was middle,with the rate of 0.22%/h at the leaf-drying stage;the bound water/free water ratio,hardness and lignin content were in the middle;tobacco leaves with high nitrogen application rate:the water loss of the midrib was more,with the rate of 0.35%/h at the leaf-drying stage;the ratio of bound water to free water was small,the hardness of midrib was larger and the rate of change was faster,and the lignin content was higher.(4)Dark box test on curing characteristics of tobacco leaves with different nitrogen application rates:under dark box condition,both the yellowing and browning indices showed the trend of low nitrogen tobacco>normal nitrogen tobacco>high nitrogen tobacco,indicating that low nitrogen tobacco had easier curing potential and poorer endurable curing potential.The easy and endurable curing potential of tobacco leaves with normal nitrogen application rate were in the middle.The high nitrogen tobacco leaves had harder curing potential and better endurable curing potential.The water loss rate of low nitrogen tobacco leaves was slower than their yellowing rate,while the water loss rate of tobacco leaves with normal and high nitrogen application rates was faster than their yellowing rate.The coordination between water loss and yellowing of normal nitrogen tobacco leaves was the best,while that of high nitrogen tobacco leaves was relatively the worst.(5)The curing characteristics of tobacco leaves with different nitrogen application rates during curing:1)water loss and yellowing characteristics of tobacco leaves:during the curing process,the water loss of tobacco leaves with low nitrogen was more,and the water loss rate at yellowing stage was 0.20%/h;The chlorophyll reduction was 89.47%,and the degradation rate was 1.24%/h,which indicated that the easy curing potential was good,the activity of polyphenol oxidase was high,and the endurable curing potential was poorer.The water loss of tobacco leaves with normal nitrogen application rate was medium,and the water loss rate was 0.13%/h at yellowing stage.The chlorophyll degradation amount was 87.35%,and the degradation rate was 1.21%/h,indicating that the easy curing potential was medium.The water loss rate of high nitrogen tobacco leaves was 0.08%/h at yellowing stage.The chlorophyll degradation amount was 85.64%,and the degradation rate was 1.19%/h,which indicated that the easy curing potential was poor,the activity of polyphenol oxidase was low,and the endurable curing potential was better.2)Antioxidant characteristics of tobacco leaves:during the curing process,antioxidant enzyme activities of low-nitrogen tobacco leaves were lower,malondialdehyde content was higher,and chemical composition coordination of tobacco leaves after curing was poorer.The activity of antioxidant enzymes was higher and the content of malondialdehyde was lower in tobacco leaves with normal nitrogen application rate.The chemical composition coordination of tobacco leaves was better.The activity of antioxidant enzyme and the content of malondialdehyde in the leaves of high-nitrogen tobacco leaves were in the middle,and the coordination of chemical components in the cured tobacco leaves was poorer.(6)Correlation analysis of midrib characteristics of tobacco leaves with different nitrogen application levels and leaf curing characteristics:correlation analysis showed that the midrib indices of tobacco leaves were significantly correlated with that of leaf curing characteristics with different nitrogen application rates.Nitrogen had a significant influence on tobacco leaf quality and its physiology,the tobacco leaves with 105 kg/hm2 application rate had better tobacco midrib development and water retention capability.In the curing process,the midrib had better capability of adjust water loss,making better coordination between water loss and yellowing of tobacco leaves and greater endurable curing potential. |
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