Changes Of Moisture Migration And State Of Flue-cured Tobacco Upper Leaves During Curing

In order to in-depth consummate the basic theory of tobacco drying and provide support for the optimization of tobacco curing process.Tobacco upper leaves with stalks and without stalks(conventional tobacco leaves)were used as experimental materials in this study.The water transfer and water status of tobacco leaves at key temperature points during curing were studied by means of nuclear magnetic imaging,apoplastic dyeing,nuclear magnetic resonance spectroscopy and abbe refract meter.The main results are as follows:(1)The moisture of tobacco leaves was transported from leaf base midrib xylem through the nearest lateral vein to the edge of the leaf,another part of moisture migrated forward along the midrib through the next adjacent lateral vein to the edge of the leaf,and met with the moisture of previous lateral vein.In this moisture transfer behavior,moved forward in turn and finally reached the tip of the leaf.The water migration rate of the midrib xylem of the fresh tobacco leaves gradually decreased from the leaf base to the tip.The closer to the base of the leaf,the higher the water migration rate of the lateral vein xylem was.The lateral migration rate of the lateral vein xylem was higher than that of the longitudinal migration rate of the midrib xylem.Tobacco leaf attached at the morphological upside,which was benefit for the water migrated from the stalk's morphological underside via the xylem of the midrib to the lamina during curing,however,if tobacco leaf attached at the morphological downside,the water can't migrated from the stalk's morphological upside via the xylem of the midrib to the lamina.(2)The water migration rate of the midrib xylem of tobacco leaves with stalks decreased gradually during curing.The moisture migration efficiency of midrib with lateral veins was higher than that of without via lateral veins.The drying delay of the midrib and lamina increased with the increase of the stalks' length.The extreme example is that the water migrated from midrib to stalks in the later curing stage of tobacco leaves with a 2 centimeters length stalks,the water migration between stalk and midrib of tobacco leaves with stalks was completely stopped at 68 ?,which reflected that the midrib had been dried.(3)Three water status were detected in the stalks and lamina based on the nuclear magnetic resonance spectroscopy(MRS),which were bound water,immobilized water and free water.While,two water states were detected in the midrib which were immobilized water and free water.Notably,the water mobility in the stalk and midrib was stronger than that in the lamina.The drying of tobacco lamina was the process of removing water in different states one by one based on their mobility.Free water had a higher degree of freedom,so this water component was easier to be lost from the vacuole at first.Then,it was the turn of immobile water in the cytoplasm,which is probably removed by conversion to free water,and the bound water is usually difficult to remove.(4)The free water content of lamina for tobacco leaves with stalks was higher than that of the conventional leaves,but there was no significant difference between their relaxation time.The free water of stalks is the stem of the drying delay for tobacco lamina during curing with stalks.When tobacco leaves were attached at the morphological upside of stalks,the free water of stalks has a significant effect on delaying the drying of lamina during curing.However,when tobacco leaves were attached at the morphological downside of stalks,there only was a significant effect on delaying the drying of lamina in yellowing stage.What's more,there was a significant increasing effect on delaying the drying of lamina with the increasing length of the stalks.