Comparison of N uptake and internal use efficiency in two tobacco varieties

To explain the observation in field experiments that tobacco variety CB-1 was more nitrogen(N)-efficient than K326, the influence of two N levels on growth, N uptake and N flow within plants of the two tobacco varieties was studied. Xylem sap from the upper and lower leaves of both tobacco varieties cultured in quartz sand was collected by application of pressure to the root system. CB-1 took up more N with smaller roots at both high(HN, 10 mmol L-1) and low(LN, 1 mmol L-1) N levels, and built up more new tissues in upper leaves especially at LN level,than K326. Both varieties showed luxury N uptake, and CB-1 accumulated significantly less NO-3in new tissues than K326, when grown at the HN level. At both N levels, the amount of xylem-transported N and phloem-cycled N from shoot to root in K326 was greater than those in CB-1, indicating higher N use efficiency in CB-1 shoots than in K326 shoots. The major nitrogenous compound in the xylem sap was NO-3irrespective of N level and variety. Low N supply did not cause more NO-3reduction in the root. The results indicated that the N-efficient tobacco variety CB-1 was more efficient in both N uptake by smaller roots and N utilization in shoots, especially when grown at the LN level.

Nicotine Concentration in Leaves of Flue-cured Tobacco Plants as Affected by Removal of the Shoot Apex and Lateral Buds

It is believed that the nicotine concentration in tobacco is closely correlated with the amount of nitrogen （N） supplied. On the other hand, N uptake mainly occurs at the early growth stage, whereas nicotine concentration increases at the late growth stage, especially after removing the shoot apex. To identify the causes of the increased nicotine concentration in tobacco plants, and to compare the effects of different ways of mechanical wounding on nicotine concentration, field experiments were carried out in Fuzhou, Fujian Province in 2003 and 2004. Excision of the shoot apex had almost no influence on N content in the plant; however, it caused dramatic increases in nicotine concentration in leaves, especially in the middle and upper leaves. An additional increase of the nicotine concentration was obtained by removal of axillary buds. The wounding caused by routine leaf harvests, however, did not change the leaf nicotine concentration, and neither did reducing leaf harvest times. The present results revealed no direct relationship between N supply and nicotine concentration in tobacco leaves, and indicate that not all kinds of mechanical wounding were capable of stimulating nicotine synthesis in tobacco plants. Since nicotine production is highly dependent on the removal of apical meristems and hence on the major sources of auxin in the plant, and application of 1-naphthylacetic acid onto the cut surface of the stem after removing the shoot apex markedly decreased the nicotine concentration in different leaves and the total nicotine content in the plant, the results suggest that decreased auxin supply caused by removal of the shoot apex as a kind of mechanical wounding might regulate nicotine synthesis in the roots of tobacco plants.

PM_(2.5)and water-soluble inorganic ion concentrations decreased faster in urban than rural areas in China

We investigated variations of PM_(2.5)and water-soluble inorganic ions chemical characteristics at nine urban and rural sites in China using ground-based observations.From 2015 to 2019,mean PM_(2.5)concentration across all sites decreased by 41.9μg/m~3with a decline of 46%at urban sites and 28%at rural sites,where secondary inorganic aerosol(SIAs)contributed to 21%(urban sites)and 17%(rural sites)of the decreased PM_(2.5).SIAs concentrations underwent a decline at urban locations,while sulfate(SO_(4)^(2–)),nitrate(NO_(3)^(–)),and ammonium(NH_(4)^(+))decreased by 49.5%,31.3%and 31.6%,respectively.However,only SO_(4)^(2–)decreased at rural sites,NO_(3)^(–)increased by 21%and NH_(4)^(+)decreased slightly.Those changes contributed to an overall SIAs increase in 2019.Higher molar ratios of NO_(3)^(–)to SO_(4)^(2–)and NH_(4)^(+)to SO_(4)^(2–)were observed at urban sites than rural sites,being highest in the heavily polluted days.Mean molar ratios of NH_(3)/NH_xwere higher in 2019 than 2015 at both urban and rural sites,implying increasing NH_xremained as free NH_(3).Our observations indicated a slower transition from sulfate-driven to nitrate-driven aerosol pollution and less efficient control of NO_(x)than SO_(2)related aerosol formation in rural regions than urban regions.Moreover,the common factor at urban and rural sites appears to be a combination of lower SO_(4)^(2–)levels and an increasing fraction of NO_(3)^(–)to PM_(2.5)under NH_(4)^(+)-rich conditions.Our findings imply that synchronous reduction in NO_(x)and NH_(3)emissions especially rural areas would be effective to mitigate NO_(3)^(–)-driven aerosol pollution.