Effect of salinity stress on photosynthesis and vegetative sink in tobacco plants

The effects of salinity stress on vegetative growth in tobacco (Nicotiana tabacum) plants were examined in terms of the source-sink relationship. The effect of salinity stress on plant growth was continuously measured using a micromorphometric technique under greenhouse conditions. This technique is less cumbersome and more precise than conventional techniques used to measure plant growth under salinity conditions. The effect of salinity stress on photosynthetic rate, C-13 partitioning and sugar and Na+ concentrations was also measured. Salinity stress was imposed by irrigating the plants with NaCl solution at 50 and 100 mmol L-1. Stem diameters of the control plants started to decrease around 07:00 Am and reached a minimum at 03:00 Pm. Salinity stress did not change the diurnal pattern of stem diameter dynamics, but increased the amplitude of daytime shrinking, and consequently stem growth estimated by the difference in stem diameter at night was inhibited. The effect of salinity stress on stems, a major sink, appeared within a few hours of treatment, whereas the effects on photosynthetic rate were observed after a lag period of 3-4 days of salt application. Salinity stress also reduced C-13 export rate, but increased leaf sugar concentration and Na concentration in all plant parts. These results suggest that salinity impairs sink activity earlier than source activity. Based on the results of this study, the utilization of micromorphometric techniques for early diagnosis of growth in salt-prone environments is recommended.