Evaluation of relationship between leaf temperature and salinity tolerance and pattern of shoot sodium accumulation in wheat cultivars

The understanding of salt tolerant mechanisms is crucial for development of varieties with high salinity tolerance. Crop growth is being limited by the osmotic effect of the salt around the root and the toxic effect of the salt within the plant. The osmotic stress has a greater effect on growth rate than the ionic stress. The osmotic stress immediately reduces stomatal conductance with the onset of salinity and is one of the initial causes of a decline in crop growth. As leaf temperature correlate with stomatal conductance, this can be used to asses osmotic stress tolerance in plants. Much of the recent experiment to improve the salt tolerance in wheat has focused on sodium exclusion in plant tissue as a criterion for salinity tolerance but in some work there was no significant relationship between sodium exclusion and salt tolerance. However, pattern of sodium accumulation in shoot is different in wheat cultivar through time. The aim of this study was to evaluate the potential use of leaf temperature in screening for salt tolerance and pattern of sodium accumulation through time and its relationship with some physiological characters and salt tolerance.

In this greenhouse experiment, two bread (Arg and Tajan) and a durum (Behrang) wheat cultivars were evaluated in terms of leaf temperature and patterns of Na+ accumulation under three salinity levels (Control, 100 and 200 mM NaCl). Treatments were imposed when the leaf 2 was fully expanded. Leaf temperature of plants were acquired in the greenhouse 6 days after imposing the salt treatments. Plants were harvested after salt treatment every 15 days for 60 days and sodium concentration was measured at each harvest. Chlorophyll content, K+ and Na+ root and shoot and shoot dry weight, 45 days after salt treatment, were measured.

Shoot and root Na+ concentration was increased in response to increasing salinity but in roots this increase was greater than shoots. Salt stress decreased shoot dry weight in bread wheat cultivars (32%) and durum wheat (63%). In the highest salinity level, after 45 days exposure to salt, shoot K+/Na+ ratio was reduced in all genotypes to a similar extent (about 60%) and in this level of salinity higher reduction in chlorophyll content occurred in Behrang (75%) cultivar. Shoot K+/Na+ ratio showed the most reaction to salinity. Differences in leaf Na+ concentration between two bread wheat cultivars was more obvious in 45 days after salt treatments but in other growth stages were less obvious. Results showed that differences between sodium concentrations in two bread wheat cultivar through time, was remarkable 45 days after treatments. At 200 mM NaCl, Arg showed the lowest and non significant increased in leaf temperature (0.87◦ C) but in Tajan (1.74◦ C) and Behrang (4.30◦ C) this increased were significant. Salt concentration of 100 mM NaCl had no effect on leaf temperature in cultivars.

Different patterns of Na+ accumulation may be exist in wheat cultivars through time. Differences in sodium transport rates from roots to shoots may cause different patterns of sodium accumulation through time in wheat. This preferential in early growth stages, stomatal factors is more important than chlorophyll content and as leaf temperature is largely a function of stomatal conductance, this can be used to assess osmotic stress tolerance. Thermography could be acquire accurate technique to distinguish differences in salt tolerance in wheat cultivars. The shoot K+/Na+ ratio, cannot be used as a reliable indicator of salt tolerance in wheat. Greater tissue tolerance in wheat cultivars was associated with prolonged retention of chlorophyll in leaves and this resulted in less reduction in shoot growth.