Effect of salt stress on photosynthetic properties of sugar beet under green house and field conditions

Determination of chlorophyll content and chlorophyll fluorescence has the potential needed to analyze the efficiency of photosynthesis in plants against environmental stresses, especially salinity, and its simplicity facilitates stress evaluation. In order to study the response of different photosynthetic characters to salinity stress at different sugar beet growth stages, two separate experiments were designed and conducted. Six sugar beet genotypes were evaluated under two treatments including non-stress (control) and salinity with 16 dsm-1 electrical conductivity in the greenhouse and field conditions. Samples were collected at four and eight- leaf (establishment) stages in the greenhouse and at leaf development (16-leaf) and physiological maturity (40- leaf) stages in the field. Photosystem (II) efficiency, evapotranspiration, stomatal conductance, photosynthesis, respiration, and chlorophyll a and b content were measured at sampling stages. The highest impact of salinity on photosynthetic traits at various growth stages was observed at the second growth stage (8- 10- leaf or establishment stage). Leaf transpiration rate, stomatal conductance, and total chlorophyll content had significant correlation with root and sugar yield. During the early stage of sugar beet growth in the salinity treatment, with reduction in initial fluorescence and without influence on maximum fluorescence, the photosystem (II) efficiency increased but during the establishment stage, reduction in all chlorophyll fluorescence parameters resulted in significant decrease in photosystem (II) efficiency which caused damage to photosynthetic system and reduction in total content of chlorophyll a and b. Salinity tolerance of genotype 7219 was accompanied by a decrease in transpiration and stomatal conductance but genotypes BP Karaj and 7233- p.29*MSC2, displayed tolerance to salinity by transpiration reduction and chlorophyll fluorescence increase. Genotype 452 was susceptible to salinity stress and showed no salinity tolerance mechanism. Finally, it was shown that in addition to genotype, different growth stages are effective on salinity stress tolerance. In genotype selection, physiological mechanisms of stress tolerance in various growth stages are also important.