Functional Activity of the Photosynthetic Apparatus in Tobacco and Arabidopsis Plants Exposed to Chilling Temperatures

Photosynthetic activities of a chilling-sensitive species, Nicotiana tabacum L. (cv. Samsun), and a chilling-resistant plant, Arabidopsis thaliana Heynh. (L) (ecotype Columbia), were examined in young seedlings exposed to low temperatures. Functional parameters were determined after 1- to 6-day exposure of tobacco and arabidopsis plants at 8 and 2°C, respectively. In tobacco leaves sampled by the end of the experiment, the chlorophyll content decreased by 30%, the ratio of variable to maximum Chl a fluorescence (Fᵥ/Fₘ) decreased by 5%, and the rate of net (apparent) photosynthesis diminished almost twofold. The concentration of soluble sugars in tobacco leaves increased by 30% approaching 130 mg/g dry wt. In arabidopsis, unlike tobacco, the content and proportions of photosynthetic pigments, as well as the Fᵥ/Fₘ ratio changed insignificantly upon cooling, whereas net photosynthetic rates decreased to a lesser extent (by a factor of 1.6). The content of sugars in arabidopsis leaves increased fourfold by the end of the low-temperature treatment and reached 90 mg/g dry wt. Thus, the photosynthetic apparatus in chill-sensitive and cold-resistant plants responded differently to chilling temperatures. The data provide evidence that the photosynthetic apparatus in arabidopsis is well preserved upon chilling and can function effectively at cold-hardening temperatures. The thylakoid processes and the reactions of photosynthetic carbon metabolism in leaves of heat-loving tobacco plants are more sensitive to chilling temperatures and undergo changes in a more or less coordinated manner, helping chloroplasts to avoid the excessive generation of reactive oxygen species.