Objectives Drought stress is one of key environmental factors of limiting the growth and productivity of plants. However, which physiological processes to make plant more tolerant to drought stress remains unclear. The purpose of this study was to investigate the special physiological responses of drought-tolerant tobacco (Nicotiana tabacum L.) adapted to drought stress.

Methods A hydroponic culture experiment was conducted to analyze effect of drought stress (mimicking moderate drought-stress condition by application of 2.5% PEG) on five tobacco cultivars with contrasting responses to drought stress in this study. We investigated plant growth, including dry weight of shoot and root, formation and elongation of lateral roots in five tobacco cultivars. Meanwhile, we analyzed net photosynthetic rate and gas exchange parameters through Li-Cor 6400 photosynthetic analysis system and leaf temperature through an infrared camera (SC 620, FLIR Systems, Inc., USA) with a spectral sensitivity ranging from 7.5 mm to 13 mm and a spatial resolution of 0.65 mrad.

Results Significant differences were observed in plant growth among five tobacco cultivars under drought stress compared with the control treatment. Compared with the control treatment, two tobacco cultivars (Dapingban and Nongda 202) kept the similar shoot and root dry weight as those under drought stress. However, three other tobacco cultivars showed the drought-inhibited shoot and root growth with the decrement ranging from 66% to 83%. Accordingly, the two tobacco cultivars were considered as drought-tolerant tobacco cultivars and the three others as drought-susceptible cultivars. Further result showed that although drought stress decreased the elongation of first- and second-order lateral roots in five tobacco cultivars, formation of first- and second-order lateral roots was increased by drought stress only in two drought-tolerant tobacco cultivars. Interestingly, although significant decrement of shoot biomass induced by drought stress wasn’t observed in two drought-tolerant tobacco cultivars, the net photosynthetic rate was decreased by drought stress relative to control treatment. The decrement of net photosynthetic rates of two drought-tolerant tobacco cultivars was less than three drought-susceptible tobacco cultivars. Furthermore, compared with the control treatment, the stomatal conductance and transpiration rate were decreased only in three drought-susceptible cultivars. After number of first-order lateral roots under the PEG treatment was trimmed to the similar level as that under the control treatment during the experimental stage, surprisingly, the shoot dry weight, net photosynthetic rate, stomatal conductance and transpiration rate of nongda202 were decreased to comparable extents as comparison with those in three drought-susceptible tobacco cultivars under the PEG treatment. This suggested that increment of lateral root formation was the main reason for two drought-tolerant tobacco cultivars to keep plant growth under drought stress relative to the control treatment.

Conclusions The results indicated that compared with the control treatment, the formation of first- and second -order lateral roots was induced by moderate drought stress in drought-tolerant tobacco cultivars, and it resulted in higher photosynthetic characteristics in drought-tolerant than in drought-susceptible tobacco cultivars under drought stress.