Morphological, physiological, and biochemical plant responses were studied in unstressed and stressed seedlings of indica rice (Oryza sativa L.) genotypes. The effect of NB medium supplemented with a 100 mM mannitol treatment, which induced drought stress conditions, was measured for relative growth rate, cell membrane stability, antioxidant enzyme activity (superoxide dismutase [SOD], catalase [CAT], and peroxidase [POD]), and total antioxidant capacity by DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2′-azinobis[3-ethylbenzothiazoline-6-sulfonic acid] diammonium salt) assays. Results of morphological and physiological factors showed two contrasting rice groups, drought-sensitive and drought-tolerant genotypes. After drought stress, the increased rate of SOD activity was lower in drought-tolerant than in drought-sensitive genotypes, but the increased rate of CAT and POD activity and total antioxidant capacity was higher in drought-tolerant than in drought-sensitive genotypes. Increasing changes in activity levels of antioxidant enzymes and total antioxidant capacity indicated more rapid free radical scavenging compared with relative growth and cell membrane stability in drought-tolerant genotypes under shortterm treatment. The trend toward change in CAT and POD antioxidant enzyme activity and total antioxidant capacity of stressed seedlings, as well as the correlation between these changes, and the morphological and physiological responses (0.96 correlation coefficient [R 2 ] between relative growth rate and CAT; 0.93 R 2 between relative growth rate and POD enzyme activity; 0.96 R 2 between relative growth rate and Trolox equivalent antioxidant capacity values) coincided with the degree of drought tolerance.

The investigations of nanotechnology with the application on agricultural products also have been few reported, especially the plant regeneration. The effects of activated charcoal and nanocarbon on the callus induction and plant regeneration of aromatic rice were studied. Activated charcoal was added into the callus induction and regeneration medium. The presence of activated charcoal in the callus induction medium (100-500 mg L −1), activated charcoal significantly reduced the percentage of the callus induction and biomass accumulation (fresh weight, dry weight and size). Whereas, the regeneration medium supplemented with 100 mg L −1 of activated charcoal showed the highest percentage of plant regeneration (61.90%) and the ratio of the number of seedlings to the number of regenerated calli (RSR; 3.06) that derived from the callus induction medium (without activated charcoal). Moreover, the induced calli derived from the callus induction medium supplemented with nanocarbon at 5 mg L −1 showed the highest percentage of callus induction (94.70%), the percentage of green spots (95.83%), the percentage of plant regeneration (60.42%) and the RSR (3.12) when transferred the calli into the regeneration medium (without nanocarbon). After that, nanocarbon was also added into the regeneration medium. The percentage of green spots (96.08%), the percentage of plant regeneration (62.75%) and the RSR (3.16) obtained from the regeneration medium supplemented with 20 mg L −1 of nanocarbon showed the highest values. This experiment showed that the optimum concentration of activated charcoal and nanocarbon had potential to enhance the callus induction and plant regeneration frequencies in tissue culture medium of aromatic rice.

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