Impact of exogenous caffeine on morphological,biochemical, and ultrastructural characteristics of <Emphasis Type="Italic">Nicotiana tabacum</Emphasis>

Caffeine (1,3,7-trimethylxanthine) is purine alkaloid, and it is secondary metabolite produced naturally in plants. It plays a crucial role in defense and stress tolerance. A hydroponic experiment was carried out to study the morphological, biochemical, and ultrastructural effects of caffeine treatment on seedlings of tobacco (Nicotiana tabacum L. cv. Turkish). The plants were grown in a growth chamber for 14 d in Hoagland’s nutrient solution supplemented with 0 (control), 25, 50, 100, 1000, and 5000 μM caffeine. Shoot heights as well as root lengths significantly decreased in the plants treated with 1000 and 5000 μM caffeine. Total protein and sugar content in leaves increased significantly in the 5000 μM caffeine-treated plants. Moreover, electron microscopic analysis shows that in plants exposed to 1000 and 5000 μM caffeine, mesophyll cells possessed enlarged chloroplasts with disrupted thylakoid membranes associated with large starch grains and plastoglobules. Scanning electron micrographs exhibit that the vast majority of stomata in the plants treated with 1000 and 5000 μM were closed. In contrast, the 25 and 50 μM caffeine-treated plants showed an enhanced growth and no signs of injury.