Mannitol-producing tobacco exposed to varying levels of water, light, temperature and paraquat

Transgenic mannitol-producing (+mtlD) and wild-type (-mtlD) tobacco plants were exposed to water deficit, varying light intensities, low temperatures, and paraquat applications to test whether mannitol was involved in protection against abiotic stresses. In the water deficit experiment, -mtlD and +mtlD plants were fully irrigated [100% evapotranspiration (ET)] or received 25% ET for 40 d. Water deficit reduced the relative water content (RWC) of both types of plant starting on day 22 and the total stem length (TSL) of -mtlD 25% ET plants after 11 d, whereas TSL of +mtlD 25% ET was reduced only after 34 d. After 30 days of water deficit, a higher percentage of mature foliage was retained by +mtlD 25% ET plants compared to -mtlD 25% ET plants. The mannitol-1-phosphate dehydrogenase activity of +mtlD plants was not affected by water deficit. The photosynthetic rates of +mtlD and -mtlD plants were measured at PPFD levels ranging from 0 to >2,000 mmol m-2 s-1. No differences in quantum yield, saturation and compensation points, or dark respiration were observed between the +mtlD and -mtlD plants. Exposing the leaves of +mtlD and -mtlD plants to 0°C for 24 h caused significant injury to cell membranes and was similar in both plant types. The application of 0.2 mM paraquat onto expanding -mtlD leaves showed a higher percentage of necrotic leaf area compared to +mtlD leaves. The amount of mannitol produced by +mtlD plants could not provide significant osmotic protection or increases in photosynthesis, whereas it may provide a specific system to protect cells from free radical-induced damage.