Non-optimal growth temperatures and antioxidants in the leaves of Sorghum bicolor (L.) moench .2. Short-term acclimation

In pursuing previous studies on long-term acclimation to non-optimal, non-stressing growth temperatures (Badiani et al., 1993 b; Paolacci et al., 1997; Fusari et al., in press), the foliar antioxidant status and the photosynthetic capacity were evaluated in two sorghum [Sorghum bicolor (L.) Moench.] cultivars of different agroclimatic provenance, namely Aralba and ICSV 112, which were grown at the near-optimal temperature of 27 +/- 0.3 degrees C and then gradually (1 degrees C h(-1)) and transiently (up to 120 h) exposed to suboptimal, 17 +/- 0.4 degrees C, or supraoptimal, 37 +/- 0.1 degrees C, temperatures, under moderate light intensity and ad libitum water and mineral nutrition. Comparative analysis of short-term and long-term responses to nonoptimal growth temperatures suggested that: i) even realistic, i.e. limited and gradual, upward or downward shifts from the normative growth temperature, incapable of causing evident stress symptoms, might per se enhance the formation of reactive oxygen species whose effect, albeit not drastic, appear to be more noxious during the early stages of the exposure; ii) this disthermia-driven oxidative burst triggers almost immediate and extensive changes in all of the major antioxidant metabolites and scavenging enzymes. This could be aimed at preparing plant tissues in case moderate disthermia flows into authentic temperature stress; iii) certain short-term antioxidant changes persist beyond the relief of the disthermic regimes; and iv) however, in case the exposure to non-optimal growth temperatures becomes chronic, longterm adjustment processes take place, consisting of increases in protective pigments, ascorbic acid and glutathione, but without the involvement of antioxidant enzymes. Such a strategy might be aimed at keeping disthermia-dependent oxidative stress under control at the lowest possible price in terms of metabolic resources.