NONOPTIMAL 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 an d the photosynthetic capacity were evaluated in two sorghum [Sorghum b icolor (L.) Moench.] cultivars of different agroclimatic provenance, n amely Aralba and ICSV 112, which were grown at the near-optimal temper ature of 27 +/- 0.3 degrees C and then gradually (1 degrees C h(-1)) a nd transiently (up to 120 h) exposed to suboptimal, 17 +/- 0.4 degrees C, or supraoptimal, 37 +/- 0.1 degrees C, temperatures, under moderat e light intensity and ad libitum water and mineral nutrition. Comparat ive analysis of short-term and long-term responses to nonoptimal growt h temperatures suggested that: i) even realistic, i.e. limited and gra dual, upward or downward shifts from the normative growth temperature, incapable of causing evident stress symptoms, might per se enhance th e formation of reactive oxygen species whose effect, albeit not drasti c, appear to be more noxious during the early stages of the exposure; ii) this disthermia-driven oxidative burst triggers almost immediate a nd extensive changes in all of the major antioxidant metabolites and s cavenging enzymes. This could be aimed at preparing plant tissues in c ase moderate disthermia flows into authentic temperature stress; iii) certain short-term antioxidant changes persist beyond the relief of th e disthermic regimes; and iv) however, in case the exposure to non-opt imal growth temperatures becomes chronic, longterm adjustment processe s take place, consisting of increases in protective pigments, ascorbic acid and glutathione, but without the involvement of antioxidant enzy mes. Such a strategy might be aimed at keeping disthermia-dependent ox idative stress under control at the lowest possible price in terms of metabolic resources.