Short- and long-term responses of pepper seedlings to ABA exposure

Plants perceive environmental stress and use this information to modulate their regular pattern of development. Abscisic acid (ABA) mediates plant responses to many abiotic stresses. This study investigated the efficiency of short-versus long-term ABA exposure on critical morphological and physiological responses linked to functional changes in stomata, stomatal density, plant water status, photosynthetic activity and overall growth in bell pepper (Capsicum annuum L.). ABA was applied to pepper seedlings as foliar at 0, 0.1 or 1.0 mM for 5, 10 or 15 consecutive days. A depressive effect of ABA on net photosynthetic rate (A(co2)) and stomatal conductance (g(s)) compared to control was found after 5-day ABA-exposure at 1.0 mM, and after 15-day ABA-exposure at 0.1 mM and 1.0 mM ABA. Plants quickly resumed the photosynthetic activity since no effect of ABA was found after a longer recovery. Temporary higher leaf water potential was observed in seedlings treated with ABA compared to control. At 17 days after the beginning of the experiment, the number of stomata at adaxial and abaxial side was reduced at 1.0 mM ABA-rate compared to 0.1 mM ABA for each ABA-day of exposure. ABA-rate at 1.0 mM decreased leaf weight, number and area with a more pronounced effect as ABA-days of exposure increased. Stem dry weight was almost unaffected by ABA-rate at 0.1 mM, but 1.0 mM decreased stem weight by 30% after 15 day ABA -exposure. The increased specific root length at 1.0 mM ABA-rate after 5- or 15-day ABA exposure suggested that ABA induced the growth of thinner roots. The delayed effect of ABA-rate and ABA-days of exposure was measured for plants allowed to recover for 23 days after the last application for the longest treatment. Growth parameters were generally affected by ABA after longer recovery. The effect of ABA on modulation of pepper seedlings growth and physiology was dependent on rate and length of exposure but recovery duration was important as well. These results provide new evidences that application of ABA at a specific rate and length of exposure could eliminate the risk of stressing young seedlings to the point of physiological injury as previously reported for some hardening techniques.