Exogenous spermidine enhances the photosynthesis and ultrastructure of lettuce seedlings under high-temperature stress

The effects of foliar application of 1mM exogenous spermidine (Spd) on the growth, photosynthetic pigment content, chlorophyll synthesis precursor and key enzyme activity, photosynthetic characteristics, porosity change, the ultrastructure of chloroplasts and mitochondria were studied with sensitive high temperature variety 'No.3 Beisansheng' as test material.The results showed that under 35 degrees C/30 degrees C high-temperature stress, lettuce seedling growth was inhibited, the contents of chlorophyll a and b were reduced by 27.78% and 28.57% compared with the treatment CK(control), respectively, and the transformation process from porphobilinogen (PBG) to uroporphyrinogen III (urogen III) was promoted, thus blocking chlorophyll synthesis. The stomatal conductance was increased, the net photosynthetic rate was reduced by 45.15%, and the structures of chloroplasts and mitochondria were damaged. Spraying exogenous spermine on the leaves of lettuce seedlings increased the contents of chlorophyll a and b, respectively, decreased the transformation process from PBG to urogen III, and suppressed the excess accumulation of protoporphyrin IX (Proto IX), avoiding oxidative damage in the chloroplast. The stomatal density and relative opening degree was reduced, and the net photosynthetic rate was increased by 22.96%. The chloroplast structure maintained stability, and damage to mitochondria was reduced. We can conclude that, through maintaining the stability of the chloroplast and mitochondrial structure, exogenous Spd inhibits stomatal opening and density, thus improving the net photosynthetic rate and biomass in lettuce. Exogenous Spd effectively alleviates damage to lettuce at a high-temperature.