EFFECTS OF EXOGENOUS SPERMIDINE ON POLYAMINE METABOLISM IN LETTUCE (LACTUCA SATIVA L.) UNDER HIGH-TEMPERATURE STRESS

The physiological mechanism by which exogenous spermidine (Spd) alleviates high-temperature stress of lettuce (Lactuca sativa L.) was explored. The effects of exogenous Spd on the content of key enzymes of PA metabolism and endogenous polyamines (PAs) in lettuce under high-temperature stress were studied with G-S59 and P-S11 as test materials. The results showed that under high-temperature stress, the plant height, dry fresh weight of shoots and dry fresh weight of underground parts were all significantly reduced, and the growth of lettuce seedlings was inhibited. Exogenous Spd could significantly alleviate the inhibition of lettuce seedling growth by high-temperature stress. The contents of free, conjugate and bound putrescine (Put) were significantly decreased in leaves of lettuce seedlings under high-temperature stress, and in contrast, the contents of Spd and spermine (Spm) in free and bound states were significantly increased. Exogenous Spd effectively increased the Spd and Put contents but inhibited the increase in Spm content. Exogenous Spd could increase the plants Spd content by increasing the activity of diamine oxidase (DAO), polyamine oxidase (PAO), s-adenosylmethionine decarboxylase (SAMDC) and ornithine decarboxylase (ODC), and increasing the amount of upstream Spd, inhibiting the production of downstream Spm and maintaining higher endogenous Spd levels. The effects were more obvious in heatresistant varieties such as P-S11. The results showed that exogenous Spd promoted the synthesis of endogenous Put and Spd, which might have great effects on the heat tolerance of lettuce seedlings.