Abstract: Abstract：The objective of this study was to examine osmotic potential，membrane fatty acid changes
and antioxidant enzyme protection in two Kentucky bluegrass（Poa pratensis）cultivars（heat-tolerant
‘Midnight’and heat-sensitive‘Brilliant’）in response to heat stress（35/30 ℃，day/night）. The results
showed that under heat stress，‘Midnight’demonstrated significantly higher turf apparent quality than
‘Brilliant’. With the extension of heat stress，osmotic potential in‘Midnight’constantly decreased and
remained unchanged in‘Brilliant’except for an initial drop. At the same time，‘Midnight’maintained a
higher relative water content than‘Brilliant’. Linolenic acid（18︰3）content of the two cultivars gradually
decreased and linoleic acid（18︰2）and palmic acid（16︰0）gradually increased，accordingly，there was
an increase in levels of saturated fatty acid. However，the higher 18︰3 and palmitoleic acid（16︰1）
contents，correspondingly，the lower 16︰0 and stearic acid（18︰0）contents in‘Midnight’were observed
as compared to‘Brilliant’，therefore，‘Midnight’had a higher double bond index（DBI）than‘Brilliant’.
MDA content of‘Brilliant’increased quickly after 10 d of heat stress and was 1.6 times higher than that of
‘Midnight’at the end of heat stress. Furthermore，SOD activity was significantly higher and electrolyte
leakage was significantly lower in‘Midnight’than in‘Brilliant’. These results suggested that a better
osmotic adjustment，a higher SOD activity and a less MDA accumulation from membrane unsaturated fatty
acid composition in‘Midnight’provided effective protection for membrane stability against heat stress. As
a result，water deficit and oxidative stress was relieved. Meanwhile，‘Midnight’had an ability to avoiding
a dramatic change of lipid saturation level. These mechanisms conferred‘Midnight’a better heat tolerance
compared to‘Brilliant’.

Key words: Kentucky bluegrass, high temperature, osmotic potential, membrane fatty acid, lipid peroxidation