Effects Of Low Temperature Stress On Physiological Metabolism And Related Gene Expression Of Pitaya Grafted Seedlings

Pitaya(Hylocereus spp.)is a perennial succulent climbing plant of the cactus family.It prefers a warm and humid environment,but it’s not resistant to low temperatures.When the ambient temperature is lower than 5 ℃ in winter or cold spring,the young branches of pitaya are easily frostbitten,which affects the growth and yield of pitaya.In this study,two kinds of pitaya seedlings with different cold resistance,‘Baishuijing’ and ‘Hongxin’ were used as experimental materials to study the effects of osmotic adjustment substances,oxidative system,AsA-GSH recycle and cold-resistant gene expression of two kinds of pitaya branches under low temperature stress,and explore the relationship between active oxygen scavenging system and cold resistance of pitaya,aim to clarify the response of pitaya to low temperature stress and the mechanism of grafting to improve the cold resistance of pitaya.The main results are as follows:(1)During low temperature stress at-2 ℃,the soluble sugar,soluble protein and free proline content of ‘Hongxin’,‘Baishuijing’ and ‘Hongxin’/‘Baishuijing’ pitaya seedlings increased significantly.‘Baishuijing’ pitaya seedlings had higher soluble sugar and free proline content than ‘Hongxin’ pitaya seedlings.Using ‘Baishuijing’ pitaya seedlings as rootstock to graft ‘Hongxin’ pitaya,the accumulation of soluble protein and free proline in branches of ‘Hongxin’/‘Baishuijing’ pitaya grafted seedlings were increased,indicating that grafting promoted the accumulation of osmotic adjustment substances in ‘Hongxin’pitaya seedlings under low temperature stress,maintained the osmotic balance of plants.(2)With the prolongation of low temperature stress,the REC of self-rooted seedlings and grafted seedlings of pitaya fluctuated.At 48 h after low temperature stress,the MDA content and REC in self-rooted seedlings and grafted seedlings of pitaya increased,and pitaya seedlings were damaged by membranous peroxidation.MDA content in‘Hongxin’/‘Baishuijing’ pitaya seedlings were lower than ‘Hongxin’ pitaya seedlings,indicating that grafting alleviated the damage of the cell membrane system of ‘Hongxin’pitaya seedlings under low temperature stress,and maintained the cell membrane stability.(3)With the prolongation of low temperature stress,the activity of antioxidant enzymes such as SOD,POD and CAT in self-rooted seedlings and grafted seedlings of pitaya increased gradually.‘Baishuijing’ pitaya seedlings had higher antioxidant enzyme activity than ‘Hongxin’ pitaya seedlings,which maintained at a high level for a long time and had a stronger ability to scavenge ROS.However,‘Hongxin’ pitaya seedlings have only high activity of SOD and CAT,so the antioxidant capacity was relatively weak.Grafting improved the activity of SOD,POD and CAT in ‘Hongxin’ pitaya seedlings,indicating that grafting improved the antioxidant capacity of the plants.(4)Low temperature stress significantly reduced the AsA content and AsA/DHA of self-rooted seedlings and grafted seedlings of pitaya,and increased the DHA content.The AsA content and AsA/DHA of ‘Baishuijing’ pitaya seedlings were higher than ‘Hongxin’pitaya seedlings.Grafting significantly increased AsA content,AsA/DHA and GSH/GSSG in ‘Hongxin’ pitaya seedlings.Under low temperature stress,the key enzymes of AsA-GSH recycle in pitaya seedlings showed the trend of ‘Baishuijing’> ‘Hongxin’;grafting improved the APX,GR and DHAR activities in ‘Hongxin’ pitaya seedlings,but several enzymes activity was not completely consistent with changes in their gene expression.It indicated that grafting significantly increased APX,GR and DHAR activity and AsA/DHA and GSH/GSSG in ‘Hongxin’ pitaya seedlings under low temperature stress,which improved the AsA-GSH recycle capability.(5)CBF gene and ICE1 gene in self-rooted seedlings and grafted seedlings of pitaya were rapidly expressed under low temperature stress.And the CBF and ICE1 gene expression levels in ‘Baishuijing’ pitaya seedlings were significantly higher than those in‘Hongxin’ pitaya seedlings.Grafting could increase the expression of CBF and ICE1 in‘Hongxin’ pitaya,thus improved the cold resistance of ‘Hongxin’ pitaya seedlings.In summary,‘Baishuijing’ pitaya seedlings were more resistant to cold than the‘Hongxin’ pitaya seedlings,the reason was related to its ability to maintain high osmotic adjustment substances and antioxidant enzyme activity and AsA-GSH recycle ability.Grafting could improve the content of osmotic adjustment substances,antioxidant enzyme activity and AsA-GSH recycle ability of ‘Hongxin’ pitaya seedlings,thus enhanced the cold resistance of ‘Hongxin’ pitaya seedlings.