Influence of seedling growth stage on flooding injury in alfalfa

Temporary flooding of alfalfa (Medicago sativa L.) seedings frequently occurs in the midwestern USA, often resulting in total or partial stand establishment failures. This study evaluated the effect of seedling growth stage on the severity of flooding injury in alfalfa grown in the greenhouse. Alfalfa seedlings of 'WL 323', rated as highly resistant to root rot caused by Phytophthora medicaginis E.M. Hans. and Maxwell and resistant to root rot caused by Aphanomyces euteiches Drechs., were flooded for 14 d at three vegetative growth stages: (i) early seedling development (1-2 trifoliolates), (ii) early vegetative (3-6 trifoliolates), and (iii) mid to late vegetative (5-11 branches on the main stem). Root and shoot dry weights (DW) were recorded at the initiation of flooding, 0 days after flooding (DAF), 18 DAF (shoots only), and 36 DAF (18 d of regrowth from the 18 DAF harvest). The experiment was conducted twice. Flooding significantly (P < 0.01) reduced root and shoot DW, regardless of growth stage, and adversely impacted regrowth potential of alfalfa. There was a growth stage x flooding treatment interaction (P < 0.01) for root DW at 0 and 36 DAF in Trial 1, and at 0 DAF in Trial 2. Generally, flooding reduced root DW the least when seedlings were in the mid to late vegetative growth stage. Averaged over experimental trials, flooding reduced root DW at 36 DAF by 78% for early seedling, 73% for early vegetative, and 64% for mid to late vegetative growth stages, compared with unflooded controls. There was a growth stage x flooding treatment interaction (P < 0.01) for shoot DW at 0, 18, and 36 DAF in Trial 1, and at 36 DAF in Trial 2. Averaged over trials, flooding reduced alfalfa shoot DW measured at 36 DAF by 57% for early seedling, 40% for early vegetative, and 19% for mid to late vegetative growth stages, compared with unflooded controls. We conclude that older and more vegetatively advanced alfalfa seedlings recover shoot regrowth potential after temporary flooding better than do seedlings in the early trifoliolate stages.