Root respiratory costs of ion uptake, root growth, and root maintenance in wetland plants: efficiency and strategy of O2 use for adaptation to hypoxia.

Oxygen use in roots is an important aspect of wetland plant ecophysiology, and it depends on the respiratory costs of three major processes: ion uptake, root growth, and root maintenance. However, O2 allocation in wetland plants has received little attention. This study aimed to determine the O2 allocation and specific respiratory cost of each process under hypoxic conditions, to better understand the strategy and efficiency of O2 use in wetland plants. The root respiration rate, nitrogen uptake, and root growth in three Carex species with different growth rates were examined under hypoxic conditions using different N sources, and the respiratory costs of ion uptake, root growth, and root maintenance were statistically estimated. All species exhibited low specific costs and low ratios of O2 allocation for root growth (2.0 ± 0.4 mmol O2 g(-1) and 15.2 ± 2.7 %, respectively). The specific cost of ion uptake was 20-30 % lower in fast-growing species than in slow-growing species. As plant growth rate increased, the O2 allocation ratio for ion uptake increased, and that for root maintenance decreased. The cost was higher when NO3 (-) was fed, than when NH4 (+) was fed, although the pattern of O2 allocation ratios for three processes was similar for NO3 (-) and NH4 (+). Our results indicate that wetland plants primarily employ an O2 use strategy of minimising the respiratory costs of root growth, and fast-growing plants specifically use O2 to maximise ion uptake. These findings provide new insights into ecophysiological behaviours of roots in adaptation to hypoxia.