We investigated the effects of short-term root-zone hypoxic stress and exogenous
calcium application or deficiency in an anoxic nutrient
solution on
nitrogen metabolism in the roots of the muskmelon cultivar Xiyu No. 1. Seedlings grown in the nutrient
solution under hypoxic stress for 6d displayed significantly reduced plant growth and soluble
protein concentrations. However, NO₃⁻ uptake rate and activities of
nitrate reductase and
glutamate synthase were significantly increased. We also found higher amounts of
nitrate,
ammonium,
amino acids, heat-stable
proteins,
polyamines, H₂O₂, as well as higher
polyamine oxidase activity in the roots. In comparison to the reactions seen under hypoxic stress, exogenous
calcium application led to a marked increase in plant weights, photosynthesis parameters, NO₃⁻ uptake rate and contents of
nitrate,
ammonium,
amino acids (e.g.,
glutamic acid,
proline,
glycine,
cystine, γ-
aminobutyric acid), soluble and heat-stable
proteins, free
spermine, and insoluble bound
polyamines. Meanwhile, exogenous
calcium application resulted in significantly increased activities for
nitrate reductase,
glutamine synthetase, and
glutamate synthase but decreased activities for
diamine and
polyamine oxidase, as well as lower H₂O₂ content in roots during exposure to
hypoxia. However,
calcium deficiency in the nutrient
solution decreased plant weight, photosynthesis parameters, NO₃⁻ reduction,
amino acids (e.g.,
alanine,
aspartic acid,
glutamic acid, γ-
aminobutyric acid),
protein, all
polyamines except for free
putrescine, and the activities of
glutamate synthase and
glutamine synthetase. Additionally, there was an increase in the NO₃⁻ uptake rate,
polyamine oxidase activity and H₂O₂ contents under
hypoxia-Ca. Simultaneously, exogenous
calcium had little effect on
nitrate absorption and transformation, photosynthetic parameters, and plant growth under normoxic conditions. These results suggest that
calcium confers short-term
hypoxia tolerance in muskmelon, most likely by promoting
nitrate uptake and accelerating its transformation into
amino acids, heat-stable
proteins or
polyamines, as well as by decreasing
polyamine degradation in muskmelon seedlings.