[Effects of NO3- stress on photosynthetic characteristics and nitrogen metabolism of strawber- ry seedlings].

In order to explore the effects of NO3- stress on photosynthetic characteristics and nitrogen metabolism, strawberry seedlings were grown in sand culture condition under different concentrations of NO3- (64, 112 and 160 mmol · L(-1)) with the control of 16 mmol NO3- · L(-1). The results indicated that at the 8th day after treatment, with the increased NO3- concentration, the strawberry leaf net photosynthetic rate (Pn), stomatal conductance (g(s)), PS II actual photochemical efficiency (ΦPS II), PS II maximum photochemical efficiency (Fv/Fm), and photochemical quenching coefficient (q(P)) significantly decreased, and decreased by 67.7%, 68.4%, 35.7%, 23.2% and 26.9%, respectively, when NO3- concentration reached 160 mmol · L(-1) compared with the control. The non-photochemical quenching coefficient (q(N)) increased by 4.4%, 10.9% and 75.8% respectively in the treatments of 64, 112 and 160 mmol NO3- · L(-1) compared with the control. The intercellular CO2 concentration (Ci) decreased under low NO3- concentration stress and then increased under high NO3- concentration stress, while the stomatal limitation (Ls) was vice versa. With the increased NO3- concentration, the nitrate nitrogen, ammonia nitrogen, total nitrogen and Kjeldahl nitrogen contents in the strawberry leaves and roots increased, but the protein nitrogen content decreased. The activities of nitrate reductase (NR) , glutamine sybthetase (GS), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) increased at low NO3- concentration and then decreased at high NO3 concentration. Consequently, the net photosynthetic rate of strawberry seedling leaves decreased, the PS II electron transfer was blocked, the nitrogen accumulated with the increasing NO3- concentration, and the nitrogen metabolism enzyme activity decreased at high NO3- concentrations. When the NO3- concentration reached 64 mmol · L(-1) or higher in the nutrient solution, the growth of strawberry seedlings were inhibited significantly.

[Effects of NO3- stress on cucumber seedling growth and magnesium absorbing under suboptimal temperature].

The effects of NO3- stress on the growth and the magnesium absorption of cucumber seedlings were investigated after 1 and 14 d of suboptimal temperature [18 °C/12 °C (day/night)] treatment. The results indicated that the growth, net photosynthetic rate, transpiration rate, Fv/Fm and ΦPSII of cucumber seedlings were significantly inhibited by NO3- stress under suboptimal temperature. The magnesium content of cucumber seedlings was also significantly decreased compared with control treatment, especially in the IV treatment (suboptimal temperature +140 mmol · L(-1) NO3- + 1 mmol · L(-1) Mg2+), and the antagonistic impact of magnesium ion absorption on the absorption of potassium and calcium ion was observed. The magnesium deficiency symptom of cucumber seedlings could be partly alleviated by increasing the concentration of magnesium ion in the nutrient solution.

[Effects of different NO3- concentrations on the growth and antioxidant enzyme systems of strawberry seedlings].

The effects of different NO3- concentrations (CK, 16 mmol x L(-1) NO3-; T1, 48 mmol x L(-1) NO3-; T2, 96 mmol x L(-1) NO3-; T3, 144 mmol x L(-1) NO3-) on the growth properties and antioxidant enzymes activities of strawberry seedlings were studied by sand culture. The results indicated that the plant height, leaf area, dry matter accumulation and root/shoot ratio of strawberry seedlings in T1, T2 and T3 were significantly decreased compared to the control (CK) after 15 days of treatment. The plant heights in T1, T2 and T3 decreased by 16.1%, 36.8% and 43.9%, and the areas of functional leaf decreased by 19.7%, 34.3% and 47.5% respectively, compared to the control. With the increasing NO3- concentration, the malondialdehyde (MDA), proline, soluble protein content and relative electronic leakage of the strawberry seedlings first increased and then decreased, but all those parameters in T1, T2 and T3 were higher than in the control. The activities of antioxidant enzymes, superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) in both root and leaf, and catalase (CAT) in the root first increased and then decreased, but the CAT activity in the leaf gradually decreased with the increasing NO3 concentration. In T3, the activity of SOD in both root and leaf, APX in the root were lower than in the control. In sum, the osmolytes of strawberry seedlings increased and the plants become weaker under nitrate stress, and some antioxidant enzymes in strawberry seedlings increased at the low level of nitrate stress and decreased at the high level. The strawberry plants were stunted when growing under 48 mmol x L(-1) NO(3-) condition.