Effects Of Nitrate Nitrogen On Nitrogen Metabolism And Related Enzymes In Grape Seedlings Under Water Stress

Water and nitrogen are two important factors affecting the growth of grape?Vitis vinifera L.?.An important research goal is to improve the use of both factors.The objective of thishyd-roponic experiment was to determine the physiological effects of NaNO₃ on one-year-old grape?cv.Victoria?seedlings under drought stress.To create drought stress conditions,the seedlings were grown in Hoagland's nutrient solution containing 5,10,and 15%,PEG ?respectively simulating mild,moderate,and severe water stress?.Hoagland's solution with-out PEG was used as a control?CK,no drought stress?.The NaNO₃ was applied to the see-dlings and then the following variables were measured:?1?leaf chlorophyll,MDA,and pro-line contents;?2?free amino acid concentrations in different plant organs;?3?¹⁵N absorption,distribution,and the activities of enzymes related to N metabolism.The main results of this experiment were as follows:?1?Photosynthetic pigment concentrations and root system vigor increased and then gradually decreased after NaNO₃ was applied in all three drought stress treatments.The maximum concentrations of chlorophyll a?35.03 mg/g?,chlorophyll b?16.32 mg/g?,carotenoid?8.95 mg/g?and total chlorophyll?51.35 mg/g?were all observed 72 h after NaNO₃ application.These maximum values were all significantly greater than those in CK?P<0.05?.These results indicated that NaNO₃ increased the chlorophyll concentration in grape seedlings.Root activity reached a maximum(0.2613 mg·g^-1h^-1)48 h after NaNO₃ application,which was beneficial to the root activity of grape seedlings under mild stress.Leaf proline,MDA,and free amino acids reached maximum concentrations 144 h after NaNO₃ application,with values that were 4.56,2.78,and 1.14 times greater than those in CK.These results showed that NaNO₃ alleviated the negative effects of drought stress on grape seedling growth.The self protective mechanism of grapes increased resistance to environmental stress by increasing the concentration of osmotic substances in vivo.?2?Compared with the control group?CK?,grape seedlings were treated with ¹⁵N-Na NO₃.Moderate drought stress increased N derived from fertilizer?Ndff?in roots by 5.41%and in new branches by 2.07%compared with CK.The Ndff in leaves was relatively low,only0.41%.The results meant that?i?moderate water stress increased the ability of grape seedlings to absorb and utilize N fertilizer and?ii?roots and new branches had greater ability than leaves to absorb and use the fertilizer ¹⁵N.Averaged across water stress treatments,the¹⁵N distribution ratio in various grape organs was as follows:roots,53.95%;main vine,29.98%;new branches,13.44%;and new shoots,11.36%.Leaves had the lowest ¹⁵N distribution ratio?3.4%?among the plant organs.The results also showed that the ¹⁵N utilization rate of each organ differed greatly.The root system had the highest utilization rate?19.2%?,followed by the main vine and new shoots.Leaves had the lowest utilization rate?2.3%?.The results indicated that drought stress was not favorable to N fertilizer use by grape seedlings.Drought stress reduced N use efficiency.?3?The activities of NR in organs of grape seedlings were significantly higher than those in the control group?CK?after NaNO₃ was applied to different levels of water stress.The NR activity of roots and leaves decreased with time in all drought stress treatments.The NR activities remained high in the mild and moderate stress treatments.The NR activity in roots reached a maximum[163?g/?g·h?]96 h after NaNO₃ application.The NR activity reached maximum values in stems[113.53?g/?g·h?]and leaves[135.73?g/?g·h?]24 h after NaNO₃ application.The NaNO₃ application to seedlings under mild drought stress maintained GS and GOGAT activities which were similar to those in the CK treatment.However,under moderate and severe stress,the activities of GS and GGAT were relatively low and decreased as treatment time increased.The activities of GOT and GPT in drought stressed-seedlings increased and then decreased after NaNO₃ application.The activities of GOT and GPT also decreased with time.Compared with CK,NaNO₃ application had the following effects on seedling under mild drought stress:?i?greater ammonia assimilation and amino acid synthesis,?ii?better N balance,and?iii?enhanced resistance to water stress.Applying NaNO₃ to seedlings under moderate and severe drought stress,increased ammonia assimilation and reduced the inhibition of amino acid transformation so that N metabolism was balanced and the resistance of plants was increased in the short term.