Nitrate reductase in apple trees

Factors affecting the in vivo nitrate reductase assay were investigated in order to optimize the assay conditions for various tissues of MM 106 apple trees. Optimum concentrations of nitrate and phosphate for in vivo nitrate reductase activities ranged from 50 to 100 mM and were tissue-specific. The optimum pH of the assay medium was 7.5. Vacuum infiltration and the addition of 2% (v/v) n-propanol to the assay medium resulted in stimulation of nitrate reductase activity. The in vivo nitrate reductase assays for leaves and stems were linear for at least 60 minutes following an initial 30 minute lag, whereas there was no lag phase in root tissues.The effect of nitrate supply in the nutrient solution on nitrogen accumulation and nitrate reductase activity was examined. Increasing nitrate supply resulted in increases in the concentration and amount of reduced nitrogen in leaves and stems. The reduced nitrogen contents of trunk bark and trunk wood were significantly decreased when low nitrate was supplied, whereas that of roots was changes insignificantly. Most organs of MM 106 apple trees contained only a small portion of their total nitrogen as nitrate. The activity of nitrate reductase was measured using in vivo assay either with or without nitrate, namely in vivo +NO$\sb3\sp-$ and in vivo $-$NO$\sb3\sp-$ nitrate reductase assay. The in vivo $-$NO$\sb3\sp-$ assay may be the better way to estimate reduced nitrogen accumulation than the in vivo +NO$\sb3\sp-$ assay. Increasing nitrate supply resulted in significant increases of in vivo $-$NO$\sb3\sp-$ nitrate reductase activities of all organs. The proportion of leaf nitrate reductase activity to the total activity increased as the nitrate supply increased. The stems also contributed considerably to the total nitrate reductase activity accounting for as much as 23.5% of the total activity. Therefore, it can be concluded that nitrate reduction can occur in various parts of MM 106 apple trees as long as nitrate is available.Effect of darkness on nitrate reduction was also examined in MM 106 apple leaves. As the period of darkness increased, nitrate reductase activity decreased and the accumulation of nitrate increased. The decrease in nitrate reductase activity with increasing period of darkness may be related to the decrease in soluble sugars.U of I OnlyETDs are only available to UIUC Users without author permissio