Effect Of Nitrogen Stress On Citrus Vegetative Rowth And Expression Of Enzyme Genes In Cycle Of GS/GOGAT

Citrus was mainly cultivated in hilly region of South China which exhibited bad capability of keeping water and fertilizer. Many factors which influence citrus industry developing healthily and steadily were decided by the changes and current situation of market, unscientific fertilization was one of the key points which restricts the improvement of fruit quality and sustainable development of citrus industry. Unscientific fertilization in production mainly reflected in:single type of fertilizer which mainly was available inorganic nitrogen fertilizer; inaccurate amount of fertilizer; obsolete fertilizing modes; unscientific fertilizing season. Not only the aim of nitrogen supplement was not achieved based on such a long term of badly fertilization, but also the nutrition imbalance of plant body strengthened, as well as the problem of nitrogen deficiency or excess, fertilizer wasting, rising of production cost and soil environment were taken to a further step. Soil, plant body and leaf in most citrus orchard trapped into nitrogen deficiency condition in Hunan province according to the measured data, and it’s extreme harmful to establish excellent plant body and high efficiency citrus manufacture.NO3--N and NH+-N were primary nitrogen form absorbed by plants, NO3--N was reducted by nitrate reductase and nitrite reductase to transform into NH4+-N, then all NH+-N was converted into organic nitrogen stored in plant body through the GS/GOGAT cycle. Products of GS/GOGAT cycle were biosynthetic precursor substances of other amino acids, nucleotide, chlorophyll and hormone. Therefore, GS/GOGAT cycle plays very crucial role in nitrogen metabolism as was the main route of ammonium assimilation, also is of great importance to the biosynthesis of nitrogenous organic matter.Hence, studying the effect of nitrogen stress on citrus vegetative growth and GS/GOGAT cycle, through analyzing the relevance on characterization, content of nutrient elements and amino acids, relative expression of key enzyme genes in nitrogen assimilation, to give an explanation of nitrogen stress affecting on citrus growth from outside to inside, aiming to lay a theoretical foundation for finding the scientific method of fertilization, so that can provide technically guidance on decreasing nitrogen wasting, increasing the use efficiency and improving citrus production. Main results of this research were showed as follows: 1、Effect of different levels of nitrogen deficiency stress on plant growth mainly was the different relative growth rate, slight and moderate stress were beneficial to enhance the number of new shoot and plant height, as well as the root system in a short period.2、Nitrogen absorbed and accumulated efficiently under short term of nitrogen deficiency to some extent, but each part of plant body were lacking of nitrogen under long term of stress, which disturbed the growth physiology. Significant differences of nutrient elements content were presented in both various treatment and parts of plant. Content of phosphorus and potassium were increased dramatically.3、Content of free amino acids were inversely proportional to levels of nitrogen deficiency, accumulation of amino acids was performed in each part of plant body under short term of nitrogen deficiency, however, the content significantly decreased with the stress extended, particularly in leaves.4、Different down regulation of enzyme genes in GS/GOGAT cycle was exhibited when nitrogen stress was started, but then the relative expression of those genes up-regulated with the time extended and were proportional to deficiency levels. Slight deficiency showed no remarkable up or down regulation of key enzymes in GS/GOGAT cycle till the end of trial. The relative expression of enzyme CisGSl, CisNADH-GOGAT in root and CisGS1, CisGOGAT in leaf exhibited significantly up regulation.5、Leaf chlorophyll content was remarkably increased under extra nitrogen fertilization, which include48.9%-82.6%increase of chlorophyll a,49.0%-85.7%of chlorophyll b and61.5%～79.5%of total chlorophyll. Leaf net photosynthetic efficiency were also remarkably increased, the most was up to132.2%of control.6、Nitrogen assimilation key enzyme gene CrGS2displayed a continuous high level expression in the nitrogen application periods specially, both CrNADH-GOGAT and CrGOGAT had an excess expression significantly after30days treatment, and then presented a rising trend again after back to low levels. In chlorophyll biosynthesis pathway, expression of genes CrHEMA1and CrCHLH had an intense expression in30days after treatment, basically identical with genes which related with nitrogen assimilation.