The nitrogen nutrition level of apple trees directly affects photosynthesis. As early as 1935, Childers pointed out that photosynthesis was reduced by more than 60% when nitrogen was lacking. When the nitrogen content of trees is suitable, the leaf area is large, chlorophyll is rich, chloroplast volume is large, the number and layers of basal grains are large, and the utilization rate of light energy is high. At the same time, sufficient nitrogen, more young branches and leaves of fruit trees and high gibberellin content can promote stomatal opening and improve photosynthetic efficiency. Nitrogen nutrition level not only affects the growth of apple shoot, but also has a far-reaching impact on root growth and nutrient absorption.
Effect of nitrogen nutrition level on active parts of apple root system
Under the condition of low nitrogen (1mg/L), the diameter of active roots and the number of growth points per unit length increased obviously, thus forming a larger active surface of roots. When the nitrogen concentration increased to 100 ~ 400 mg/L, the above indexes decreased obviously.
Nitrogen content directly affects the growth and fruiting of apple trees. The yield of apple increases with the increase of nitrogen content, but high nitrogen level will obviously reduce the quality of fruit, such as color difference, poor taste, decreased hardness, etc., making the fruit intolerant to storage.
The absorption of nitrogen by apple plants in the annual cycle can be divided into three periods: the first period, from germination to rapid growth of new shoots, is a period of great demand for nitrogen, and nitrogen mainly comes from stored nitrogen, which is characterized by high nitrogen content in new tissues (young leaves, young fruits, stem tips, root tips, etc.). ) In early spring, amino nitrogen was dominant; The second stage, from vigorous growth of new shoots to fruit harvest, is a stable period of nitrogen nutrition, and all forms of nitrogen in various organs and tissues are at a low level; In the third stage, nutrients return from harvest to defoliation in late autumn, which is also the reserve period for roots to grow again and absorb nitrogen. At this time, the nitrogen content in roots and branches increased obviously, mainly amino nitrogen. Nitrate nitrogen, ammonium nitrogen, amide nitrogen (urea) and asparagine in soil can be absorbed by apple roots. In the past, it was thought that apple roots could not directly absorb ammonium nitrogen, but after the mid-1970s, it was proved that apple roots could directly absorb ammonium nitrogen. Applying ammonium nitrogen fertilizer at or before the physiological differentiation stage of flower buds can greatly increase the amount of flower bud differentiation, which is contrary to nitrate nitrogen promoting the growth of new shoots (or delaying the growth of new shoots) and inhibiting flower bud differentiation.
Effects of Ammonium Nitrogen and Nitrate Nitrogen on the Number of Apple Flower Bud Differentiation
When nitrogen is absorbed into the root system, nitrate nitrogen in the root system is first reduced to ammonia, and then combined with photosynthetic products transported by leaves to form asparagine and glutamine, and then further converted into aspartic acid, glutamic acid or arginine, and then transported to leaves in the form of amino acids to further synthesize polymer compounds such as protein and nucleic acid. Ammonium nitrogen can't accumulate in roots. If the apple tree has physiological obstacles, ammonium can't quickly synthesize amino acids, but ammonia poisoning will occur when it accumulates in the roots, which will prevent the roots from further absorbing ammonium ions and some other cations. Nitrate does not accumulate in the root system. If nitrate cannot be reduced to ammonia or converted to amino acids in the root system, it can be transported directly upwards and converted in the leaves.
Every winter before the apple tree goes to sleep, some nitrogen in the leaves is converted into amino acids, transported to the bark and roots of the branches, and then converted into protein nitrogen for storage. The next spring, these stored protein were hydrolyzed into amino acids, and then transported to the tissues and organs where trees thrive. The growth of apple trees in the first few weeks of spring mainly depends on the storage nutrition in bark, followed by the storage nutrition in root system, and then absorbs nutrients from soil. Nitrogen can be reused in trees. If nitrogen in trees is insufficient, protein in old leaves can be hydrolyzed into amino acids and transported to new leaves and new growth sites. Therefore, the lack of nitrogen in fruit trees begins with the symptoms of old leaves.