Relationship between the Nitrogen Content in the Leaves during Summer and Autumn Seasons and Fruit Quality of Young Satsuma Mandarin Trees

Abstract

Experiments were carried out to clarify the effects of difference in nitrogen content in leaves due to variation in nitrogen fertilizer application from May to August on the growth and quality of fruits of young satsuma mandarin trees. An additional objective was to clarify the optimum nitrogen content in the leaves during summer and autumn seasons from the point of view of improvement in fruit quality.
1. Foliar analyses revealed that leaf N changed 10 to 15 days after N application. These changes in leaf N were proportional to the amount added and the levels then persisted until the fruits were harvested in December. On September 3, the % leaf N were: N₀: 2.08; N₁: 2.78; N₂: 3.21; N₃: 3.40 and N₄: 3.73. Leaf N was inversely related to K level as were Ca and Mg contents. Fruit N content followed a trend similar to that of leaf N.
2. Differences in leaf coloration became apparent among treatments after the first of June and clear in August, when the leaves of N₀ plot were light green while those of N₃ and N₄ were dark green. Older basal leaves of N₀ plot became yellow and began abscising in August, when the fruit skin color was light green. These trees lacked vigor notably. The coefficient of correlation, r, between leaf color index and leaf N was 0.823**; that between chlorophyll content and leaf N was 0.695**.
3. The fruit yield in decreasing order by plots was: N₃, N₄, N₂, N₁ and N₀. The trend of average fruit weight from the plots paralleled that of yield. Fruit shape was unaffected by N treatments. Fruit color index revealed that fruits from N₁ and N₂ were superior to those from N₀ plot. While fruits from the No plot lost chlorophyll earlier than those from the N plots, they did not develop bright orange color and the color grade at the harvest time was less than that of others. Soluble solids of the juice was highest in fruits from N₀ and N₁ plots, decreasing with increasing N applied. Citric acid content of the juice was variable although the level in N₀ and N₁ plots was higher than others. N₀ difference in sugar/acid ratio was detected among fruits from the differentially treated trees.
It is concluded from these observations that the best quality of satsuma mandarin was attained when the leaf N in summer to early autumn ranged from 2.6 to 2.8% as was found in the N₁ plot.