Study On The Relationship Between Leaf Yellowing And Mg & B Nutrient Levels In Navel Orange (C.sinensis Osbeck) Leaves In Southern Jiangxi Province Of China

The south of Jiangxi province is the largest navel orange production region in China,which contributes 40%to the national total navel orange production and hectares. Over 90%of the hectares are Newhall navel orange and 80%of the hectares are newly planted trees.Nutrient deficiency symptoms are found rather widely in this navel orange production region.Especially during the fruit growth and development stage,leaf yellowing is more serious and simultaneously with a spectacular enlargement and split of leaf veins.The oldest leaves usually turn yellow first and then the old mature leaves, sometimes newly matured leaves,also turn yellow.It often begins with yellowish spots between main veins and along both sides of midribs,or the margins of leaves fade inward.These yellow areas enlarge and merge to form large yellow areas with a "∧" shape green area at the leaf base.Main veins and sometimes lateral veins show enlargement,corkiness and splitting.The local farmers often treat this leaf yellow symptoms as boron deficiency,but which shows no or only little effects when applying B fertilizer for many years.In order to find the causes of navel orange leaf yellowing,in this paper,Newall navel orange(Citrus sinensis Osbeck) and Skaggs Bonanza navel orange(Citrus sinensis Osbeck) trees on trifoliate orange(Poncirus trifoliata(L.)) rootstocks in typical red soil hill orchards in southern Jiangxi province were used to study the seasonal changes of Mg and B contents in fruits and in different shoot leaves,and the relationship between the contents of leaf chlorophyll and leaf Mg or B contents was also studied.1.Seasonal changes of Mg and B contents in leaf of navel orangeThe results showed that,for the same seasonal shoots,Mg contents in leaves of trees without nutrient deficiency symptoms(normal trees) were significantly higher than those in leaves of trees with nutrient deficiency symptoms(yellow trees).For different shoot leaves,on any leaf sampling date,the Mg contents were highest in the newly matured leaves,and lowest in the oldest leaves.As leaf became older and older, the leaf Mg content decreased gradually.The decrease degree of Mg contents was 87.3%in normal Newhall trees and 60.6%in yellow Newhall trees.The average leaf Mg content of Skaggs Bonanza normal trees was higher than that of Newhall normal trees,and the average leaf Mg content of Skaggs Bonanza yellow trees was also higher than that of Newhall yellow trees.The average leaf Mg contents of NewhaU and Skaggs Bonanza yellow trees were 1.25 g/kg and 1.60 g/kg,respectively.Both were below the threshold limitation of leaf Mg for sufficiency.For Newhall navel orange,as leaf became older and older,the leaf B content became higher and higher,and the variation range of leaf B contents was 58.7～166.9 mg/kg for normal trees and 72.2～105.6mg/kg for yellow trees.For Skaggs Bonanza navel orange,the leaf B content was also higher and higher as its leaf became older and older,and the variation range of leaf B contents was 61.0～93.5 mg/kg for normal trees and 56.9～87.5mg/kg for yellow trees.The average leaf B content of Skaggs Bonanza normal trees was lower than that of Newhall normal trees,and the average leaf B content of Skaggs Bonanza yellow trees was also lower than that of Newhall yellow trees.Leaf B contents in all sampled trees were at suitable or high levels for diagnosing nutrient status of navel orange trees。2.Seasonal changes of Mg and B contents in fruit of navel orangeThe fruit Mg contents of Newhall and Skaggs Bonanza decreased gradually with the growth and development of fruits,and reached the lowest contents when fruits matured. For the both varieties,as compared with the fruits from yellow trees,Mg contents in fruits from normal trees were slightly higher.However,when matured,fruits from all sampled trees approximately had the same Mg contents.From June to August,Mg contents in fruits from Skaggs Bonanza normal trees were near those in fruits from Newhall yellow trees,and lower than those in fruits from Newhall normal trees.Mg contents in fruits from Skaggs Bonanza yellow trees were lower than those in fruits from Newhall yellow trees,but there was no significant difference.Seasonal changes of B contents in fruits of NewhaU and Skaggs Bonanza navel oranges were small during fruit growth and development in despite of slight decrease when matured.For the two varieties,the B contents in fruits from normal trees were slightly higher than those in fruits from their yellow trees,but no significance existed between their normal and yellow trees.For both the normal and yellow trees,fruits of Newhall had significant higher B contents than those of Skaggs Bonanza had.3.Effects of Mg and B contents in leaf on navel orange fruit qualityFor fruit longitudinal diameter,it was significant greater in fruits from normal trees than fruits from yellow trees for Newhall navel orange,but no significant difference for Skaggs Bonanza navel orange.Titratable acid content in fruits from normal trees was significantly higher than that from yellow trees for Skaggs Bonanza navel orange.For the other fruit quality characteristics,there was no significant difference existed between fruits from normal trees and fruits from yellow trees.4.Effects of Mg and B contents in leaf on leaf chlorophyll content and the cause of leaf yellowingFor both the Newhall and Skaggs Bonanza navel oranges,the leaf Mg contents gradually decreased as leaves became older and yellower.There were significant positive correlationships between leaf relative chlorophyll contents and leaf Mg contents with the relevant R~2 values of 0.8249 and 0.8214 for Newhall and Skaggs Bonanza,respectively.However,there was no correlationship between relative chlorophyll contents and leaf B contents for the two varieties.As to other leaf nutrient elements,there was no significant difference between normal trees and yellow trees,so the main cause of navel orange leaf yellowing is Mg deficiency.5.The accumulation of Mg and B in fruit of navel orangeMg and B accumulation in fruits of Newhall and Skaggs Bonanza navel oranges significantly increased with the development of fruits.At each fruit development stage, the single fruit Mg and B accumulations were higher in fruits from normal trees than in those from yellow trees.At fruit mature stage,the accumulations of Mg and B in Newhall navel orange were 36.9-47.2mg/fruit and 1.0-1.4mg/fruit,respectively;and the accumulations of Mg and B in Skaggs Bonanza navel orange were 36.5-39.5mg/fruit and 0.7-0.8mg/fruit,respectively.