Study on diagnosis of leaf chlorosis disease of ‘Kuerle fragrant pear' (Pyrus bretschneideri Rehd. ) by pH and iron concentration in apoplast sap

Abstract: 【Objective】In oasis of Talim Basin of southern Xinjiang,China,the soil is highly calcerousand alkaline,with p H above 8.2. Under such high p H,soluble iron in the soil might be fixed and activeiron might decrease greatly.‘Kuerle fragrant pear'(Pyrus bretschneideri Rehd.) is a fruit tree species great-ly affected by iron deficient chlorosis in the area. The objective of this work was to study the relationshipbetween leaf chlorosis and p H and leaf iron concentration in xylem sap,with the final aim to explore thepossibility of early diagnosis of iron chlorosis in‘Kuerle fragrant pear'by p H and iron concentration in xy-lem sap.【Methods】The plants were six-year-old‘Kuerle fragrant pear'grafted on rootstock of‘Duli'(Pyrus betulifolia Bunge). At the different stages of growing season,the root,trunk,limb and one-year-old shoots were cut from the plants. The incision was coated with an absorbent cotton ball and a rubberbag for 24 h. The absorbent ball was taken off when it was soaked by xylem apoplastic sap. Petioles andleaf blades were also coated by absorbent balls,then packed in a rubber bag. After extrusion,an absor-bent ball was saturated in xylem apoplastic sap. All absorbent balls were sealed in Buffer tubes. The p Hvalue of samples of the leaf apoplast fluid and xylem excudate was measured by p H indicator strips andmicroelectrode P15. The leaves used to iron analysis were cleaned with 0.1% sodium dodecyl sulfonate(SDS),tap water and deionized water for three times in sequence. The other parts were stripped,weighed,sterilized for 1 h and dried for 24 h at 115 ℃ in turn. All samples were weighted again to deter-mine the water content. The leaves were grinded with agate mortar. The other parts were grinded into pow-der with a stainless steel mill and kept in a plastic bottle for further analysis. Total iron in solution was de-termined by 1,10-phenanthroline method at the wavelenth of 530 nm. Control sample was prepared and astandard curve was achieved. The curvilinear equation was C=0.240 5x+0.011 2,R²=0.997 7. Active ironwas extracted with 0.1 mol·L^-1 HCl. The sample was immersed into the extraction solvent for 24 h. Theweight ratio of sample/extraction solvent was 1∶100. Active iron was also measured using 1,10-phenanth-roline spectrometric method. According to water content of the sample,total iron content with dried basewas converted into total iron concentration per unit volume. Similar treatment was performed in the expres-sion of active iron.【Results】The result showed that the apoplastic p H of parts tested ranged from 5.0 to6.0. The value was in a normal physiological relevant p H scale. In general,for a normal tree,p H of leaffluid was not significantly different at different stages(F=1.554). But for a chlorotic tree,significant differ-ence of p H of leaf fluid were observed at different growing stages(F=2.125). The difference of apoplasticp H was most significant(F=8.442) in different parts of a chlorotic tree as well as in a normal tree(F=7.518). It revealed that apoplastic p H difference in different part was apparent. The apoplastic p H of anormal tree was slightly higher about 5.6% than that of a chlorotic tree. It was not relevant to inactivationof iron in xylem sap. Either in a chlorotic tree or a normal tree,total iron concentration in different partswere most significantly different(Fchlorotic tree 46.72,Fnormal tree=46.72). Total iron concentration inthe root was the highest(174.3-175.5 mg·L- 1). Leaf total iron concentration ranked No. 2(98.2-104.7mg·L-1). Total iron concentration in the trunk was slightly lower(72.2-75.0 mg·L-1). Total iron content ofpetioles,limb and one-year-old shoots were generally low(43.0-51.1 mg·L-1). Leaf total iron concentra-tion of a chlorotic tree was higher 6.5% over that of normal tree. Total Fe concentrations in the same partwere not markedly different between a normal and a chlorotic tree. Therefore,active Fe concentrations insix sampling parts of a normal tree were most significantly higher than those of a chlorotic tree(F=5.201-6.307). The difference remained in the whole growing season.【Conclusion】The apoplastic p H of leaf fluidin a chlorotic and a normal tree provided an important parameter for formulating iron foliar fertilizer anddiagnosing iron deficient chlorosis. There was no direct reference value of apoplastic p H in other parts forchlorosis diagnosing of‘Kuerle fragrant pear'. Iron was enriched in leaf tissue rather than in other parts.Due to“chlorosis paradox”,in the nutrient diagnosis for iron chlorosis,leaf total iron concentration didnot represent exactly iron status of a‘Kuerle fragrant pear'plant. In contrast,active Fe content was an ide-al parameter in diagnosis of iron-deficient chlorotic disorder.