Abstract:

Objective: Secondary laticifer in the trunk of a rubber tree (Hevea brasiliensis) is the major tissue to synthesize and store the natural rubber. The number of secondary laticifer rows has a significantly positive correlation with the rubber yield,and further,the differentiation of the secondary laticifer can be induced by mechanical wounding in both mature tree trunk and tender shoot. Wickham germplasm,as the key source of cultivars,little is known about the secondary laticifer differentiation of shoots responded to the mechanical wounding. In the present study,the differentiation characteristics of different Wickham germplasm shoots were studied,in order to provide early prediction of the capacity of the secondary laticifer differentiation after tapping of mature trees,and to provide a basis for identification and evaluation of the germplasm resources,and further to shorten the time needed for breeding,therefore to improve the efficiency of Hevea breeding. Method: A total of 304 accessions of Wickham germplasms were selected and treated with mechanical wounding by using a blade to scrape the tender shoot deep to the cortex. Fresh bark samples were separately collected at 5 days and 10 days after wounding,then treated with iodine-bromine reagent,and embedded in paraffin. The differentiation of the secondary laticifers were observed with an optical microscope and data on time of emergence and number of the secondary laticifers were recorded for statistical analysis. Result: There was a significant difference of secondary laticifer differentiation among the 304 different accessions of Wickham germplasms induced by mechanical wounding. 5 days after wounding,secondary laticifer(SL) were clearly formed in rows in 95 accessions of the germplasms,sparse SL cells not in rows were found in 172 accessions of the germplasms,and no SL was detected in the other 37 accessions. 10 days after wounding,however,up to 265 accessions of the germplasms displayed clear SL rows,of which 22 accessions had 2 or more SL rows. Accordingly,the number of germplasm accessions with sparse SL and no SL reduced to 37 and 2 respectively. Undoubtedly,the induction ratio of SL differentiation in the 304 germplasm accessions at 10 days was obviously higher than that at 5 days. In different breeding series,the Dafeng,Baoting,Reyan,and Daling that derived from Hainan showed a better differentiation effect. According to the sensitivity and intensity of the SL differentiation,the Wickham germplasms were classified into 4 types:sensitivity type,sensitivity-sustainability type,hysteresis type and hysteresis-sustainability type,among which the sustainability types were always the high-yield germplasms. Conclusion: Different Wickham germplasm accessions had different characteristics of SL differentiation responded to mechanical wounding. 5 days and 10 days after wounding can be used to select the sensitive and the hysteretic germplasms respectively. Among the 4 types of the Wickham germplasms,the sustainability type is the main source for superior varieties. The ability of high SL differentiation of epicormic shoot induced by wounding can be used as an effective indicator for early selection of high-yield Hevea trees.

Key words: Hevea brasiliensis, Wickham germplasm, mechanical wounding, secondary laticifer, differentiation