Commercial exploitation of the Hevea tree for latex commenced in the late 19th century (Carron et al. 1989). This latex is called ‘natural rubber’ and it has remained the base material for the production of heat-resistant polymeric products such as tyres, tubes and bearings.
The latex yield of unselected rubber was 300–400 kg
ha–1 yr–1 before commencement of genetic improvement in the 1920s (FAO 1966; Pushparajah 1995). Following controlled hybridization, the latex yield of recommended clones was increased to 1600–2000 kg ha–1 yr–1 in the 1960s and 2000–3000 kg ha–1yr–1 in the 1980s (RRIM 1977; Alika 1982; Omokhafe and Ugwa 1997).
Further improvement, however, in clonal latex yield has not yielded any significant progress over the level achieved in the 1980s (Verghese et al. 1997). This is in spite of the heterogeneous breeding population of Hevea brasiliensis as reported by Alika (1991), Chevallier (1988) and Verghese et al. (1998). It is likely, therefore, that the dwindling genetic progress is the result of selection of parental clones with recessive genes. An understanding of the mode of inheritance of latex yield could serve as a guide to selection of parental clones that are completely dominant for genes of latex yield. Hence the objective of this study was to determine the number of gene loci that control inheritance of latex yield in H. brasiliensis.
Materials and methods
One-hundred-and-thirty hybrids were obtained from nine crosses as described by Omokhafe and Nasiru (2001). The crosses were: RRIM 628 × RRIM 501, RRIM 501 × RRIM 628, RRIM 501 × PR 107, RRIM 501 × Har 1, RRIM 600 × PR 107, Tjir 1 × RRIM 600, PB 5/51 × RRIM 600, RRIM 628 × Har 1 and PB 5/51 × PR 107. Among the parental clones, RRIM 501, RRIM 600, RRIM 628 and PB 5/51 were selected in Malaysia, PR 107 and Tjir 1 were from Indonesia, and Har 1 was selected in Africa. Three of the parental clones (RRIM 501, RRIM 600, RRIM 628) have latex yields of 1450–1650 kg ha–1 yr–1. Another set of three clones (PB 5/51, PR 107 and Tjir 1) has a yield range of 1000 to 1100 kg ha–1 yr–1, while PR 107 has a mean latex yield of 950 kg ha–1 yr–1.
The hybrids were cloned by budding and planted in 1967 in single row trials at 5.5 m × 5.5 m spacing. There were nine trees per hybrid per row. The hybrids were evaluated for latex yield in four years from 1980 to 1983. Latex was obtained through tapping, without stimulation, as described by Omokhafe (2001). In each month, two samples of coagulated latex, referred to as cup-lumps, were collected for each hybrid and air-dried for 21 days. Monthly samples were weighed on ±0.01 g precision weighing balance. Samples were not taken in February each year as this is the period of severe defoliation of Hevea leading to a break in tapping in Nigeria.
A Mg is a Megagram = 1 x 10^6 grams or a metric ton. It is used to avoid the confusion with different standards for ton in several countries.
Mg C ha-1 yr-1 is the equivalent of metric tons (or a thousand kilos) of Carbon produced by hectare per year.
