Abstract

Pollen viability was high in both E. guineensis (61.8%) and E. oleifera (66.9%), but low in the interspecific hybrid (6.2%). It was also higher than 50% in the backcrosses to E. guineensis , but tended to be low when the E. oleifera participation in the backcross was higher than 25%. These results indicate that pollen viability should not be an obstacle to obtaining a good fruit set in commercial backcrosses of E. oleifera to E. guineensis .

Introduction

A wild hybrid of E. guineensis and E. oleifera (OxG) has generated, through backcrosses to E. guineensis , several populations that have been extensively studied in Costa Rica, due to their outstanding features in terms of yield and stem growth. These "compact" palms have shorter stems and leaves than commercial E. guineensis crosses (Sterling et al . 1988), which characteristics are desirable since both planting density and the useful life of the plantation can be increased. It is a well-known fact that pollen viability is high in E. guineensis and E. oleifera , but is low in the hybrid of these two species (Hardon and Tan 1969). Since compact palms are a valuable commercial genotype, it was important to determine pollen viability in order to foresee any potential problems in getting a good fruit set.

Materials and Methods

The study was conducted from October 1997 to March 1998 in the facilities of the Oil Palm Research Program of ASD de Costa Rica in Coto (South Pacific coast). The station is located at 8 meters above sea level and receives an annual average precipitation of 4118 mm. The dry season runs from January to March, and average mean minimum and maximum temperatures are 21.2 and 31.8 °C.

Pollen was collected from inflorescences in palms planted between 1979 and 1995. Sample size was estimated with 85% confidence and 15% precision ( Table 1 ). One day before anthesis the inflorescence was placed within a bag, the mouth of which was tied with a rubber band around the peduncle to prevent contamination by pollinating weevils. Inflorescences were cut when at least 50% of the flowers were open. The pollen was shaken within the bag and passed through a 100-mesh screen, and then dried in a container with silica gel for 18 hours at room temperature.

Pollen germination was determined following the methods of Turner and Gilbanks (1974). A growth medium was prepared with 1.2-g agar and 11-g sacarose in 100 ml of distilled water and then boiled for 10 min. Around 5 cc of the medium was poured into 5-cm diameter Petri dishes. The dry pollen was dusted on the so-lidified medium using the device in Fig. 1, which allowed for a good distribution of the grains over the surface of the medium. The plates were then covered to maintain a high relative humidity and incubated at 35°C for two hours. Finally, each plate was examined under a microscope (100X) to determine the percentage of germination in a sample of 250 grains.

Results and discussion

Pollen germination was high in both E. guineensis (61.8%) and E. oleifera (66.9%), but low in the hybrid (6.2%) ( table 1 ). backcrosses to E. oleifera , Bx1xEo and Bolx1, genetically similar to the OxG hybrid, also had a low percentage of germination ( Fig.2 ). Rajanaidu and Yap (1985) had found 93.6 % germination in E. guineensis and 18.8% in the OxG hybrid.

The reasons for a poor germination in the hy-brid are not well known. Incomplete pairing of the chromosomes has been observed, as well as some difficulties during pollen liberation (which could be related to anther malformation). It has also been suggested that pollen degenerate between meiosis and anthesis (Hardon and Tan 1969; Tan 1967; Arnaud 1979; Schwediman 1983).

The pollen in populations resulting from backcrosses to E. guineensis showed a germination of more than 50% ( Fig. 2 ). The backcrosses Bx1, Bx2 and Bx3 had values of 54.6%, 84.1% and 88.1% respectively ( Table 1 ).

For selfings of backcrosses (Bx1F1 and Bx1F2) the germination values were 73.2% and 68.7% respectively, which indicates that inbreeding did not have a negative effect. Germination data were similar among parents and descendants, which can be interpreted as the absence of recessive genes affecting this trait ( Fig. 2 ). Backcrosses to E. oleifera showed a pollen germination of less than 10% ( Fig. 2 ). Similar results were obtained by Chaves (1989).

It can thus be concluded that pollen viability should not be an obstacle to commercially ex-ploiting OxG hybrids backcrossed to E. guineensis .