Abstract

Five commercial oil palm varieties ( Elaeis guineensis Jacq.) were planted and their vegetative growth (roots and aerial portion) was measured at different times, while in a prenursery. Digital photographs were also taken of the plants showing what was considered to be the typical appearance of each variety at each measuring time.

Seed weight seemed to be positively associated with initial growth, but it was also clear that other genetic factors controlled growth rate and development. Deli x Yangambi was the most vigorous variety, followed by Deli x AVROS, Deli x Calabar, Deli x Ekona, and finally, Deli x La Mé. The growth of mature palms of these varieties in commercial plantings closely mirrors early vegetative growth in the prenursery.

Growth rate data indicate that plants should be transplanted from the prenursery to the main nursery before they are three months old, in order to reduce the risks of causing severe transplanting shock. Maximum growth rate (both for the aerial part of the plants and the root system) was obtained three months after sowing.

The information presented below may serve as a reference to evaluate the commercial performance in a prenursery of any of the five oil palm varieties studied.

Introduction

The choice of planting material from a recognized source, together with a sound agronomic management of the crop, are two key aspects to guarantee the success of a commercial oil palm plantation (Rajanaidu 1997).

During the first weeks after they emerge, the plantlets live essentially from the food reserves in the endosperm. Approximately 80% of these reserves are depleted within three months (Corley 1976). The morphological changes associated with this early development, and the final establishment of the plants in the soil have been described in the literature (Hartley 1988). 

The most popular oil palm variety is the Deli x AVROS cross (usually called AVROS), though other varieties are also available on the market that may be more suitable for planting under particular growth and environmental conditions. AVROS performs very well in optimum environments for the economical exploitation of the crop. AVROS plants have a vigorous vegetative growth and large bunches and fruits with a high oil content.

The Deli x Calabar cross (Calabar) performs well in regions with low solar radiation and a moderate water deficit, where other varieties, such as AVROS, may have problems of a fast stem growth that limit the plantation's economical life span. 

Deli x La Mé has been second in popularity to AVROS. Its main advantages are slow stem growth and very good adaptability to moderate water deficit. The Deli x Yangambi crosses show a vigorous vegetative growth (which may be undesirable under certain conditions), but the fruits have a high oil content and the plants adapt well to water deficit and moderately low temperatures (ASD 2000). Deli x Ekona crosses have a low to moderate tolerance to drought and low temperatures and an excellent oil content in the fruits.

A comparison of vegetative growth at the prenursery stage has not been documented for some of these new commercial oil palm varieties. However, a reference growth pattern for any particular variety can be used for the detection and timely correction of any deviation that might occur in a commercial prenursery. Since the aerial development of the plants is closely related to root growth, the latter must also be included in a reference growth pattern. 

The availability of a reference pattern for the early stages of the plant's development can also be very useful for commercial seed sellers, enabling them to follow the performance of their products and take the most appropriate steps in the event of a complaint about the quality of the seeds supplied.

Materials and methods

Work was carried out at a commercial prenursery located in Coto, near the Pacific coast of Costa Rica (8 m above sea level, 4118 mm annual rainfall, with a drier period between January and March and maximum and minimum temperatures of 32 and 21 °C).

All five commercial oil palm crosses were supplied by ASD de Costa Rica, and were planted in soil with a mean texture, used to fill prenursery polyethylene bags (15x23 cm), that were arranged in square blocks with eight bags on each side. Crosses (treatments) were arranged as a complete block design with four replications. Data were collected from 36 central plants in each block of 64 plants.

The prenursery stage coincided with the low rainfall period, so water was supplied with micro sprinklers. Shade was supplied by a synthetic cloth (saran) to allow in 50% of incoming light. When the plants were two months old they each received one-gram of DAP fertilizer (18-46-0), and this application was repeated every other week thereafter. No other chemicals were used during the prenursery stage.

Before planting, each germinated seed was characterized according to the length of the radicle, and the number of lateral roots ("adventitious roots"). Each unit of 36 plants (blocks) was divided into six groups, which were sacrificed successively on the six occasions when data were gathered ( Table 1 ). Each plant was removed from its bag and the soil was carefully washed off the roots. 

The first growth evaluation was carried out one month after sowing, and then every two weeks approximately. Even though it is not commercially viable to maintain a prenursery beyond three months of age, we kept this prenursery until some plants were four months old, when the final evaluation was conducted.

The radicle, adventitious roots and primary roots were measured from the insertion at the base of the plant (bulb) to the apex. Adventitious (lateral) roots were those that have a tendency to grow horizontally and originate in the area where plumule and radicle seem to converge.

The diameter of the base of the plant (bulb) was measured with a caliper at the widest point. The youngest leaf was the first leaf to open completely. Already open leaves were defined as all those below the youngest one, including this. Dry weight was determined by drying the tissue in a forced air oven. 

A Tukey test (P=0.05) was used to statistically compare the means for the different variables. Data on dry weight and leaf area were also used to estimate other physiological and morphological parameters.

Results and discusion

Seed size. This variable was not considered as a covariable, though there were differences in the initial growth of some genetic materials that were apparently related to seed size, as already documented by Rao et al . (1999). According to these authors, the effect of seed size on initial plant growth disapears over time.

Seeds for this experiment were obtained from several mother palms, which may also have affected the mean size of the seeds. In addition, it is known that the average size of the seeds decreases as the male parent shifts from Yanbambi to AVROS, Calabar, Ekona and La Mé. Finally; palm age determines seed weight and size (Alvarado, A.). Seeds of the AVROS and Yangambi materials were harvested from 19 years-old palms, La Mé and Calabar from 12 years-old palms, and Ekona from 13 years-old palms.

Radical system

Lateral (adventitious) roots. Germinated seeds had one or more of these roots, and the number increased after sowing, reaching a maximum during the measurement taken when plantlets were 37 days old. La Mé and Yangambi produced statistically more of these roots ( Fig. 1 ). The early sprouting of these roots, together with a fast growth of the radicle, may be an adaptation to ensure that the young palm is rapidly anchoraged in its natural habitat, which may be at high risk of flooding (Hartely 1988).

The number of lateral roots developed during the early stages of the plantlets' growth does not seem to be associated with the vigor of the plant afterwards. However, it could be an important factor during these early stages, particularly when the seed is exposed to infection by seed-borne (or soil-borne) opportunistic pathogens. These pathogens may attack the growing point of the roots and radicle, may reduce the emergence rate, and may favor the appearance of abnormal plantlets. The non-formation of new lateral roots was associated with an increase in the rate of elongation of the radicle after the measurement taken 37 days after planting.

Radicle. The elongation rate of the radicle was quite fast during the first month after planting, reaching more than 15 cm in length during this period ( Fig. 2 ). After two months, it had reached over 23 cm (the height of the prenursey bags), so it spiraled at the bottom, or pushed through the drainage holes in the bag. This happened despite the fact that a large part of the volume of soil in the bags had not been yet explored by the root system. The growth pattern of the radicle was very similar in all five crosses, and no statistical differences were found.

Primary roots. The development of this type of root was not evident in any cross until the evaluation conducted 51 days after sowing. When plants were two months old, all crosses had at least one primary root, and the development rate of these roots increased after three months. Calabar and Yangambi showed a tendency to be more precocious in producing these roots, and to produce more of them ( Fig. 3 , Fig 4a , Fig 4b , Fig 4c , Fig 4d and Fig 4e ).

The rate of elongation of the primary roots followed a linear tendency, and the length reached approximately 10 cm when plants were 77 days old, a period that coincided with the initial development of secondary roots. Yangambi and Calabar showed a clear tendency to produce more primary roots than the rest of the materials. The length of the primary roots measured at 126 days after planting was statistically different between crosses, and the cross Yangambi had the longest of such roots. 

The length and number of each of the primary roots present at the different measurement times are shown in figure 4. The precocity of the Yangambi crosses contrast with the data from the Ekona and La Mé crosses.

Aerial growth

The five crosses can be divided into two groups based on the width of the plants at the base ("basal bulb"): AVROS, Calabar and Yangambi showed the best growth ( Fig 5 ). The growth rate of the bulb increased from day 77 (or some time before) after planting, which may be associated with the beginning of the activity of the primary roots and the newly emerged leaves. 

The first leaves characterized as completely open appeared in all crosses during the measurement taken 77 days after planting ( Fig. 6 ). Corley (1976), working with a particular genetic material, had determined that the plantlet became independent from the seed reserves approximately 45 days after planting, when the first leaf blade had expanded completely, and the second leaf was only partially open.

Dry weight accumulation by the plant is the result of nutrient uptake and photosynthetic activity. The Yangambi crosses show the advantages of an early growth and development ( Fig.7a , Fig.7b , Fig.7c , Fig.7d , Fig.7e ). 

The ratio between dry weight of the aerial part and root portions of the plants increased with age at about the same pace until plants reached about three months of age, when the aerial portion showed a significant increase over the root portion. 

Physiological parameters. Data taken 63 days after planting indicated that Calabar and La Mé had negative values with regard to biomass accumulation ( Fig. 8 ). This could be due to the use of seed reserves for initial growth. Aside from this, it must be considered that sampling was destructive, so different plants were used in each measurement. However, the negative values of biomass increments for Calabar and La Mé may still indicate greater dependency of seed's nutritional reserves in these crosses. This fact was also evident in the relative growth index. Early growing crosses such as Yanbambi and AVROS cease to depend on seed reserves (which are also larger) earlier.

The ratio between total leaf area and total dry weight ranged between 70 and 80, from 77 days after planting ( Fig. 9 ). This period may be associated with an increase in photosynthetic activity, which occurred after approximately two months, when some leaves were completely open. The ratio between aerial and root dry weights tended to increase during this period. The ratio between root and total plant dry weights diminished with age, indicating an emphasis on aerial growth. 

Data from these commercial crosses indicate that plants should be transplanted from the prenursery to the main nursery when they are nearly three months old. Delaying this operation carries a real risk of causing more transplanting stress, which adversely affects growth in the nursery.

Comparing the growth of the different crosses became difficult since none of them showed absolutely clear tendencies that could be separated from the others. This was due in part to variations among plants, the small number of plants sacrificed, and the destructive method used. However, it was quite clear that Calabar and La Mé have a very similar growth pattern during the initial stages of growth. At the other extreme of vigorous growth were Yangambi and AVROS. Ekona was closer to the slower growing materials. The growth of all these five crosses during the prenursery stage is closely related to their growth as mature palms in commercial plantings.

An individual plant, considered showing the typical appearance of the group was selected to be photographed to illustrate both the aerial and root growth ( Fig.10 , Fig.11 , Fig.12 , Fig.13 and Fig.14 ). The numerical information of each growth variable in each date is shown in Table 2 , Table 3 , Table 4 , Table 5 , and Table 6 . 

References

ASD de Costa Rica S.A. 2000. Las variedades de palma aceitera producidas por ASD. Panfleto publicitario. 9 p.

Corley, R. 1976. Germination and seedling growth. In. Oil Palm Research. Corley, R., Hardon, J. and Wood, B. (Eds.). Elsevier Scientific Publishing Co. The Netherlands. pp. 23-36.

Hartley, C. 1988. The Oil Palm. 3d. Ed. Longman. UK.

Rajanaidu, N. 1997. Sourcing of oil palm DxP planting material. The Planter, 73 (853): 161-162.