Introduction

The crops for biofuels correspond to a wide range of plant species, that present differences in terms of productivity, oil quality, adaptability to climate change, and other ecosystem benefits, such as the ability increase or keep the fixation of carbon in biomass and the soil. The energy projections for Costa Rica anticipate that the country’s traditional energy sources will not be able to continue increasing after 2032. Therefore, it is necessary to promote research on plant species such as Jatropha curcas (jatropha), that being a plant from Mesoamerican origins with a high quality oil, has a considerable potential in the energetic field.

In Costa Rica, the National Biofuels Programme has the overall objective of developing a biofuel industry that contributes to the energy safety and efficiency, the climate change mitigation, the reactivation of the farming sector, and the local and national socioeconomic development (12). Their plan of action includes a proposal for an environmental sustainability model; which would allow the development of different agricultural resources for biofuel and biomass production. Therefore, the National Biofuels Programme (2008) includes a proposal of areas with farming potential for selected bioenergy crops; according to the soil type, altitude, as well as other guidelines for the linkage with industry, industrialization, commercialization, and marketing.

During the last 6 years, a multidisciplinary research group from the country’s state universities has developed systematic studies on the Jatropha curcas (jatropha) crop (7) for its commercial use as an oil source for biofuel production.

The studies on jatropha domestication have been performed in collaboration with businessmen and farmers that have been involved in farming jatropha, used as hedge rows or for the generation of small plantations to obtain seeds. One of the main limitations at a global level is the absence of improved high-performance varieties, as well as the lack of flower synchronization and the irregular fruit production.

Globally, there is a considerable interest in jatropha (Jatropha curcas) as an oleaginous plant, for its use as an energy source (1) (3) (5) (8) (12), since the good oil characteristics can be exploited for biodiesel production. The oil can also be used as a baseline for soap manufacturing. Additionally, the residue for seed pressing is considered a good fertilizer and can also be used for biogas production (4). This oleaginous plant species is characterized for its resistance to drought, it is a fast growing plant species, of easy propagation, and a short life cycle (1) (6) (7) (9). Its yield depends on plant-associated factors such as the distribution of the dry biomass; the female/male flower proportion; the weight and size of the seeds; the oil content of the seeds; the oil quality; toxicity; flower induction; and the flower synchronization. Other factors relate to the crop site, such as the number of branches, flowers, fruits and seeds (3). These factors are strongly influenced by the environmental and genetic effects, the adequate farming locations and the appropriate agricultural management techniques for the crop (8). In regards to the genetic effect, the origin assays and the studies using molecular markers have shown a low genetic variability (4) (3) (11) (2). The study aims to establish a pruning method that ensures the Jatropha curcas (jatropha) farmer, a potentially profitable crop.

The objective of this study is to analyze the effect of pruning as part of the agricultural management of Jatropha curcas on the floral expression.

Materials & Methods

The study was developed in the Estación Experimental Fabio Baudrit, found at La Garita, Alajuela, Costa Rica located at 800 msnm, starting during the month of October, 2013. The soil was mechanically prepared for farming (ploughed, racked and hilllocked). A total of 80 seeds for the Comayagua accession were planted, directly and with a 4 meters distance between rows and between plants. The irrigation was performed twice a week until January, 2014, and from February on, the irrigation was reduced to once a week until the end of the study. The dead plants were substituted by others, grown in the greenhouse, and which have same age. Additional jatropha plants were planted around the assay field, to eliminate the border effect. In the first development stages, a manual weeding was performed, but when the plants reached a height of 40cm, a scythe was used.

On February 2014, a bifactorial experimental design was established, with random complete blocks and 4 repetitions, and identifying each block as corresponded. Treatment 1 and 2 consisted of plants which were pruned; while treatments 3 and 4 presented unpruned jatropha plants (figure 1). From that day on, the height of the plants was measured and the respective blocks were pruned when the plants reached a height of 40cm. The same was done when the branches resulting from that initial pruning reached 50cm. After that, pruning was left until after the plant’s flowering and fruiting stages; when the plants received their first formation pruning, and were left with a maximum height of 1.30 m (figure 2). The number of inflorescences, female flowers, green and ripe fruits were counted; and from the harvested fruits, the number of seeds were also counted and their fresh and dried weight was also determined.

The data from the dates of the first and second semester of 2014 and 2015 presenting the highest yields of green fruit, inflorescences and female flowers was used for the analysis. The results obtained from the four treatments were evaluated though an analysis of variance (ANOVA) following a random block experimental design, in which it was evaluated whether there were any significant differences found between blocks (to discard any significance in the results due to this variable) and between treatments. When differences were found, a Tukey test with a significance of 0.05 was performed. In regards to growth, an ANOVA was performed once again for each date, to define the differentiation between treatments in terms of growth. All the statistical analysis was performed using the software STATISTICA 9.0.

Results & Discussion

The plants’ growth rate was of 1 cm per week in the initial stage, with flowering occurring 7 months after planting, which was performed in November, 2013.

No significant differences were found in the growth of the jatropha plants through time; however the growth was not evaluated during 2015, since after performing the plant pruning, there is a loss in the growth rate since this plants must start their growth from the point where they were pruned (figure 3).

In figure 4, below it is possible to observe the average number of inflorescences per plant, according to the treatment; where during the first two harvests, Treatment 1 and 2 did not present any inflorescences, due to the fact that pruning leads to stress and the plant development focuses on branch and leaf growth, rather than generating inflorescences.

Based on the results observed, it can be stated that the inflorescence production from May 2015 was higher than the one reported in August of the same year; but Treatment 1 and 2 increased their yield.

The analysis of variance performed with the data from the dates which presented the maximum inflorescence productivity, is shown in table 1, below.

The pruned plants produced very few fruits in their first harvest; however the plants from Treatments 3 and 4, which were not pruned, produced more female flowers and more fruits, since their growth was not interrupted (figure 5).

The data analysis showed that the inflorescence production only presented significant differences between treatments, and not between blocks, during the first two flowering periods (table 2). It is evident that during the first growth stages, the unpruned plants produce a greater number of inflorescences per hectare, but as the plants grow, this difference becomes less obvious. This change can be due to the fact that the plants have the required nutrients accumulated through the photosynthesis, which provide vigor to the first leafs; it was determined that on average 27 leafs per branch were sufficient for the plant to achieve the development of the first floral sprouts.

Figure 5 shows the average female flowers produced per plant, where it can also be observed that the pruned plants do not present female flowers in the first year of growth; but in the second year, there is an increase in the number and they exceed the unpruned plants.

The behavior of the plants and their response to the treatment vary in terms of the emergence of female flowers through time; therefore, significant differences between treatments were found in the first two, as well as the last date of flowering, but not on the third date assessed, as shown table 2.

The production of green fruit behaved similar to the production of inflorescences and flowers, where in the first year the pruned jatropha plants did not present a considerable fruit production, but in the second year the production was higher than that of the unpruned plants (figure 6).

In regards to the fruit production, there are significant differences found between treatments, but not between blocks (table 3). These differences vary along time, since the pruned plants response starts to be observed in the second year of growth.

Figure 7 shows the values of mean monthly rainfall (mm/month) and average solar radiation (MJ/ m²), obtained from the meteorology station located at the Estación Experimental Fabio Baudrit of the Instituto Meteorológico Nacional (IMN). As it can be observed, the values of higher solar radiation occur in the months of February and March, while for 2015, these values are found during March and April. For the rainfall, a reduction was evidenced in the months of July, 2014 and on January and March of 2015; while a considerable increase in rainfall took place on September, 2014 (figure 7).

Acknowledgement

The authors would like to acknowledge PhD. Charles Aker, as the general project manager, for his technical support and the arrangements in the search for external funding, with the proposal presented for Costa Rica, Mexico, Nicaragua, Colombia and Brazil, named: “Regulation of the flowering of Jatropha curcas to improve the sustainability of biofuel feedstock production by farmers in Latin America and the Caribbean” and approved by LAC-Brazil.

The authors would also want to express their gratitude to PhD. Werner Rodríguez, Director of the Estación Experimental Fabio Baudrit and Engineer Jesús Hernández (MAG-UCR Agreement), for their technical and logistics assistance in the execution of the assays.

References

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