Introduction
The world demand for woody biomass for energy generation is increasing rapidly (Rakos 2008; Spelter and Toth 2009; Sikkema et al. 2011). Woody biomass from short-rotation crops can contribute to secure renewable and sustainable energy around the world owing to their potential to produce high biomass in short time periods, especially in tropical countries with plentiful rain (Hendrati 2016). Many fast-growing species have high wood quality for energy and an ability to re-sprout for multiple harvests, which is important for economic success. Multipurpose species provide multiple environmental and rural development benefits (Singh et al. 2010) and with genetic improvement, further improvement in yield and efficiency of production are anticipated. Studies on genetic improvement of Calliandra calothyrsus for wood energy indicated high heritability value of wood volume (h2 = 0.5) and an increased yield of 75% for wood volume (Hendrati 2016). This paper describes research on the genetic improvement for wood energy of Leucaena leucocephala, which is self-fertile and therefore less variable than Calliandra calothyrsus, which is not self-fertile. Nevertheless, as 2 subspecies of Leucaena leucocephala (ssp. glabrata and leucocephala) are present in Indonesia, there is some ability for outcrossing, so genetic gain is achievable.
Materials and Methods
Genetic improvement of L. leucocephala ssp. glabrata (imported leucaena) was initiated by collecting genetic material from 10 established orchards, including cv. Tarramba, during the period 2015‒2016. They were from Subang and Majalengka (West Java), Brebes (Central Java), Sleman, Bantul and Kulon Progo (DIY), Bangkalan Madura (East Java), Bali (Bali Island), Menado (North Sulawesi) and Kupang (East Nusa Tenggara). At most sites, the species was grown by villagers for forage, fuelwood and human consumption (seed). Leucaena leucocephala ssp. glabrata was preferred over common local leucaena (L. leucocephala ssp. leucocephala) because it has better growth and is more tolerant of psyllids (Heteropsylla cubana) than the more susceptible ssp. leucocephala. The range in elevations from which these samples were collected was 0‒500 masl and precipitation ranged from 800 to 3,050 mm/yr. Open-pollinated half-sib seeds from 80 trees (considered as families hereafter), selected as the best performers in the orchards, were collected. Leucaena leucocephala is considered to be a cross-pollinating species but up to 10% selfing is known to occur. Consequently, the collected seeds were considered F1, although some seed may have resulted from self-pollination. A long-term breeding strategy was planned as shown in Figure 1. Progeny tests were established at 2 locations, Wonogiri and Brebes, Central Java (Table 1). This phase of the program is represented by the box ‘Progeny test F1 on 2 sites’. Distance between individual mother trees (families) within each population was 70‒100 m to avoid inbreeding. Seedlings in the nursery were measured for both stem diameter and height after 4 months and again 6 months after transplanting into the field. Biomass yield after 6 months in the field was estimated using a biomass index (BI; basal diameter2 × height; Stewart and Salazar 1992). Data from the nursery and from the field were analyzed by using analysis of variance and Duncan’s Multiple Range Test.
Early results
After 4 months of growth in the nursery, seedlings were ready for transplanting. At this time, variations in both height and stem diameter were obvious (Table 2). Bantul, Bali, Menado and Kulon Progo populations had diameters comparable with that of cv. Tarramba; for height, only the Bali population was similar to cv. Tarramba. The Subang population always recorded the lowest values for both characters.
1Means followed by the same letter are not significantly different (P>0.01) by Duncan’s Multiple Range test. Source: Hendrati and Hidayati (2018).
Variations between families were re-examined after 6 months in the field (Table 3). While there were significant differences between the 80 families for stem diameter, height and biomass yield (Table 3), results for the best 8 performers for each parameter were not significantly different (P>0.05) (Table 4). Tarramba did not fall within this group for stem diameter but 3 Tarramba families fell in the top 8 families for both height and biomass yield (Table 4).
Discussion
Environmental factors were relatively uniform in the nursery and in the field. Therefore, growth was assumed to be influenced more by genetic potential than by environmental conditions. Variations in terms of growth (diameter and height) both in the nursery and in the field were expected to optimize selection to achieve genetic gain during the improvement program. While some families showed promise in terms of diameter and others were outstanding in terms of height, wood biomass as indicated by the biomass index was most important and families, which scored well in this parameter, are of most interest. Some Indonesian populations and families were comparable with those from cv. Tarramba, which is known for its superior growth compared with other cultivars (Rengsirikul et al. 2011).
Significant correlations between heights of families in the nursery and in the field indicated that good height in the nursery might indicate good height in the field. Families with high ratings for biomass production will be evaluated in terms of wood volume and quality for energy at the appropriate age to supplement current growth assessments. Outstanding families will progress through the breeding program.