Physiological responses of abaca (Musa textilis) to light, water and nutrient availability
Zusammenfassung der Projektergebnisse
The present research was conducted to gain quantitative knowledge about the reaction of the growth, morphology and physiology of abaca (Musa textilis) to shading, irrigation, fertilisation and their interactions. The project involved a cooperation of the Visayas State University (Baybay, Leyte, Philippines), the University of Hohenheim (Stuttgart, Germany) and, later, the Bern University of Applied Sciences (Zollikofen, Switzerland). Abaca is a perennial herb native to the rainforests of Southeast Asia. It is grown for its fibres, which are characterised by high specific tensile strength. They are used for a variety of purposes, from banknote to compound material production. The Philippines are the most important supplier of abaca fibres, the market for which has grown substantially over the last decade, albeit with major fluctuations on the demand and supply sides. Being adapted to shady conditions, abaca is suited for integration into the early growth phase of agroforestry systems. The development and dissemination of such systems offers great potential for sustainable land use, since they can be both economically productive and valuable in terms of biodiversity and ecosystem services. In South-East Asia, they are a valid option for the rehabilitation of degraded Imperata cylindrica grasslands. However, while multispecies production systems have been developed and promoted in tropical countries for many years, knowledge about the physiology and interactions of the involved species remains patchy. This study was initiated to improve our understanding of the processes of light interception, leaf area expansion, photosynthesis and biomass allocation in abaca as affected by light, nutrient and water supply under field conditions. A field experiment was run during 2008 and 2009 in a farmer’s field close to Ormoc City, Leyte, Philippines. A 4x2x2 split-split-plot design was chosen, where shading was the main plot (unshaded, 30%, 40% and 50% artificial shading, in relation to photosynthetically active radiation), NPK fertilisation (with and without) was the subplot, and irrigation (with and without) the sub-subplot. Data collection included non-destructive measurements e.g. of plant allometry as well as sequential destructive harvests at three-month intervals. Crops took twelve months to reach maturity for fibre harvest. Analyses of variance revealed significant and positive effects of shading and NPK fertilisation on the values of growth parameters at final harvest, including total fibre dry mass (DM) and fibre length, pseudostem DM, root DM, leaf DM, aboveground DM, total plant DM, plant height, leaf area and rooting depth (shading only) of abaca. Supplemental irrigation had no significant effects on any of the measured parameters. The effects of shading and fertilisation on abaca growth mostly set on at or before three months after planting (not including growth in the nursery, where all plants were treated equally) and were consistently visible during the growth period. No significant interactions between the effects of light, water and nutrient supply on growth parameters were found. Unlike fibre DM and length, fibre tenacity and colour were not affected by any factor. Highly significant (p ≤ 0.01) positive correlations linked all components of abaca DM, plant height, rooting depth, leaf area, green leaf N content and green leaf P content. Highly significant negative correlations linked green leaf K content with root DM, leaf DM and leaf area. All correlations between growth parameters were consistently evident from three months after planting. The observed superior productivity of shaded abaca contradicts the findings of some studies, part of which were conducted in subtropical environments. It is in line with the practical experience of abaca growers, as well as several studies on Musa x paradisiaca, which also performed well at ca. 50% natural or artificial shading. Possible explanations include reduced photorespiration, and thus increased net primary production in shaded abaca, as well as photoinhibition in unshaded plants. Other hypotheses, e.g. growth limitation due to lack of nutrients or water, or changes in plant allometry due to shading, are not supported by our data. On the basis of this research, combined with knowledge about other Musa spp., the optimum degree of shading for the investigated abaca cultivar (Laylay) can be expected to be close to 50% under tropical conditions. At substantially higher levels of shade (67% or less), light saturation may not be attained. While shading by mature coconut and (dense) mixed tree stands exceeds 50%, this level can very likely be established in agroforestry systems during the first years of tree growth. The finding that shaded and properly fertilised abaca can yield more and longer fibres, without reductions in tenacity or colour, than crops grown in the open will be useful in the dissemination and optimisation of agroforestry systems including abaca. Such systems should not only contribute to conserving biodiversity and healthy ecosystems, but also to improved productivity and enhanced resilience of production to adverse market and weather conditions. This applies particularly to smallholdings in the Philippines, where abaca directly and indirectly contributes to the livelihoods of an estimated 1.5 million people.
