Root growth potential and seedling morphological attributes of narra (Pterocarpus indicus Willd.) transplants
Introduction
Inability to recognize seedling quality as a measure of planting performance in the field has led to the failure of many reforestation and plantation projects. Strategies for assessing quality of planting stocks need to move away from the traditional approach of looking only at the sound morphological characteristics of seedlings (Grossnickle et al., 1991). These morphological indices often fail to account for differences in seedling physiology (Gazal and Kubiske, 2004). Hence, holistic assessment of stock quality requires the integration of both morphological and physiological attributes of seedlings. Consideration of both factors provides a more effective appraisal of the fitness of seedlings for field planting.
Root growth potential (RGP = root growth capacity = root regeneration potential) is a physiological attribute that can be easily measured and used to assess seedling quality. It is defined as a gauge of the ability of a seedling to produce new roots when growing in an ideal environment (Ritchie, 1984). Knowledge of the RGP and familiarity with the plant root system is key to understand ecological fundamentals that influence seedling quality (Bohm, 1979).
The ability to absorb water is an important factor affecting the establishment of tree seedlings. Increased access to soil water is highly dependent on new roots produced after planting (Burdett, 1987, Colombo and Asselstine, 1989). Hence, the optimal level of water stress to favor the root growth of tree seedlings is important in setting the standard for using RGP as an indicator of planting stock vigor.
Narra is one of the most valuable and commonly used reforestation species in the Philippines. It is propagated by seeds and cuttings but seedling stocks are mainly used for reforestation and rehabilitation of denuded land areas (Rise, 1995). Because of its high wood quality, this species is an excellent source of timber and wood material for furniture in southern Asia. Narra grows throughout the Philippines and other tropical countries such as India, Borneo, Celebes, New Guinea and the Caroline Islands (Rise, 1995). As a nitrogen fixer, narra is well adapted to low nutrient and available water conditions. The potential for using RGP as an indicator of seedling quality and performance of narra needs to be explored. Hence, this study was conducted to evaluate two measures of RGP (number and length of new roots) and provide a basis for interpreting RGP as a seedling performance attribute of narra under different levels of soil moisture.
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Materials and methods
The study was conducted from September to November 1997 in a greenhouse at the University of the Philippines Los Baños (UPLB). Narra pods were collected from a stand located on Mount Makiling at the UPLB campus. Only mature pods from phenotypically desirable trees were collected. To facilitate germination and root formation, seeds from the pods were extracted and air-dried for a week. No pre-germination treatment was applied to the seeds.
The soil medium used in the seed boxes was composed of an
Results
Significant differences in diameter, shoot biomass, total biomass and quality index remained until the 14-day test period (Table 1). However, no significant differences in seedling morphological traits were evident at the 21-day test period (data not shown). During the 7-day test period, the number of new roots was highest in the 50 and 75% of field capacity treatments (Fig. 1a). The length of new roots was lowest in the 100% of field capacity treatment (Fig. 1b). Both root-collar and stem
Discussion
In this study, the relationship between number and length of new roots as measures of RGP behaved similarly in response to moisture regimes. Response of narra transplants to different levels of moisture can already be seen at the 7-day test period. This result suggests that narra transplants quickly respond to the limited amount of moisture as reflected in their RGP.
The immediate response of narra to a drought condition could be attributed to the nature of its root system. Narra produces many
Practical implication
Failure of many reforestation projects in the Philippines is primarily due to planting of poor quality seedling stocks and poor environmental and soil conditions of the land areas where the seedlings are planted. In such situation, careful selection and planting of only viable and vigorous seedlings are crucial to ensure high survival in the field. Thus, a simple and reliable nursery grading practice to monitor seedling quality is highly needed.
RGP measurement period for narra transplants need
Acknowledgements
We would like to acknowledge the National Research Council of the Philippines (NRCP) for the financial resources it provided to conduct the research. Also the Institute of Renewable Natural Resources, University of the Philippines Los Banos College of Forestry and Natural Resources for providing the facilities and equipment during the course of the experiment. To KARD-KC, Elmo Bado, Amelita Luna and Brenda Gazal for the help in data collection in the field and greenhouse. We would also like to
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