Oil Palm Root Study Using the Mini-Rhizotron Technique

Transcription

1 Oil Palm Bulletin 48 (May 2004) p Oil Palm Root Study Using the Mini-Rhizotron Technique Oil Palm Root Study Using the Mini-Rhizotron Technique Mohd Haniff Harun* and Mohd Roslan Md Noor* ABSTRACT The mini-rhizotron technique was used to study oil palm root turnover and longevity. Preliminary data collected over a three-month observation period was not sufficient to determine oil palm root longevity since the total root length was not constant. Root length replacement was high in tertiary roots about 11% more than secondary roots. Root observation has to be done over a long extended period in order to take account of the changing season that can affect root turnover and longevity. Further work is needed to determine the optimal number of observation tubes to represent the soil bulk proliferated by oil palm roots. ABSTRAK Teknik mini-rizotron telah digunakan untuk mengkaji hayat dan penggantian akar sawit. Maklumat awal bagi tempoh pemerhatian tiga bulan didapati tidak cukup untuk penentuan hayat akar sawit kerana jumlah panjang akar adalah tidak tetap semasa cerapan. Penggantian panjang akar, iaitu penggantian akar tua oleh akar muda, adalah tinggi pada akar tertiari sebanyak 11% berbanding dengan akar sekunder. Cerapan perlu dilakukan pada tempoh yang panjang bagi mengambil kira kesan perubahan cuaca yang akan mempengaruhi hayat dan penggantian akar sawit. Penambahan bilangan tiub pemerhatian serta lokasinya perlu dikaji bagi memastikan ia cukup untuk mewakili kawasan tanah yang diterokai oleh akar sawit. Keywords: oil palm roots, mini-rhizotron, root turnover, root longevity. INTRODUCTION Roots, which are often referred to as the hidden half of plants, have an important role in crop development and adaptation to difficult environments. They not only anchor the plants in the soil, but are also involved in the uptake of nutrients and water. Understanding oil palm root dynamics is important in order to comprehend root interaction with the environment and crop yield. For several crop species, up to 40% of the net carbon fixed is allocated below ground (Gorissen, 1996; Gregory et al., 1996). Quantification of root growth, death and subsequent decomposition, is essential for estimation and modelling of assimilate partitioning. Root turnover is not only a source of energy input into the soil but is also related to ecological processes, i.e. environmental adaptation of plants, nutrient acquisition, plant competition, interactions of plants with soil organisms, and soil structure and formation. Root turnover can be considered as a trade-off between the cost of maintaining old roots, versus shedding roots and growing new ones. Maintaining roots that are inefficient in acquiring water or nutrients is a disadvantage. Root turnover is also affected by the competition for carbon amongst various plant parts. Thus, root life span can vary because of differences in plant genotype, soil or management practices. Caldwell (1977) suggested that at least 25% of the root system die and is replaced annually, accounting for as much as 50%-80% of net production for a variety of perennial plant systems. Roots differ widely in their life span and there seems to be little relationship between the longevity of roots and the plants on which they grow. Some fine lateral roots on trees live only a week or two, whereas other roots live as long as the plant. In general, a large amount of assimilates is used in the replacement of shortlived roots. Some of the causes of this short life are heredity, pest attacks (e.g. nematodes, small *Malaysian Palm Oil Board, P. O. Box 10620, Kuala Lumpur, Malaysia. 17

2 Oil Palm Bulletin 48 insects and fungi) and unfavourable conditions (e.g. drought or flooding). Conventional root sampling techniques, like coring, are destructive, and due to the large spatial variability, a very large number of replicate samples are necessary in order to follow the development of a root system. They are also labour intensive and not always feasible. The mini-rhizotron system is a viable and promising alternative to core sampling for oil palm root study. Measurements of the dynamics of root growth and decay can be done by repeated observations of the small soil surfaces exposed in mini-rhizotrons, well distributed over the soil profile. Root growth and development are monitored using observation tubes that are buried in the soil. Root images can be photographed using a telescopic macro lens camera system. The images may be translated into digital pictures to quantify root measurements with a computer (Smucker et al., 1987) or counted with a grid on a monitor or photograph (Vos and Groenwold, 1987). In order to implement the technique, the immediate surroundings of the observation tube have to be representative of the soil bulk. Currently, information on oil palm root turnover and root life span under Malaysian soil conditions is very limited. This study investigates the use of the mini-rhizotron technique for determining oil palm root turnover and root life span. MATERIALS AND METHODS The mini-rhizotron technique used was modified from the system developed by Pratiknyo and Wagimin (1997) on rubber trees in Indonesia. Observation tubes were made out of 140 cm long by 8 cm diameter PVC pipes that were installed in the soil at the study site (Figure 1). A window of 8 cm wide by 112 cm long was made in each observation tube. Soil was prevented from collapsing into the tubes by inflating a modified motorcycle inner-tube placed inside the observation tube. The modified motorcycle innertube was removed from the tube during root observation. In this study, observation tubes were buried at three positions around a nine-year-old oil palm at Bangi, Selangor, i.e. within the weeded circle, in the middle of the inter-row and at the frond pile (Figure 2). Each tube was installed at an angle of about 35 o to the soil surface, facing the palm studied. An end-cap was used to cover the exposed tube opening. A clay garden pot was also placed over the tube end as an additional protection against damage during bunch harvesting and maintenance activities. Root observations were made using an endoscope constructed out of a rigid aluminium tube of about 7 mm diameter, with a mirror at one end and a camera system (Nikon FM2 with a 300 mm lens and 2x teleconverter) attached to the opposite end (Figure 3). A 6-volt lamp and the camera flash lamp were placed near the mirror to aid focusing and photographing the root image. A camera tripod was used to support the weight of the endoscope system during root observation (Figure 4). A simple mechanical method was included for automatically registering the observation depth within the mini-rhizotron (Poelman et al., 1996). The attached measuring tape unrolls as the endoscope is inserted into the ground, which enables the depth to be recorded on the image (Figure 5). The tape scale captured on the photograph was also used in calibrating the root lengths. Root observations were made at weekly intervals and root images were collected at 5 cm intervals along each observation tube window. Visual inspection of a series of photographs taken at different time intervals from the same observation tube at the same soil depth, allows quantification of the new root lengths and the lengths of roots that had disappeared since the previous observation. Following de Willigen and van Noordwijk (1987), the root length replacement ratio for a growing season (r l ) and the turnover of root length during a growing season (t l ) can be defined as: r l = L n (e) / L r (e), t l = L d (e) / L t (e), where, L n (e) = cumulative length of new roots since the start of the growing season (cm/cm 2 ); L r (e) = standing root length in the observation plane at the end of the growing season (cm/cm 2 ); L d (e) = cumulative length of dead and/or disappeared roots at the end of the growing season (cm/cm 2 ); and L t (e) = cumulative length of roots observed during the growing season (cm/cm 2 ). Root images collected over a three-month period were used for calculating the root length replacement ratio and turnover. Root branching and diameter were used to identify the oil palm 18

3 Oil Palm Root Study Using the Mini-Rhizotron Technique Figure 1. Mini-rhizotron observation tube made from 8 cm diameter x 1.4 cm long PVC tube with a 8 cm wide x 112 cm long window; motorcycle rubber inner tube with a 1 m wooden stick inside and both ends sealed. Figure 2. Placement of mini-rhizotron observation tubes at the study site: a=weeded circle, b=frond pile and c=inter-row. 19

4 Oil Palm Bulletin 48 Figure 3. Endoscope system modified from Poelman et al. (1996): 1=SLR camera with 300 mm telephoto lens and 2x tele-converter, 2=7 cm diameter aluminium tube, 3=12 volt halogen lamp and flash lamp, 4=mirror at 45 angle, 5-measuring tape and 6=observation aperture. Inner tube Tripod Endoscope Battery Observation tube Figure 4. Root endoscope being used to make root observations in the field. 20

5 Oil Palm Root Study Using the Mini-Rhizotron Technique Depth scale Location and date label Flash bulb Secondary root Figure 5. Oil palm root image taken with the root endoscope. root classes. Root classes were determined according to their diameter, i.e. primary roots with diameter of 6.0 mm, secondary 2.0 mm and tertiary 1.0 mm (Tinker, 1976). Root length was determined by the intersection method (Tennant, 1975). RESULTS AND DISCUSSION The first root image was observed a month after installation at the frond pile. Preliminary results of the root length replacement ratio (r l ) and turnover (t l ) obtained over a three-month period are shown in Table 1. Both the r l and t l ratios were expressed on a per week basis. The root length replacement ratio obtained for primary roots was and for secondary roots No data were available for the tertiary and quaternary roots. The r l ratio for perennial crops gives information on the average longevity of individual roots, if the standing root length is constant from year to year (de Willigen and van Noordwijk, 1987). However, the r l ratio obtained cannot be used to determine the oil palm root longevity because the standing root length was not constant over the three-month observation period. Root length turnover obtained for secondary roots was and for tertiary roots No data were obtained for the primary roots as no dead primary roots were observed. The root length turnover of tertiary roots was about 11% greater than secondary roots. The more rapid replacement of tertiary roots indicates their importance in absorption of nutrients and water. Death of older fine roots and their replacement represents a considerable energy cost to the plant and has been shown to correspond to the availability of soil water and nutrients (Huck et al., 1987; Smucker, 1993). The data collected over the three-month period does not reflect the annual seasonal variation, i.e. wet and dry seasons. Therefore, data collections over a more extended period are needed to determine annual oil palm root longevity and turnover. Further work is required to determine the number of observation tubes and their locations around the palm needed in order to give results representative of the bulk soil. The root camera design also needs a few improvements, such as reducing its diameter (<7 mm), designing a rigid support to hold the root camera, and installing a better flash unit. CONCLUSION The preliminary results obtained from the minirhizotron study show that the weekly root length turnovers of secondary and tertiary oil palm roots were about 17% and 19%, respectively. This implies that the death of old feeder roots (i.e. secondary, tertiary and quaternary roots) and their replacement may impose a significant energy drain to the oil palm in terms of dry matter partitioning. Further data collection over 21

6 Oil Palm Bulletin 48 TABLE 1. PRELIMINARY RESULTS OF MINI-RHIZOTRON OBSERVATIONS: ROOT LENGTH REPLACEMENT RATIO AND ROOT LENGTH TURNOVER. BOTH ARE EXPRESSED ON A PER WEEK BASIS FROM STUDIES OF A NINE-YEAR-OLD OIL PALM AT MPOB RESEARCH STATION, BANGI, SELANGOR Root class Root Cum. length Standing Root length Cum. length diameter of new roots root length replacement of dead roots (mm) (cm cm -2 ) (cm cm -2 ) ratio, r l (cm cm -2 ) Cum. length of roots Turnover of observed root length, t l (cm cm -2 ) Primary n a n a n a Secondary Tertiary 1 n a n a n a Note: na: insufficient data/not observed. an extended period is required in order to obtain the annual rate of root turnover for oil palm. The mini-rhizotron technique was suitable for determining the root turnover and longevity of oil palm roots under field conditions. ACKNOWLEDGEMENT The authors would like to thank the Director- General of MPOB for permission to publish this paper. We also like to thank the Crop Physiology staff, especially to Mr Maurad Ahmad and Mr Abdullah Badrishah for their excellent technical assistance and to the late Mr Mohd Ariffin Jurmi, the Manager of MPOB Research Station, Bangi, for his cooperation. Comments from Dr I E Henson were very much appreciated during the preparation of this paper. REFERENCES CALDWELL, M M (1977). Root structure: the considerable cost of below ground function. Topics in Plant Population Biology (Solbrig, O T ed.). Columbia University Press, New York, USA. p de WILLIGEN, P and van NOORDWIJK, M (1987). Roots, Plant Production and Nutrient Use Efficiency. Ph.D thesis, Agricultural University Wageningen. 282 pp. GORISSEN, A (1996). Elevated CO 2 evokes quantitative and qualitative changes in carbon dynamics in a plant/soil system: mechanisms and implications. Plant and Soil, 187: GREGORY, P J; PALTA, S A and BATTS, G R (1996). Root systems and root: mass ratio carbon allocation under current and projected atmospheric conditions in arable crops. Plant and Soil, 187: HUCK, M G; HOOGEMBOOM, G and PETERSON, C M (1987). Soybean root senescence under drought stress. Mini-Rhizotron Observation Tubes: Methods and Applications for Measuring Rhizosphere Dynamics (Taylor, H M ed.). American Society of Agronomy, Madison, WI, USA. p POELMAN, G; van de KOPPEL, J and BROUWER, G (1996). A telescopic method for photographing within 8x8 cm mini-rhizotron. Plant and Soil, 185: PRATIKNYO, P S and WAGIMIN, A W (1997). Mini-rhizotron plus endoscope: a tool to observe root growth and decay. International Workshop on Biological Management of Soil Fertility on Acid Upland Soils in the Humid Tropics. Brawijaya University, Malang, Indonesia July p SMUCKER, A J M (1993). Soil environment modifications of root dynamics and measurement. Annual Review of Phytopathology, 31: SMUCKER, A J M; FERGUSON, W P; DEBRUYN, W P; BELFORD, R L and RITCHIE, J T (1987). Image analysis of video recorded plant root systems. Mini-Rhizotron Observation Tubes: Methods and Applications for Measuring Rhizosphere Dynamics (Taylor, H M ed.). American Society of Agronomy Spec. Publ., Madison, WI, USA. p TENNANT, D (1975). A test of a modified line intersect method of estimating root length. J. Ecol., 63: TINKER, P B (1976). Soil requirements of the oil palm. Developments in Crop Science (I): Oil Palm Research (Corley, R H V; Hardon, J J and Wood, B J eds.). Elsevier Scientific Publishing Company, The Netherlands. p

7 Oil Palm Root Study Using the Mini-Rhizotron Technique VOS, J and GROENWOLD, J (1987). The relation between root growth along observation tubes and in bulk soil. Mini-Rhizotron Observation Tubes: Methods and Applications for Measuring Rhizosphere Dynamics (Taylor, H M ed.). American Society of Agronomy Spec. Publ., Madison, WI, USA. p