Long term performance of certain ortets and hybrid clones of Hevea brasiliensis in a high altitude region in Kerala , India

Long term performance of four hybrid Hevea clones and ten ortet selections including nine from a high elevation site within the traditional rubber growing tract of Kerala was studied. Significant variability was observed among the hybrids and ortets for all major agronomic traits. Growth of rubber trees was invariably poor in the high altitude region and tappability by the tenth year of planting was less than 50 per cent. The hybrid clones RRII 203 and RRIC 100 showed highest growth vigour with 80 per cent tappability by the 13th year of planting. Among the ortets, P 270 and Iritty 1 recorded the highest tappability of 64 per cent. The ortet P 270 was the best performer with the highest girth at the opening (61 cm), girth increment during immaturity (5.5 cm) on tapping (3.4 cm) and bole volume (0.1 m3). RRII 203 was the highest yielding clone with 56 g tree-1 tap-1 over eight years of tapping. High annual yield (48 g tree-1 tap-1) and lean season yield of P 270 combined with high drc on par with the highest yielding clone RRII 203 indicated the specific adaptability of this ortet to high elevation areas. The ortets P 213 and Iritty 1 and hybrid clone RRIC 100 were the other promising clones exhibiting growth adaptation and high yield potential under high altitude conditions.


Introduction
The traditional rubber growing belt in peninsular India is situated at altitudes up to 300 m MSL. However, due to the compelling demand for area expansion, the species is now extended to the tropical high altitude areas in the traditional region as well as subtropical highlands and high latitude areas of the non-traditional region in North East India. The variable productivity due to the spatial distribution of the crop within the traditional region was earlier discussed by Vijayakumar et al. (2000) and Nair et al. (2010). Rubber cultivation at high altitudes is normally not advisable due to multiple constraints including low-temperature stress, irregular rainfall pattern, high wind velocity, low humidity during summer months, disease epidemics and altered soil physical properties (Priyadarshan et al., 2005). Chattopadhyay (2015) highlighted the physiographic and ecological limit for growing rubber in Wayanad district, a high elevation area (>900 m above MSL) within the traditional rubber growing tract of India. Gahlod et al. (2017) have identified 32.5 per cent of the area comprising 69158 ha as marginally suitable for cultivation whereas 35 per cent of the area was observed to be unsuitable for rubber in this region. They also observed that marginally suitable areas for rubber cultivation could be brought under moderately suitable areas by adopting strategies to combat chilling stress and associated stress factors. At the same time, the area under rubber in Wayanad has shown a progressive increase over the last few decades (Karunakaran, 2013). This situation warrants identification of specifically adapted clones suitable to high altitude regions for profitable exploitation of the crop.
Recently, to formulate a region-specific recommendation of Hevea clones, the traditional belt of South India was further divided into seven regions of which tropical high altitude region of Wayanad and Idukki has been treated separately . These areas are characterised by high altitude above 450 MSL, heavy South West monsoon with a mean annual rainfall of 4000 mm, very low temperature in January with over-hanging mist favourable for high powdery mildew infestation, which is followed by a dry spell of four months. Clones that can withstand low temperature and tolerance to powdery mildew will only be suitable for cultivation to sustain growth and yield in the region. The present study is the first step in this direction in Wayanad.
Screening of old seedling trees (ortets) in a small village Panamaram in Wayanad, Kerala State, India a high elevation site, was undertaken in the early 1990s based on growth, yield and tolerance to Oidium heveae. Another ortet selection from North Kerala, i.e., Iritty 1, was also identified during the same period. These ortets were further evaluated in Ambalavayal in Wayanad district (>900 m above MSL), along with four hybrid clones viz., RRII 105, RRII 203, RRIC 100, RRIC 102 and PB 86 a primary clone (ortet). The performance of these clones with respect to immature growth, test tap yield in the early mature phase and incidence of powdery mildew disease has already been reported by Lakshmanan et al. (2006). The results of long term evaluation of these clones over 21 years are discussed in this paper to identify clones with high yield and desirable secondary attributes suited to high altitude farming.

Layout and field upkeep
A small scale evaluation trial was laid out in 1995 in the Regional Research Station of the Kerala Agricultural University, Ambalavayal, Wayanad district (latitude 11 o 37'N, longitude 76 o 12'E; 974 m above MSL) Kerala State, India. The minimum temperature in the region ranged from 14 to 21 o C and maximum from 21 to 31 o C. The experiment was laid out in a randomised complete block design with three replications, comprising of ten ortet selections viz., P 1, P 2, P 90, P 121, P 155, P 213, P 270, P 280, P 296 and Iritty 1 and five popular clones viz., RRII 105, RRII 203, RRIC 100, RRIC 102 and PB 86. Out of the ten ortets, nine were from Panamaram in Wayanad and one from Iritty, Kannur district (Iritty 1). The details of test clones are given in Table 1. All cultural operations were undertaken as per the recommended package of practices (Rubber Board, 2019).

Growth measurements
The girth was monitored annually from the 4 th year after field planting, by measuring the trunk girth at 125 cm height from the bud union. Annual girth increment for the immature and mature phases was calculated from annual girth data. The percentage of tappable trees attaining a girth of 50 cm at a height of 125 cm from the bud union was recorded during the 10 th year of planting. At the end of the study period, i.e., 21 years after field planting, clear bole height was measured as the distance from the bud union to the first forking. Clear bole volume was calculated from the data on bole height and corresponding girth of trees following the method given by Chaturvedi and Khanna (1982).

Dry rubber yield and yield components
Trees were opened for tapping in the 10 th year of planting under the S/2 d3 6d/7 tapping system. Tree wise yield was recorded by cup coagulation method (Nair et al., 2012), and dry weight of cup lumps was expressed as grams per tree per tap (g tree -1 tap -1 ). The yield reduction in summer was calculated by deducting the lean season yield (Feb-May) from the annual yield and expressed in percentage. Monthly variation in yield was plotted from yield data over the first eight years of tapping. The volume of latex (mL tree -1 tap -1 ) and dry rubber content (%) was determined on a seasonal basis during lean and peak yielding period. Disease incidence was monitored, and preventive measures were adopted whenever essential. Integrated use of systemic and non-systemic fungicides was used to avoid the chances of building up resistance in the pathogen against fungicides (Edathil et al., 1998). Analysis of variance for all the characters was worked out as per the standard procedure and the observed differences among different treatments compared based on CD values.

Results and discussion
Ortet selection involves systematic screening of vast seedling plantations obtained through natural genetic recombination, identification and multiplication of elite trees, followed by field evaluation and selection of outstanding genotypes. Clones developed through ortet selection are called primary clones. In India, from the early screening of high yielding mother trees, RRII 5 was a promising ortet for latex and timber yield, and RRII 33 was noted for its tolerance to abnormal leaf fall (Marattukalam et al., 1980). Later, several ortets were added to the pipeline (Mydin et al., 2005;Mercykutty et al., 2013;John et al., 2013). The present ortets selected from Wayanad are relatively recent.

Girth and tappability
The percentage of tappable trees assessed from the 10 th year of planting revealed wide clonal variation in the number of trees attaining tappability. RRII 203 attained a tappability of 53.3 per cent by 10 th year after planting, followed by RRIC 100 (46.7 %). The other hybrid clones and ortets failed to attain tappable girth of 50 cm by 10 th year. By the 13 th year of planting, the hybrid clones RRII 203 and RRIC 100 attained the highest tappability were the other clones with a high girth of 72 cm. Girth increment (GI) rate before tapping varied from 5.5 cm (P 270) to 3.3 cm (P 90). Five ortets from Panamaram, Iritty 1, RRII 203 and RRIC 100 showed GI at immaturity on par with P 270. The trend of high GI at immaturity for P 270 was continued in the mature phase also with the clone exhibiting the highest GI rate on tapping (3.4 cm) followed by P 280 and P 213. The GI on tapping of all the hybrid clones was poor in the region, except RRII 203 and PB 86 ( Table 2). The growth trend of the ortets and hybrids for a long term period of 21 years is plotted in Figure 1. Among the hybrid clones, the differences in girth were not significant until the 6 th year after planting. The girth of ortets P 270, P 90, P 296, P 1, P 213 and Iritty 1 was comparable and on par with that of the hybrid clones till the 6 th year. Among the popular clones, RRII 105 recorded the lowest girth. Even though RRII 105 is a highly adapted high yielding clone for the traditional tract, its growth and yield reduction under cold stress was reported (Sethuraj et al., 1989;Reju et al., 2007). From the 10 th year after planting the ortet, P 270 showed the highest girth of 49.4 cm and was closely followed by RRII 203 (48.7 cm) and RRIC 100 (48 cm). Overall, P 270, closely followed by RRII 203, maintained the best growth throughout the study period. Compared to ideal growing conditions, relatively poor growth and high clonal variability for growth has been reported from other abiotic stress-prone areas also .
Crotch height is yet another highly variable trait. The clear bole height of trees usually varies from 4-5 m. In the present study, irrespective of clones, a lower first forking height ranging from 2.2 m to 2.9 m was noticed. However, timber yield in terms of clear bole volume showed significant clonal variation (Table 2). Highest bole volume was recorded by P 270 and RRII 203 (0.1 m 3 ). P 213 and P 280 were ranked the second best with 0.08 m 3 . RRII 203 is categorised as a latex timber (LT) clone (Thomas et al., 2003) in the traditional region which has also recorded vigorous growth in the cold prone high altitude areas of non-traditonal NE India (Reju et al., 2007). High variability in bole volume was reported within the traditional region itself. The conducive climatic conditions in Kanyakumari region promoted better growth and high timber output, whereas Padiyur region in Kannur district of North Kerala faced with seasonal drought produced the least timber yield . The average bole volume of the clones evaluated in the high elevation site in this study is much lower than that observed in other parts of Kerala.

Yield and yield components
Mean dry rubber yield in the two virgin panels along with mean yield over eight years of tapping and lean season yield is presented in Table 3. Yield in the BO-1 panel varied from 19.3 g tree -1 tap -1 (RRIC 102) to 57.8 g tree -1 tap -1 (RRII 203). Three ortets P 213, P 270 and Iritty 1 along with RRIC 100 recorded yield on par with RRII 203. However, in the BO-2 panel PB 86 and RRIC 100 were the high yielders with 67-63 g tree -1 tap -1 . P 213, P 270 and Iritty 1 maintained a rising trend in yield whereas RRII 203 recorded a slight drop in yield in the BO-2 panel. RRII 203 recorded the highest yield of 56.6 g tree -1 tap -1 over eight years of tapping. Three ortets P 213, P 270 and Iritty 1 along with RRIC 100 and PB 86 recorded yield in the range of 45.4 to 53.6 g tree -1 tap -1 which was on par with RRII 203. Three ortets viz., P 213, P 270, Iritty 1, and two hybrids viz., RRIC 100 and PB 86 exhibited a rising trend in yield whereas RRII 105, RRIC 102 showed a declining trend in yield over the years. Yield during the summer months also showed a similar trend. RRII 203 recorded the highest lean season yield (46 g tree -1 tap -1 ). Three ortets P 213, P 270 and Iritty 1 along with RRIC 100 and PB 86 recorded summer yield on par with RRII 203. RRII 203 is a proven hardy clone combining high growth vigour with moderately high yield in the traditional region . The specific adaptation of RRII 203 to high altitude areas has also been reported earlier (Reju et al., 2007). RRIC 100, a high yielding clone from Sri Lanka, was reported to be cold susceptible in the high latitude areas of sub-Himalayan West Bengal (Meti et al., 2003). However, the average minimum temperature in Wayanad does not go below 15 °C to adversely affect the growth and yield of trees as compared to the very low-temperature stress experienced in Northern West Bengal. PB 86 is one of the oldest primary clones and is a prime progenitor of many of the modern-day clones (Priyadarshan, 2003a) including RRII 203. Monthly variations in yield were more predominant during the wet months (Fig. 2). The yield pattern of high yielders and low yielders were distinct and consistent throughout the year. The high yielders including hybrids PB 86, RRII 203, RRIC 100 and ortets Iritty 1, P 270 and P 213 showed significantly higher yield compared to low yielders. An increasing trend in yield was observed for all the clones/ortets from May with peak yield in July followed by a declining trend except for RRII 105 and RRIC 100 which showed a peak yield in June followed by a declining trend thereafter. The second peak in yield was observed during November for all clones/ortets. Meteorological data from the study site (Lakshmanan et al., 2006) indicated that the yield trend observed coincide with the rainfall pattern of the region. The decline in yield was noticed during the winter period (November to February) for all clones and ortets, which may be attributed to the elevation induced low winter temperature stress coupled with the onset of wintering. According to Priyadarshan (2003b), temporal yield variations in stress-prone areas are more prominent due to the influence of fluctuating weather variables on specific adaptation.
The volume of latex and dry rubber content (drc) per cent are among the major component traits contributing to high rubber yield. Iritty 1 and RRIC 100 were the best performers in terms of the total volume of latex (258 mL tree -1 tap -1 ) ( Table 4). P 213, P 270, RRII 203 and PB 86 recorded latex volume on par with the superior clones. Dry rubber content of clones varied from 30 per cent (RRII 105) to 41.5 (P 270) and clonal variations was significant. Except for P 121, P 155, RRII 105 and RRIC 102, all the ortets and hybrids recorded relatively high drc.

Fig. 2. Seasonal variation in yield
Overall performance of clones was assessed by a rank-sum method (Kang, 1988), based on mean values for growth, annual yield, and yield components, to identify elite clones (Table 5). P 270 emerged as the top-ranking clone with superior performance for major agronomic traits. It is noteworthy that P 270 registered vigorous growth comparable to the modern clone RRII 414 and its yield was on par with RRII 105, in a clonal nursery evaluation in Central Kerala (Meenakumari et al., 2018) indicating the phenotypic plasticity of this clone. Two ortets, P 213 and Iritty 1 and hybrids RRII 203 and RRIC 100 were the other top-ranking clones. Of these, Iritty 1 is reported to show a high degree of tolerance to powdery mildew disease, whereas P 213 and RRIC 100 showed good disease control after prophylactic treatment. RRII 203 and P 270 were, however, found susceptible to powdery mildew disease (Edathil et al., 2011). It appears that the interaction of multiple stress factors affects the long term performance of the clones in the high altitude region of Wayanad. Further, large scale/onfarm evaluation of the selected ortets and hybrids is however essential to bring out the best clones suited to the region.

Conclusion
The effect of high elevation induced lowtemperature stress varied among the 15 clones under study with respect to growth and latex yield. Two ortets P 270 and Iritty 1 and two-hybrid clones RRII 203 and RRIC 100 were selected for high yield and growth adaptation under the agroclimatic conditions in Wayanad. Even though ortet P 213 recorded high mean yield, the clone showed low tappability indicating high growth variability. PB 86 was a high yielder in the region with medium girth. The promising performance of P 270 in Central Kerala region also renders adaptive plasticity for this clone. The top-ranking clones may be considered for further large scale/on-farm evaluation in the region.