The relationship between long term yield of seven Hevea clones and climatic parameters such as minimum temperature (T_min), maximum temperature (T_max), sunshine hours (s), rainfall (R) and relative humidity [RH1 (morning) and RH2 (afternoon)] was studied under a typical tropical humid climatic regime. Of the seven genotypes, RRII 105 was significantly high yielding (68.0 g/t/t), while Gl 1 (40.5 g/t/t) and RRIM 600 (43.8 g/t/t) registered relatively low yields. Climatic parameters were regressed on yield. T_max was found to be the most important influencing factor with a negative relationship in each clone (slopes from -7.299 to - 3.353). T_max showed less monthly variability when compared to RH in the corresponding period. RH was seen to be low during the winter and pre-monsoon seasons. Yield was found to be the most important function of T_max (R²= 0.21). Most of the clones showed an increase in yield with time mainly during the monsoon season (slopes from 4.94 to 2.56 per year). The R² ranged from 0.30 (RRII 105) to 0.76 (RRII 203) during the monsoon season. The clone RRII 208 did not show any increase in yield, while RRIM 600 showed a varied response of yield with time over different seasons. The study indicated the moderate level of decrease in yield with increasing T_max. However, rising temperature in the long-term climate change scenario, the yield decline in rubber clones need to be evaluated further in large scale.

Water deficit stress will be one of the most important environmental factors that will affect crop productivity in the coming years and prolonged drought conditions will have adverse effects on photosynthesis and growth of plants. In this study, five modern Hevea clones and three check clones were compared for their intrinsic tolerance to water stress conditions. The results indicated that the clone RRIM 600 grown in open condition had the least decline in CO₂ assimilation rate followed by RRII 430, fifteen days after drought imposition while RRII 414 was found to be severely affected. Similar trend was noticed in the glasshouse grown plants 10 days after drought treatment. The effective quantum yield of PS II (Φ PS II) was maintained better in RRIM 600 and RRII 430 while other clones showed drastic decline in PS II activity with drought stress. RRII 430 and RRII 422 showed stable Φ PS II activity under drought conditions. PS I activity was not much affected in stressed plants both in the glasshouse as well as in PEG solution. Among the modern clones, RRII 422, RRII 429 and RRII 430 showed high PS I activity under moisture stress, probably indicating an increased rate of cyclic electron flow around PS I when the PS II function is impaired. Gas exchange and fluorescence data revealed that the clone RRII 430 was more likely to endure drought stress than the others in the RRII 400 series. Gen-Next clones should have better tolerance to changing climatic conditions and the results of the present study help in this direction.

In vitro experiments were carried out to identify the most intrinsic tolerance traits for abiotic stresses like drought, temperature, light etc. using two contrasting Hevea clones. Combination of stresses was found more deleterious than single stress causing an increased membrane injury, leaf chlorophyll bleaching and reduction of PS II photochemical activity. Attempts were made to identify a few vital drought tolerance traits using poly ethylene glycol (PEG) with known drought tolerant (RRIM 600) and drought susceptible (RRII 105) clones. Leaf samples of clone RRII 105 upon incubation with PEG (60%) and light together (350 µmol m-2 s-1) exhibited more membrane injury in the drought susceptible clone RRII 105 (60.5%) than in the known drought tolerant clone RRIM 600 (44.2%). Experiments with in vitro drought and light stress decreased the chlorophyll content of leaves and enhanced chlorophyll bleaching more in clone RRII 105. Similarly the dark adapted Fv/Fm ratio in exposed leaf samples of clone RRII 105 was found more inhibited than clone RRIM 600. The drought tolerance traits thus identified in laboratory were validated in young plants grown in nurseries. In young plants under acute drought stress, the reduction in chlorophyll content (24%) and mid-day water potential (25%) were more in clone RRII 105 than in clone RRIM 600 (8% and 15%, respectively). In similar stress condition, the PS II quantum yield declined by 41% in clone RRII 105 and 17% in clone RRIM 600. Under extreme drought, the greenish nature of leaves diminished and turned yellowish due to photo- bleaching causing reduction in total functional leaf area of plant leading to drying and senescence of leaves. Intrinsic drought tolerance traits thus identified were employed to screen large number of Hevea germplasm collection. Leaf traits such as leaf yellowing and senescence that normally occur in summer under acute drought and high light stress in field were empirically scored in 3772 wild germplasm accessions. Accessions that were exhibiting less senescence and yellowing were ranked top as the most drought tolerant ones. From the large pool of wild germplasm 165 accessions were identified as intrinsic drought tolerant ones and 121 as most susceptible ones. The leaf samples of selected accessions were exposed to in vitro drought assay using PEG with light and an inverse relationship was observed between percent leaf yellowing under summer drought in the field and leaf PS II activity measured under in-vitro drought and high light stress. Thus, leaf yellowing is an easy proxy measure of PSII activity during drought and high light stress.

Natural rubber cultivation in India faces adverse effects of drought and cold stresses, especially in the non- traditional rubber growing areas. As the genetically divergent germplasm accessions are an excellent repository of various useful traits including stress tolerance, systematic screening of wild germplasm for drought and cold tolerance with the ultimate objective of developing location-specific clones for non- traditional rubber growing areas holds much importance. With this objective, during the year 2007, a field evaluation of 30 Hevea clones comprising Amazonian accessions and Wickham clones was initiated at Regional Research Station (RRS), Dapchari located in the North Konkan region of Maharashtra state in India. This region experiences high temperature (exceeding 40 ⁰C in April-May), high light intensity and very low soil moisture during the summer months. The check clones in the experiment were RRII 105, RRII 430, RRII 414, RRII 208, RRIM 600 and Tjir 1. Growth parameters along with annual and summer girth increment were worked out. Leaf yellowing and drying were also used as criteria to assess drought susceptibility. Based on this data, the growth response of wild Amazonian clones and hybrid Wickham clones were assessed and compared with that of the check clones. The growth performance of the modern clone, RRII 430 and RRII 414 was assessed for the first time under Dapchari conditions. Clone RRII 430 was found to be more suitable than RRII 414 for establishment in a drought prone region based on juvenile growth performance.

A propagation technique which will promote deep growth of taproot and well-developed lateral roots, is very important under a warmer and drier agro-climate influenced by global warming. Polybag plants with improper root system may not be able to withstand the various stresses characteristic of dry agro-climate. Vigorous seedlings of polycross origin propagated by means of seed-at-stake planting seem to be the best planting technique for the future. Advanced planting materials produced by the new propagation technique of in situ budding of stocks raised in root trainers may also perform well under an agro-climate influenced by global warming.

Drought is one of the most important manifestations of climate change as far as the rubber growing regions in India are concerned. The effect of type of planting material and water conservation techniques like tillage, mulching and in situ water harvesting on growth of immature rubber and storage of soil moisture in two ongoing experiments are discussed. In the nursery experiments, growth parameters of planting materials raised through direct-seeding in polybags followed by in situ budding and planting budded stumps in polybags were compared. The results indicated significant difference in the growth performance depending up on the type of planting material. It was observed that the planting material produced through direct- seeding with an intact root system was significantly superior in stem diameter, plant height, number of whorls, fibrous root and dry matter compared to plants raised through budded stumps. In the field experiment, direct-seeded green budded plants were integrated with moisture conservation practices like tillage, mulching, silt pits and enhanced fertilizer application along with the use of organic manures. It was observed that the soil moisture storage during summer was significantly increased compared to the plots where plants were raised from budded stumps following the current package of practices. A significantly higher leaf area index (LAI) and root length density (RLD) were maintained by the direct-seeded green budded plants under integrated management. The growth of the direct-seeded green budded plants under integrated management was significantly superior to other treatments. Therefore introduction of planting materials with a good root system and adoption of good agricultural practices (GAPs) could play an important role in adapting plants to future extremes of climate change.