Evaluation of unconstrained and constrained mathematical functions to model girth growth of rubber trees (Hevea brasiliensis) using young agemeasurements

No attempt has been made to date to model growth in girth of rubber tree （Hevea brasiliansis）. We evaluated the few widely used growth functions to identify the most parsimonious and biologically reasonable model for describing the girth growth of young rubber trees based on an incomplete set of young age measurements. Monthly data for girth of immature trees （age 2 to 12 yearsi from two locations were sub- jected to modelling. Re-parameterized, unconstrained and constrained growth functions,of Richards （RM）, Gompertz （GM） and the monomo- lecular ＇model ^（MM） were fitted to data. Duration of growth was the firsf constraint introduced. In the stagel We attempted a population aver- age （PA） model to capture the trend in growth. The best PA model was fitted as a subject specific （SS） model. We used appropriate error vari- ance-covariance structure to account for correlation due to repeated measurements over time. Unconstrainecl functions underestimated the asymptotic maximum that did not reflective carrying capacity of the locations. Underestimafions were attributed to the partial set＇ of meas- urements made during the early growth phase of the trees. MM proved superior to RM and GM. In the randomcoefficient models, both Gf and Go appeared to be influenced by tree level effects. Inclusion of diagonal definite positive matrix removed the correlation between random effects. The results were similar at both locations. In the overall assessment MM appeared as the candidate model for studying the girth-age relationships in Hevea trees. Based on the fitted model we conclude that, in Hevea trees, growth rate is maintained at maximum value at to, then decreases until the final state at dG/dt 〉 0, resulting in yield curve with no period of accelerating growth. One physiological explanation is that photosynthetic activity in Hevea trees decreases as girth increases and constructive metabolism is larger than destructive metabolism.

Evaluation of Brazilian wild Hevea germplasm for cold tolerance：genetic variability in the early mature growth

Hevea brasiliensis is a commercially cultivated species for its natural rubber（NR） latex in South East Asian countries. To meet the ever-increasing demand, NR cultivation has been extended to non-traditional regions due to the limited scope of further expansion in traditional rubbergrowing areas in India. These areas are often confronted with various abiotic stresses, especially high and low temperatures, which cause reduction in plant growth, thereby increasing its uneconomical immaturity period. Eighteen wild Hevea accessions along with two modern clones RRII203 and PB 235 and two check clones RRIM 600 and Haiken 1 were evaluated in the early mature growth phase.The site was at Nagrakata, West Bengal, the sub-Himalayan cold-prone region of India. In Hevea, crop production is governed by two major factors, growth-vigor and production capacity. Growth-vigor is of special importance because the production of rubber is a process linked with the early growth of the plant, which results in early tappability and early economic gains. The genotypes exhibited highly significant clonal differences（P = 0.05） for all the growth traits. Tappability percentage in the seventh year, ranged from 0.33 %（AC 3074, AC 3075, AC 3293） to 89.67 %（RO 2727）. The most vigorously growing accession（RO2727） reached tappabile girth early in the seventh year when the girth of plant ranged from 22.38 cm（AC 3293） to53.12 cm（RO 2727）. The general mean was 43.32 cm, and the similar growth trend was exhibited by these accessions in the tenth year also. Annual girth increment（cm a-1） over3 years ranged from 1.81 cm（AC 3075） to 6.80 cm（RO2727）. The mean winter girth increment（cm a-1） over4 years ranged from 0.13 cm（AC 3075） to 0.96 cm（RO2727） as compared to the check clone RRIM 600（1.11 cm）and Haiken 1（1.10 cm）. Wide differences between the phenotypic coefficient of variation（50.29） and genotypic coefficient of variation（24.82） were observed for winter girth increment. Girth in the tenth year recorded the highest heritability（87 %）. Girth was significantly correlated with the other growth traits. The top 30 % of the potential accessions showing high growth vigour and early tappability under cold stress were identified. These ecotypes/selections have high potential value for the development of coldtolerant clones for these regions and also in broadening the genetic base of the present-day cultivated rubber.

Leaf Photosynthetic Activity and Antioxidant Defense Associated with Sub1 QTL in Rice Subjected to Submergence and Subsequent Re-aeration

The influence of submergence on photosynthesis and antioxidant capacities in rice varieties Swarna and Swarna-Subl with or without Sub1 QTL were evaluated under control, simulated complete submergence and subsequent re-aeration. The leaf photosynthetic rate and stomatal conductance decreased in the both varieties during the progression of submergence as compared to the control plants, but significant varietal differences were observed after 1 d of submergence. Submergence also altered the PSII activity, as reflected in a decrease in the values of Fo, Fm and Fv/Fm and degradation of chlorophyll, more in Swarna than in Swarna-Subl. During early submergence period, the activities of superoxide dismutase （SOD）, ascorbate peroxidase （APX）, dehydroascorbate reductase （DHAR）, glutathione reductase （GR） against reactive oxygen species were increased in the both varieties. However, with the progress of submergence period （after 7 d）, the activities of SOD, catalase （CAT）, APX, guaiacol peroxidase （GPX）, GR and DHAR declined, more in Swarna than in Swarna-Subl. During re-aeration, Swarna-Subl showed significant increase of above antioxidant enzymes but not in Swarna, Swarna-Subl improves photosynthetic activity, showing higher photosynthetic rate compared to Swarna under submergence and subsequent re-aeration because of less degradation of chlorophyll, higher stomatal conductance, and efficient PSII activity along with better antioxidant protection from oxidative damage.

Divergent DNA Methylation Patterns Associated with Abiotic Stress in Hevea brasiliensis

Cytosine methylation is a fundamental epigenetic mechanism for gene-expression regulation and development in plants. Here, we report for the first time the identification of DNA methylation patterns and their putative relationship with abiotic stress in the tree crop Hevea brasiliensis （source of 99% of natural rubber in the world）. Regulatory sequences of four major genes involved in the mevalonate pathway （rubber biosynthesis pathway） and one general defense-related gene of three high-yielding popular rubber clones grown at two different agroclimatic conditions were analyzed for the presence of methylation. We found several significant variations in the methylation pattern at core DNA binding motifs within all the five genes. Several consistent clone-specific and location-specific methylation patterns were identified. The differences in methylation pattern observed at certain pivotal cis-regulatory sites indicate the direct impact of stress on the genome and support the hypothesis of site-specific stress-induced DNA methylation. It is assumed that some of the methylation patterns observed may be involved in the stress-responsive mechanism in plants by which they adapt to extreme conditions. The study also provide clues towards the existence of highly divergent phenotypic characters among Hevea clones despite their very similar genetic make-up. Altogether, the observations from this study prove beyond doubt that there exist epigenetic variations in Hevea and environmental factors play a significant role in the induction of site-specific epigenetic mutations in its genome.