Rubber latex yield is affected by interactions between antecedent temperature, rubber phenology, and powdery mildew disease.

Rubber (Hevea brasiliensis) latex production is crucial to the local economy, yet Xishuangbanna's climate is considered sub-optimal for rubber cultivation. The prevalence of the powdery mildew disease (Oidium heveae) in this region has decreased the annual latex yield by 20%. Rubber latex yield is influenced by several factors, including temperature, disease, other biotic conditions, and plantation management. However, the interrelationships and potential influencing networks between rubber latex yield and these factors are rarely quantitatively assessed, and understanding their impacts on latex yield could inform better management practices. To address this gap, we investigated the effects of temperature, phenology, and powdery mildew disease on rubber latex yield in March using observational data on daily rubber latex yield combined with detailed phenology, powdery mildew, and temperature data from 2004 to 2010 in a state farm in the Xishuangbanna, Yunnan, China. We found that the critical influencing periods of daily temperature difference (or diurnal temperature difference) on the rubber latex yield were during Nov 27-Jan 19 and Jan 21-Mar 17. Partial least square regression analysis and variance partitioning analysis were conducted on the 35 phenological variables, eight powdery mildew-related variables, and two climatic variables. The most influential factors were identified as the factors of the daily temperature differences during Jan-Mar, the duration of leaf flushing phenology, and mean and maximum percentage of leaves infected by powdery mildew. Subsequent canonical correlation analysis and linear regression found that temperature difference directly affected the rubber latex yield and indirectly affected the yield through phenology and powdery mildew disease. Raised daily temperature differences from Jan to Mar had the greatest impact, leading to a higher rubber latex yield. Our comprehensive quantitative assessment revealed the relative importance of antecedent daily temperature differences, phenology, and powdery mildew disease as well as their complex interconnections in influencing rubber latex yield. Our findings are essential to future studies on both powdery mildew disease and rubber latex yield, and also develop rubber latex models.

The powdery mildew disease of rubber (Oidium heveae) is jointly controlled by the winter temperature and host phenology.

Rubber powdery mildew disease (Oidium heveae) is a serious threat to natural rubber production (Hevea brasiliensis) in some rubber developing regions of the world. Both phenological- and meteorological-related factors have been reported influencing the powdery mildew disease. However, few studies have investigated the effects of both phenological- and meteorological-related factors on the disease. The objective of this study is to quantify the contributions of phenological- and meteorological-related factors to affect the disease. We used the partial least squares (PLS) regression method to comprehensively quantify the effects of thirty-five phenological related factors and six meteorological factors on the infection level of powdery mildew of rubber trees over 9-year records (2003-2011). The relative contributions of significant factors were further investigated by the variation partition analysis. We found that the most influential variables were the mean temperature during winter and the duration of leaf development to maturation which explained 32 and 26% of the variations in the infection level. We found the controlling role of winter mean temperature, for the first time, on the infection level of powdery mildew. The controlling role of winter temperature may have directly increase the infection level when winter temperature is high and indirectly increase the infection level through prolonging the duration of leaf development to maturation, although the duration itself had smaller influences. We detected a warming trend of the winter temperatures from 2003 to 2011, which indicates that the infection level of powdery mildew will be increased if the winter warming continues.

In situ 13CO2 labelling of rubber trees reveals a seasonal shift in the contribution of the carbon sources involved in latex regeneration.

Rubber trees (Hevea brasiliensis) are the main source of natural rubber, extracted from latex, which exudes from the trunk after tapping. Tapped trees require large amounts of carbon (C) to regenerate the latex after its collection. Knowing the contribution of C sources involved in latex biosynthesis will help in understanding how rubber trees face this additional C demand. Whole crown 13CO2 pulse labelling was performed on 4-year-old rubber trees in June, when latex production was low, and in October, when it was high. 13C content was quantified in the foliage, phloem sap, wood, and latex. In both labelling periods, 13C was recovered in latex just after labelling, indicating that part of the carbohydrate was directly allocated to latex. However, significant amounts of 13C were still recovered in latex after 100 d and the peak was reached significantly later than in phloem sap, demonstrating the contribution of a reserve pool as a source of latex C. The contribution of new photosynthates to latex regeneration was faster and higher when latex metabolism was well established, in October, than in June. An improved understanding of C dynamics and the source-sink relationship in rubber tree is crucial to adapt tapping system practices and ensure sustainable latex production.

Contrasted effects of temperature during defoliation vs. refoliation periods on the infection of rubber powdery mildew (Oidium heveae) in Xishuangbanna, China.

Rubber powdery mildew caused by the foliar fungi Oidium heveae is one of the main diseases affecting rubber plantations (Hevea brasiliensis) worldwide. It is particularly serious in sub-optimal growing areas, such as Xishuangbanna in SW China. To prevent and control this disease, fungicides causing serious environmental problems are widely used. Strong correlations between the infection level and the temperature variables were reported previously, but they were related to monthly data that did not allow unraveling the patterns during the entire sensitive period. We correlated the infection level of powdery mildew of rubber trees recorded over 2003-2011 with antecedent 365 days daily temperature variables using partial least squares (PLS) regression. Our PLS regression results showed that the infection level of powdery mildew responded differently to the temperature variables of the defoliation and refoliation periods. Further analysis with Kriging interpolation showed that the infection level increased by 20% and 11%, respectively, per 1 °C rise of the daily maximum and mean temperature in the defoliation season, while it decreased by 8% and 10%, respectively, per 1 °C rise of the daily maximum and temperature difference in the refoliation season. This pattern was likely linked to the effects of temperature on leaf phenology. It seems highly possible that the infection level of powdery mildew increases, as increasing trends of maximum temperature and mean temperature during the defoliation continue.

Increased light-use efficiency sustains net primary productivity of shaded coffee plants in agroforestry system.

In agroforestry systems, shade trees strongly affect the physiology of the undergrown crop. However, a major paradigm is that the reduction in absorbed photosynthetically active radiation is, to a certain extent, compensated by an increase in light-use efficiency, thereby reducing the difference in net primary productivity between shaded and non-shaded plants. Due to the large spatial heterogeneity in agroforestry systems and the lack of appropriate tools, the combined effects of such variables have seldom been analysed, even though they may help understand physiological processes underlying yield dynamics. In this study, we monitored net primary productivity, during two years, on scales ranging from individual coffee plants to the entire plot. Absorbed radiation was mapped with a 3D model (MAESPA). Light-use efficiency and net assimilation rate were derived for each coffee plant individually. We found that although irradiance was reduced by 60% below crowns of shade trees, coffee light-use efficiency increased by 50%, leaving net primary productivity fairly stable across all shade levels. Variability of aboveground net primary productivity of coffee plants was caused primarily by the age of the plants and by intraspecific competition among them (drivers usually overlooked in the agroforestry literature) rather than by the presence of shade trees.

Concurrent starch accumulation in stump and high fruit production in coffee (Coffea arabica).

In coffee, fruit production on a given shoot drops after some years of high yield, triggering pruning to induce resprouting. The timing of pruning is a crucial farmer's decision affecting yield and labour. One reason for fruit production drop could be the exhaustion of resources, particularly the non-structural carbohydrates (NSC). To test this hypothesis in a Coffea L. arabica agroforestry system, we measured the concentrations of NSC, carbon (C) and nitrogen (N) in leaves, stems and stumps of the coffee plants, 2 and 5 years after pruning. We also compared shaded vs full sun plants. For that purpose, both analytical reference and visible and near infrared reflectance spectroscopy (VNIRS) methods were used. As expected, concentrations of biochemical variables linked to photosynthesis activity (N, glucose, fructose, sucrose) decreased from leaves to stems, and then to stumps. In contrast, variables linked more closely to plant structure and reserves (total C, C:N ratio, starch concentration) were higher in long lifespan organs like stumps. Shading had little effect on most measured parameters, contrary to expectations. Concentrations of N, glucose and fructose were higher in 2-year-old organs. Conversely, starch concentration in perennial stumps was three times higher 5 years after pruning than 2 years after pruning, despite high fruit production. Therefore, the drop in fruit production occurring after 5-6 years was not due to a lack of NSC on plant scale. Starch accumulation in perennial organs concurrently to other sinks, such as fruit growth, could be considered as a 'survival' strategy, which may be a relic of the behaviour of wild coffee (a tropical shade-tolerant plant). This study confirmed that VNIRS is a promisingly rapid and cost-effective option for starch monitoring (coefficient of determination for validation, R2val = 0.91), whereas predictions were less accurate for soluble sugars, probably due to their too similar spectral signature.

Effect of tapping activity on the dynamics of radial growth of Hevea brasiliensis trees.

Rubber tree (Hevea brasiliensis Müll. Arg.) radial growth dynamics were monitored with displacement sensors, together with latex production, to investigate three aspects of the dual production of latex and wood: (1) the usefulness of fine-scale dendrometric measurements as a physiological tool to detect water shortage through radial growth; (2) the dynamic aspects, both at the seasonal and at the multi-year scale, of the competition between latex and wood production; and (3) the spatial distribution of radial growth rates around the tapping cut. Radial growth of untapped control trees started with the onset of the rainy season and lasted until the onset of the dry season, ceasing completely during the driest period. Displacement sensors provided a sensitive means of detecting water shortage, with a clear correlation between diameter variations and changes in water availability (both daily evapotranspiration and monthly rainfall) over the whole annual cycle. However, the correlation was significantly disturbed in tapped trees. After resumption of tapping, the radial growth rate dropped sharply within two weeks and the effect persisted throughout the whole season, so that the cumulative growth of tapped trees was about half that of untapped trees, with the cumulative growth deficit reaching 80% for the period from mid-June to November. This long-known negative impact of tapping on growth was much stronger in the second year of tapping than in the first, whereas latex production increased significantly between the first and second year of tapping. The increased latex production, which could not be ascribed to climatic conditions, shows that the establishment of an artificial latex sink is a progressive, long-term process likely involving many aspects of metabolism. As expected, ethylene significantly increased latex production in both years; however, ethylene had no effect on the growth rates of tapped trees. Radial growth was differentially affected at different locations around the tapping cut, with growth rates significantly lower in the tapped panel than in the untapped panel, and higher above the cut than below the cut. Thus, caution is needed when deriving whole stem wood production from girth measurements at one location on the stem, especially from girth measurements made close to the tapping cut. This also provides new evidence for the location of the latex regeneration area in the tapped panel, below the cut.

Carbon isotope composition of latex does not reflect temporal variations of photosynthetic carbon isotope discrimination in rubber trees (Hevea brasiliensis).

Latex, the cytoplasm of laticiferous cells localized in the inner bark of rubber trees (Hevea brasiliensis Müll. Arg.), is collected by tapping the bark. Following tapping, latex flows out of the trunk and is regenerated, whereas in untapped trees, there is no natural exudation. It is still unknown whether the carbohydrates used for latex regeneration in tapped trees is coming from recent photosynthates or from stored carbohydrates, and in the former case, it is expected that latex carbon isotope composition of tapped trees will vary seasonally, whereas latex isotope composition of untapped trees will be more stable. Temporal variations of carbon isotope composition of trunk latex (δ(13)C-L), leaf soluble compounds (δ(13)C-S) and bulk leaf material (δ(13)C-B) collected from tapped and untapped 20-year-old trees were compared. A marked difference in δ(13)C-L was observed between tapped and untapped trees whatever the season. Trunk latex from tapped trees was more depleted (1.6‰ on average) with more variable δ(13)C values than those of untapped trees. δ(13)C-L was higher and more stable across seasons than δ(13)C-S and δ(13)C-B, with a maximum seasonal difference of 0.7‰ for tapped trees and 0.3‰ for untapped trees. δ(13)C-B was lower in tapped than in untapped trees, increasing from August (middle of the rainy season) to April (end of the dry season). Differences in δ(13)C-L and δ(13)C-B between tapped and untapped trees indicated that tapping affects the metabolism of both laticiferous cells and leaves. The lack of correlation between δ(13)C-L and δ(13)C-S suggests that recent photosynthates are mixed in the large pool of stored carbohydrates that are involved in latex regeneration after tapping.

A simple framework to analyze water constraints on seasonal transpiration in rubber tree (Hevea brasiliensis) plantations.

Climate change and fast extension in climatically suboptimal areas threaten the sustainability of rubber tree cultivation. A simple framework based on reduction factors of potential transpiration was tested to evaluate the water constraints on seasonal transpiration in tropical sub-humid climates, according pedoclimatic conditions. We selected a representative, mature stand in a drought-prone area. Tree transpiration, evaporative demand and soil water availability were measured every day over 15 months. The results showed that basic relationships with evaporative demand, leaf area index and soil water availability were globally supported. However, the implementation of a regulation of transpiration at high evaporative demand whatever soil water availability was necessary to avoid large overestimates of transpiration. The details of regulation were confirmed by the analysis of canopy conductance response to vapor pressure deficit. The final objective of providing hierarchy between the main regulation factors of seasonal and annual transpiration was achieved. In the tested environmental conditions, the impact of atmospheric drought appeared larger importance than soil drought contrary to expectations. Our results support the interest in simple models to provide a first diagnosis of water constraints on transpiration with limited data, and to help decision making toward more sustainable rubber plantations.

Impact of tapping and soil water status on fine root dynamics in a rubber tree plantation in Thailand.

Fine roots (FR) play a major role in the water and nutrient uptake of plants and contribute significantly to the carbon and nutrient cycles of ecosystems through their annual production and turnover. FR growth dynamics were studied to understand the endogenous and exogenous factors driving these processes in a 14-year-old plantation of rubber trees located in eastern Thailand. FR dynamics were observed using field rhizotrons from October 2007 to October 2009. This period covered two complete dry seasons (November to March) and two complete rainy seasons (April to October), allowing us to study the effect of rainfall seasonality on FR dynamics. Rainfall and its distribution during the two successive years showed strong differences with 1500 and 950 mm in 2008 and 2009, respectively. FR production (FRP) completely stopped during the dry seasons and resumed quickly after the first rains. During the rainy seasons, FRP and the daily root elongation rate (RER) were highly variable and exhibited strong annual variations with a total FRP of 139.8 and 40.4 mm(-) (2) and an average RER of 0.16 and 0.12 cm day(-) (1) in 2008 and 2009, respectively. The significant positive correlations found between FRP, RER, the appearance of new roots, and rainfall at monthly intervals revealed the impact of rainfall seasonality on FR dynamics. However, the rainfall patterns failed to explain the weekly variations of FR dynamics observed particularly during the rainy seasons. At this time step, FRP, RER, and the appearance of new FR were negatively correlated to the average soil matric potential measured at a depth of between 30 and 60 cm. In addition, our study revealed a significant negative correlation between FR dynamics and the monthly production of dry rubber. Consequently, latex harvesting might disturb carbon dynamics in the whole tree, far beyond the trunk where the tapping was performed. These results exhibit the impact of climatic conditions and tapping system in the carbon budget of rubber plantations.

Differential gene expression in different types of Hevea brasiliensis roots.

Three types of roots (taproots, first order laterals and second order laterals) were functionally characterized on 7-month-old in vitro plantlets regenerated by somatic embryogenesis in Hevea brasiliensis. A histological analysis revealed different levels of differentiation depending on root diameter. A primary structure was found in first and second order lateral roots, while taproots displayed a secondary structure. The expression of 48 genes linked to some of the regulatory pathways acting in roots was compared in leaves, stems and the different types of roots by real-time RT-PCR. Thirteen genes were differentially expressed in the different organs studied in plants grown under control conditions. Nine additional other genes were differentially regulated between organs under water deficit conditions. In addition, 10 genes were significantly regulated in response to water deficit, including 8 regulated mainly in lateral roots types. Our results suggest that the regulation of gene expression in lateral roots is different than that in taproots, which have a main role in nutrient uptake and transport, respectively.