Vulnerability to water-stress-induced embolism of stems, petioles, and leaf midribs was evaluated for two rubber clones (RRIM600 and RRIT251). The xylem conduits were relatively vulnerable to cavitation with 50% of embolism measured for xylem pressures between -1 and -2 MPa. This feature can be related to the tropicalhumid origin of the species. A distinct basipetal gradient of vulnerability was found, leaf midribs being the least vulnerable. Substantial variation in vulnerability to cavitation was found between the two clones only at the petiole level. A correlation was found between the stomatal behavior and the development of cavitation. Stomata were nearly closed when the xylem pressure reached the point of xylem dysfunction. Stomata may thus contribute to controlling the risk of cavitation. However, for one clone a poor correlation was found between stomatal regulation and petiole vulnerability. This was consistent with a high degree of embolism measured in the petioles after a soil drought event. Therefore, xylem cavitation might represent a promising criterion to evaluate the performance of rubber clones under drought conditions.

<p style="text-align: justify;">Selection for drought-tolerant clones has become a major challenge in rubber breeding programs undertaken to ensure the sustainability of natural rubber production, as rubber plantations are expanding in drought-prone areas. Xylem vulnerability to cavitation is a trait related to drought-induced mortality. It can be rapidly evaluated without subjecting plant materials to drought stress, making it useful in large-scale screening for drought tolerance in the near future. We first compared the most widely used techniques for measuring vulnerability to cavitation (air pressurization and Cavitron) on this species, and the effect of sample conditions (size, age and sunlight exposure), in order to ensure reliable analysis. Secondly, ten rubber clones were compared for their xylem vulnerability to cavitation in branches and petioles, and for other traits related to drought response, including stomatal response and leaf shedding occurring during a simulated drought. We also tested the plasticity of vulnerability to cavitation on two clones grown in three locations with contrasting precipitation regimes. We found no clonal variability and a small phenotypic plasticity for xylem vulnerability to cavitation in branches. However, clonal differences in xylem vulnerability to cavitation were found in petioles, and clones also showed differences in stomatal response and in leaf shedding behavior in response to a simulated drought. Our study suggests a genetic canalization for vulnerability to cavitation in organs critical for survival, such as branches, whereas there are clonal differences for traits related to drought avoidance: vulnerability to cavitation of petioles, leaf shedding behavior and stomatal response. The insights gained in this study for screening rubber tree clones for drought tolerance is also discussed. </p>

Hydraulic efficiency, xylem pressure, and stomatal conductance were measured in two Hevea clones having contrasting growth performances in their immature phase. Hydraulic efficiency was estimated by the xylem resistance on a leaf area basis on stem, petiole segments, and whole branches. The fast-growing clone exhibited significantly higher xylem efficiency, higher stomatal conductance, and higher xylem pressure. The difference in xylem pressure between the two clones was quantitatively consistent with the differences in hydraulic resistance and leaf transpiration rates. Our results suggest that variations in xylem efficiency may explain variations in stomatal conductance and xylem pressure, and hypothetically, growth performance between Hevea clones.Résumé : L'efficience hydraulique, la pression hydrostatique du xylème et la conductance stomatique ont été mesurées chez deux clones de l'Hevea ayant des potentialités de croissance contrastées pendant leur phase immature. L'efficience hydraulique a été estimée par la conductance hydraulique du xylème par unité de surface foliaire, pour les tiges, les pétioles et les branches entières. Les résultats montrent que le clone ayant la plus forte croissance possède également la meilleure efficience hydraulique, la conductance stomatique la plus élevée et des pressions de xylème moins négatives. Les différences de pressions entre les deux clones sont quantitativement cohérentes avec les différences de résistances hydrauliques et de transpiration foliaires. Nos résultats suggèrent que des variations d'efficience hydraulique pourraient expliquer des variations de conductance stomatiques, de pressions de xylème, et, hypothétiquement, de potentialités de croissance entre clones de l'Hevea.

The geometrical structure of plant canopies has many implications for plant functioning, microclimatic conditions, and plant-pathogen/herbivore interactions. Plant geometry can be described at several scales. At the finest scale, canopy structure includes the shape, size, location, and orientation of each organ in the canopy. This data set reports the three-dimensional (3-D) geometry of a set of fruit and rubber trees at the leaf scale. A 3-D magnetic digitizer was used to measure the spatial coordinates and the orientation angles of each leaf, namely, the midrib azimuth and inclination angles, and the rolling angle of leaf lamina around the midrib. In addition, for most trees, branching or flush order is given as well as the ranking of leaves along branches and the leaf identity of leaflets in compound-leaved trees. Leaf length was also measured for most trees. Leaf width was measured or estimated based on allometric relationships. Leaf area was derived from allometric relationships with leaf length and width. The data set includes the 3-D geometry of six trees: one apple, two mangos, two rubbers, and one walnut. Plant height ranged from 1.6 m for mango trees to 5.3 m for the large rubber tree. The number of leaves ranged from 895 for the small rubber tree to 26,283 for the apple tree. Total leaf area ranged from 3.6 m2 for the small rubber tree to 36.4 m2 for the apple tree. Most of the data were used to show how canopy geometry determines light interception and subsequently plant primary production and fruit yield. The data set was also used to test the quality of innovative methods for canopy structure description at tree scale. The apple tree was used to study the thermal environment of a leaf miner insect, to show how canopy geometry leads to a strongly heterogeneous risk of mortality, in particular under heat-wave conditions. Overall, our data set provides explicit plant architectures suitable for spatial modeling of plant physiological ecology and plant-herbivore interactions, allowing us to determine the mechanisms through which climate impacts biological and ecological processes involved in these functions

Rubber-growing soils in southern Thailand are usually deficient in both macro-and micronutrients. Omission pot trial is an excellent tool for nutrient assessment because it can indicate the most limiting nutrient and the order of limitation. Maize is generally used as a test plant, but the difference in nutrient response of maize and rubber is not clearly understood. An omission pot trial with 10 treatments (All, -N, -P, -K, -Mg, -S, -Zn, -Cu, -B, and -Lime) was conducted. The soil samples were limed with Ca(OH) 2 to pH 6, except for that used for the -Lime treatment. Equivalent amounts of 400 kg ha -1 of N, 120 of P, 175 of K, 75 of Mg, 100 of S, 6 of Zn, 4 of Cu and 2 kg ha -1 of B, were added in the All treatment. Nutrient X was omitted in the -X treatment. Maize and rubber were grown as test plants. The plant growth indices were measured after 30 days for maize and after 9 months for rubber. The limiting nutrients of both plants were N, P, and Ca (lime). Rubber growth in the field, which received government-recommended and omission-based fertilization, were not different.