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.

Carbohydrate reserve storage in trees is usually considered a passive function, essentially buffering temporary discrepancies between carbon availability and demand in the annual cycle. Recently, however, the concept has emerged that storage might be a process that competes with other active sinks for assimilate. We tested the validity of this concept in Hevea brasiliensis Müll. Arg. (rubber) trees, a species in which carbon availability can be manipulated by tapping, which induces latex regeneration, a high carbon-cost activity. The annual dynamics of carbohydrate reserves were followed during three situations of decreasing carbon availability: control (no tapping), tapped and tapped with Ethephon stimulation. In untapped control trees, starch and sucrose were the main carbohydrate compounds. Total nonstructural carbohydrates (TNC), particularly starch, were depleted following bud break and re-foliation, resulting in an acropetal gradient of decreasing starch concentration in the stem wood. During the vegetative season, TNC concentration increased. At the end of the vegetative season, there were almost no differences in TNC concentration along the trunk. In tapped trees, the vertical gradient of starch concentration was locally disturbed by the presence of the tapping cut. However, the main effect of tapping was a dramatic increase in TNC concentration, particularly starch, throughout the trunk and in the root. The difference in TNC concentration between tapped and untapped trees was highest when latex production was highest (October); the difference was noticeable even in areas of the trees that are unlikely to be directly involved in latex regeneration, and it was enhanced by Ethephon stimulation, which is known to increase latex metabolism and flow duration. Thus, contrary to what could be expected if reserves serve as a passive buffer, a decrease in carbohydrate availability resulted in a net increase in carbohydrate reserves at the trunk scale. Such behavior supports the view that trees tend to adjust the amount of carbohydrate reserves stored to the level of metabolic demand, at the possible expense of growth.

The hypothesis that root apical diameter may be used to evaluate root growth potential was tested. Temporal variations in the apical diameter of individual roots of rubber seedlings (Hevea brasiliensis) were studied together with their elongation patterns, using root observation boxes under controlled conditions. This study confirmed the overall positive correlation between apical diameter and growth rale. Moreover, the two parameters, varied in the same way during the life of a given root. For roots with short growth duration, there was a parallel quick decrease in both apical diameter and elongation rate, whereas roots that grew for longer periods showed synchronous fluctuations for both parameters. Since the mean values for the secondary roots within a root system exhibited the same trends, variations in apical diameter and elongation rates should depend on factors influencing the whole root system. When related to shoot rhythmic growth, both apical diameter and elongation rates were depressed during the periods of leaf growth. These effects were enhanced and/or prolonged by shading, hence reinforcing the hypothesis that this development depends on assimilate availability. Such results can be interpreted in terms of a source‐sink relationship within the whole plant by considering the apical diameter, representing the size of the meristem related to the number of rneristematic cells, as an indicator of each root's growth potential.

The aim of this study was to assess the temperature response of photosynthesis in rubber trees (Hevea brasiliensis Müll. Arg.) to provide data for process-based growth modeling, and to test whether photosynthetic capacity and temperature response of photosynthesis acclimates to changes in ambient temperature. Net CO 2 assimilation rate (A) was measured in rubber saplings grown in a nursery or in growth chambers at 18 and 28°C. The temperature response of A was measured from 9 to 45°C and the data were fitted to an empirical model. Photosynthetic capacity (maximal carboxylation rate, V cmax , and maximal light driven electron flux, J max ) of plants acclimated to 18 and 28°C were estimated by fitting a biochemical photosynthesis model to the CO 2 response curves (A-C i curves) at six temperatures: 15, 22, 28, 32, 36 and 40°C. The optimal temperature for A (T opt ) was much lower in plants grown at 18°C compared to 28°C and nursery. Net CO 2 assimilation rate at optimal temperature (A opt ), V cmax and J max at a reference temperature of 25°C (V cmax25 and J max25 ) as well as activation energy of V cmax and J max (E aV and E aJ ) decreased in individuals acclimated to 18°C. The optimal temperature for V cmax and J max could not be clearly defined from our response curves, as they always were above 36°C and not far from 40°C. The ratio J max25 /V cmax25 was larger in plants acclimated to 18°C. Less nitrogen was present and photosynthetic nitrogen use efficiency (V cmax25 /N a ) was smaller in leaves acclimated to 18°C. These results indicate that rubber saplings acclimated their photosynthetic characteristics in response to growth temperature, and that higher temperatures resulted in an enhanced photosynthetic capacity in the leaves, as well as larger activation energy for photosynthesis.

The growth pattern of the root system of young rubber trees (Hevea brasiliensis) was studied in relation to shoot development over a period of 3 months. Temporal and spatial variations in elongation and branching processes were examined for the different root types, by means of root observation boxes. Shoot growth was typically rhythmic. Root development was periodic and related to leaf expansion. Root elongation was depressed during leaf growth, whereas branching was enhanced. Consequently, highly branched areas with vigorous secondary roots alternated along the taproot with poorly branched areas with shorter roots. Root types were not affected to the same degree by shoot competition: during leaf expansion, taproot growth was just depressed but remained continuous, the emergence and elongation rates of secondary roots were significantly affected and the elongation rates of tertiary roots fell to zero. These results were consistent with the hypothesis that root growth is related to competition for assimilates and to the sink strength of the different root types, whereas root branching appeared to be promoted by leaf development.