Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (K leaf ) to irradiance. The effects of light and temperature on K leaf are described. Under dark conditions, K leaf was low, but increased by 400% upon exposure to light. In contrast to dark conditions, K leaf values of light-exposed leaves responded to temperature and 0.1 mM cycloheximide treatments. Furthermore, K leaf was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that K leaf dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low K leaf in the dark was associated with down-regulation, whereas high K leaf in the light was associated with up-regulation of JrPIP2. Light responses of K leaf and aquaporin transcripts were reversible and inhibited by cycloheximide, indicating the importance of de novo protein biosynthesis in this process. Our results indicate that walnut leaves can rapidly change their hydraulic conductance and suggest that these changes can be explained by regulation of plasma membrane aquaporins. Model simulation suggests that variable leaf hydraulic conductance in walnut might enhance leaf gas exchanges while buffering leaf water status in response to ambient light fluctuations.

Canopy CO concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO]) throughout the canopy (0.02-38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO] in the upper and middle canopy decreased on average 7-10 μmol mol below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO] did not differ between the wet and dry seasons. In contrast, [CO] below 0.7 m were generally higher during the dry season, resulting in larger [CO] gradients. Supporting this observation, soil CO efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO] immediately above the forest floor of about 50␣μmol mol. Temporal and spatial variations in [CO] were reflected in changes of δC and δO values. Tight relationships were observed between δC and δO of canopy CO during both seasons (r > 0.86). The most depleted δC and δO values were measured immediately above the forest floor (δC = -16.4‰; δO = 39.1‰ SMOW). Gradients in the isotope ratios of CO between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δC, and between 1.0‰ and 3.5‰ for δO. The δC and calculated c /c of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δC within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c /c. Plotting 1/[CO] vs. the corresponding δC ratios resulted in very tight, linear relationships (r = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δC of respired sources were close to the measured δC of soil respired CO and to the δC of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δ, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season.

The Cavitron spinning technique is used to construct xylem embolism vulnerability curves (VCs), but its reliability has been questioned for species with long vessels. This technique generates two types of VC: sigmoid 's'-shaped and exponential, levelling-off 'r'-shaped curves. We tested the hypothesis that 'r'-shaped VCs were anomalous and caused by the presence of vessels cut open during sample preparation. A Cavitron apparatus was used to construct VCs from samples of different lengths in species with contrasting vessel lengths. The results were compared with VCs obtained using other independent techniques. When vessel length exceeded sample length, VCs were 'r'-shaped and anomalous. Filling vessels cut open at both ends with air before measurement produced more typical 's'-shaped VCs. We also found that exposing segments of 11 woody species in a Cavitron at the pressure measured in planta before sampling considerably increased the degree of embolism above the native state level for species with long vessels. We concluded that open vessels were abnormally more vulnerable to cavitation than intact vessels. We recommend restricting this technique to species with short conduits. The relevance of our conclusions for other spinning techniques is discussed.

Overall, the results suggest that cavitation resistance is a causal factor of tree mortality under extreme drought conditions.