RESUMEN
A link exists between hydraulic traits and leaf habit. However, few attempts have addressed a possible link between hydraulic traits and altered leaf habit in introduced ranges. Within its native range, the Amazon rainforest, Hevea brasiliensis (Willd. ex A. Juss) Muell. Arg. is an evergreen but it becomes drought-deciduous in non-native ranges. The reason for this change remains poorly understood. The hydraulic-related traits, gas exchange rates and water status of H. brasiliensis and the co-occurring evergreen Drypetes indica (Muell. Arg.) Pax et Hoffm. were examined in Xishuangbanna, China. The water potential at turgor loss point in both species almost overlapped, but the water potential at which leaf relative water content reached 70% was more negative in D. indica. The water loss rate from excised leaves was quicker in H. brasiliensis. Leaf and stem hydraulic conductivity were more susceptible to drought-induced embolisms in H. brasiliensis than in D. indica. Vessels were significantly wider in H. brasiliensis but D. indica had more vessels. H. brasiliensis displayed higher rain-season gas exchange rates than D. indica. During the dry season, low soil water potential rendered water transport inefficient in H. brasiliensis; this effect was less pronounced in D. indica. D. indica has traits that help prevent hydraulic failure but has a low photosynthetic capacity. The opposite was found for H. brasiliensis. The results suggest that a combination of hydraulic traits, gas exchange characteristics and water status during the dry season might trigger a change in the leaf habits of H. brasiliensis in introduced ranges.
RESUMEN
The response of plants to drought has received significant attention, but far less attention has been given to the dynamic response of plants during recovery from drought. Photosynthetic performance and hydraulic capacity were monitored in seedlings of Hevea brasiliensis under water stress and during recovery following rewatering. Leaf water relation, gas exchange rate and hydraulic conductivity decreased gradually after water stress fell below a threshold, whereas instantaneous water use efficiency and osmolytes increased significantly. After 5 days of rewatering, leaf water relation, maximum stomatal conductance (g(s-max)) and plant hydraulic conductivity had recovered to the control levels except for sapwood area-specific hydraulic conductivity, photosynthetic assimilation rate and osmolytes. During the phase of water stress, stomata were almost completely closed before water transport efficiency decreased substantially, and moreover, the leaf hydraulic pathway was more vulnerable to water stress-induced embolism than the stem hydraulic pathway. Meanwhile, g(s-max) was linearly correlated with hydraulic capacity when water stress exceeded a threshold. In addition, a positive relationship was shown to occur between the recovery of g(s-max) and of hydraulic capacity during the phase of rewatering. Our results suggest (i) that stomatal closure effectively reduces the risk of xylem dysfunction in water-stressed plants at the cost of gas exchange, (ii) that the leaf functions as a safety valve to protect the hydraulic pathway from water stress-induced dysfunction to a larger extent than does the stem and (iii) that the full drought recovery of gas exchange is restricted by not only hydraulic factors but also non-hydraulic factors.