Whole-plant transpiration in Populus sp.: its determination, nocturnal effects and influence by form of nitrogen

Abstract

Water is a crucial factor in the life of land plants. Transpiration (E) is inevitable and it is viewed as the universal cost of accessing a CO2-rich atmosphere. Although we now know that plants lose water at night, it had been assumed that stomata remained closed after dark. Renewed interest has encouraged more research on night-time E. An overlooked challenge in this area is that of measuring water loss. Most of the current research relies on leaf-level measurements using gas analysers. This presents many complications and although the instruments are simple to operate, it is not a trivial task to measure E properly, especially at night. I present a system that includes instructions to build the hardware platform and the software package, which utilises balances to measure E simultaneously and continuously from multiple specimens. The system is particularly suited to measuring nocturnal E of whole plants since it requires minimal interaction and it is not intrusive for the plant. With this system, a survey of whole-plant nocturnal E was carried out on four species of Populus with a range of habitat preference from riparian to upland. This survey characterises night-time water loss in relation to habitat, showing that Populus from drier environments may curtail night-time E more readily than riparian species. The survey also investigates the effect of drought on nocturnal E, arguing that night-time sap flow may be associated with one or more physiological functions. Lastly, the effect of two forms of N (NO3- and NH4+) on whole-plant conductance is analysed in two hybrid poplars with different growth potentials. NH4+ is shown to depress day-time, but not night-time conductance in comparison with NO3-; likely due to a change in stomatal function. Root anatomy was radically changed in a species-dependent manner when grown under different N forms. Some of these changes reflect different metabolic needs associated with each N form, while others may have a direct bearing on plant hydraulics.