Estimation of Whole-Plant Resistance to Gaseous Exchange Independent of Leaf Temperature Measurement

Abstract

For studies into the uptake of mercury vapor by wheat (Tridcum aesdvum), a simple theory and plant chamber were employed to esdimate total leaf resistance of whole plants to water vapor exchange.The estimates were independent of kaf temperature, for which mean values were Indbectly determined. The approach involved the measurement, at steady-state conditions, of the net change in water vapor flux per unt of leaf surface (Aqp) The uptake of mercury vapor by plants is a long established phenomenon (7). This is the cause for some environmental concern since mercury vapor is released to the atmosphere by a number of industrial processes, including the combustion of fossil fuels such as coal (2). The quantitative data available to assess any hazard posed by plant uptake and accumulation of such mercury, however, are negligible. This paper reports, in part, the results of a study into factors influencing the uptake of mercury vapor by wheat.The entry of mercury vapor into plants can be expected to follow the same major transfer pathways as water vapor and CO2 and, as such, is subject to conventional transfer resistance analysis (9). This entails determination of transfer resistances to water vapor which in turn can be related to other gases by means of appropriate diffusion coefficients. Such studies are often confined to single-leaf chambers, one reason being the theoretical necessity to measure leaf temperature, a heterogeneous (3) and difficult entity to characterize accurately. In this study, however, it was desired to examine mercury uptake and subsequent distribution on a whole-plant basis. An alternative theory was therefore sought whereby total leaf resistance to gaseous exchange of whole plants and groups (1) where q, is the flux of water vapor per unit of leaf surface (g cm-2 sec-1), CA is the ambient water vapor concentration (g cm-3), and CL is the water vapor concentration (g cm-3) at evaporative surfaces within the leaf and is assumed to be saturated at leaf temperature. Total leaf resistance is considered to consist of several component resistances, the principal of which are the parallel stomatal (r,) and cuticular (r,) resistances, which in turn are considered in series with an external boundary layer resistance (ra). Differentiation of component resistances was not undertaken in this work.The value of rL determined in the above manner is subject to error in the measurement of both leaf temperature and CA (9, 13), as well as to uncertain error in the assumption that CL iS saturated at leaf temperature. In an attempt to reduce these errors, Jarvis (9) proposed a theory that estimates leaf resistance independently of CL and hence leaf temperature. As such, the theory is particularly suited to whole-plant studies.The approach, expanded here, involves inducing a small change in ambient water vapor concentration such that in addition to equation 1,The assumption is that rL is independent of change in ambient humidity.

Uptake of Mercury Vapor by Wheat

“…The first assumption may not be particularly valid (3), and could likely lead to the greatest errors in estimation of rL.Hg in instances of narrowest stomatal opening. Such (2).…”