Non-destructive measurements such as Gs and recording of pot weight for the calculation of FTSW was undertaken each day on all twelve plants.
nummularia (0.5 mol m-2 s-1 at 100% FTSW) exhibited significantly higher rates of Gs than C.
nummularia began to reduce as the fraction of transpirable soil water (FTSW) declined below 80%, while C.
exhibited a 20% decline at 45% FTSW (co-incident with the level of soil water availability at which Gs values fell), and pronounced reductions of 60% and 125% at FTSW levels of 15 and 5%, respectively (Fig.
Drought stress treatment was determined based on the value of FTSW
(Hainemann et al., 2011).
The basis of this screen was the threshold FTSW at which [N.sub.2] fixation activity initially decreased.
The FTSW values for the threshold in decline of ARA ranged from 0.11 to 0.28 among these eight genotypes.
In this genotype, the threshold for transpiration was 0.40 FTSW while the threshold for ARA was shifted to a low value of 0.19 FTSW, or a [N.sub.2] fixation threshold that was 0.21 FTSW less than transpiration.
Abbreviations: ARA, acetylene reduction assay; FTSW, fraction transpirable soil water.
The relationship between NTR and FTSW for each hybrid in response to a drying soil was first evaluated using the equation described by Muchow and Sinclair (1991) for maize:
In each experiment, a single curve was generated for each hybrid by combining the NTR and FTSW data from the individual plants.
To determine the FTSW point at which stomata began to close, the plateau regression procedures of PC SAS (SAS Institute, Inc., 1988) were employed.