5. Conclusion
Differences in maintenance of RWC may be related to performance under stress, particularly when lines of widely different maturity and adaptation are compared, and stress is severe. In such an experiment, however, it is difficult to conclude that differences in RWC are the major cause of differences in performance. Among germplasm of more similar type, differences in grain yield can be found that do not appear to be related to differences in leaf water status, either in the upland control condition or under water deficit imposed near flowering. In one case, it appeared that the maintenance of RWC was necessary but not sufficient to ensure good yield. On the basis of these results, it may be concluded that other mechanisms of tolerance to water deficit should be considered as avenues to greater yield, in addition to those mechanisms that improve plant water status. In the studies reported here, leaf RWC under stress does not meet the criteria of high heritability plus significant correlation with the target trait, which would be necessary to justify its use as a secondary selection target for improved grain yield under reproductive stage water deficit.