Maize (Zea mays L.) yield is often restricted by low soil water availability, particularly late in the growing season. To increase yields, genetic options for more effective use of available soil water are being explored. One option is to select genotypes that have restricted transpiration rate under high vapour pressure deficit (VPD) conditions so that soil water is conserved for use later in the growing season. While genetic variation for this trait has been identified within several crop species, such variation has never been explored in maize. The objective of this study was to examine transpiration rate of 35 single-cross hybrids to determine whether hybrids can be identified that express limited transpiration under high VPD. Two sets of experiments were undertaken in which plants were exposed to a range of VPD in chambers. A two-phase transpiration response was observed in 11 hybrids in which there was a threshold VPD above which transpiration rate was restricted. The VPD threshold varied from 1.7 to 2.5 kPa among these hybrids. Eight hybrids were included in both sets of experiments, and the same results were obtained in both experiments, indicating that expression of the trait was consistent.

No abstract

This study compared maize, sorghum and pearl-millet, leading C 4 cereals, for the transpiration rate (TR) response to increasing atmospheric and soil water stress. The TR response to transiently increasing VPD (0.9-4.1 kPa) and the transpiration and leaf area expansion response to progressive soil drying were measured in controlled conditions at early vegetative stage in 10-16 genotypes of each species grown in moderate or high vapor pressure deficit (VPD) conditions. Maize grown under moderate VPD conditions restricted TR under high VPD, but not sorghum and pearl millet. By contrast, when grown under high VPD, all species increased TR upon increasing VPD, suggesting a loss of TR responsiveness. Sorghum and pearl-millet grown under high VPD reduced leaf area, but not maize. Upon progressive soil drying, maize reduced transpiration at higher soil moisture than sorghum and pearl millet, especially under high VPD, and leaf area expansion declined at similar or lower soil moisture than transpiration in maize and sorghum. It is concluded that maize conserves water by restricting transpiration upon increasing VPD and under higher soil moisture than sorghum and millet, giving maize significantly higher TE, whereas sorghum and pearl millet rely mostly on reduced leaf area and somewhat on transpiration restriction.

Persistence of green leaves during seed fill, referred to as a stay‐green trait, has been investigated in sorghum [Sorghum bicolor (L.) Moench] as an approach to increasing yields under water‐limited conditions. An hypothesis to explain the observation of stay green in some sorghum genotypes and not in others is that the genotypes expressing the trait employ mechanisms to increase availability of soil water during seed fill. In this study, the expression of two mechanisms resulting in soil water conservation to allow greater water availability during seed fill was explored among 12 sorghum genotypes. One mechanism is an earlier decrease in transpiration with soil drying so that the rate of soil water loss is decreased earlier in the soil drying cycle. The second mechanism is a limitation on transpiration rate at high vapor pressure deficit (VPD) so that soil water is conserved on days when midday VPD is high. Field studies were undertaken to identify seven genotypes that consistently expressed the stay‐green trait and five genotypes that did not exhibit this trait. The range of the threshold for the decrease in transpiration rate with soil drying was similar among the two sets of genotypes. Similarly, the expression of the limited transpiration rate under high VPD was found for both sets of genotypes. There was no evidence in these studies that the stay‐green trait was closely linked with either mechanism of water conservation.

1147 RESEARCH P lants are sessile organisms that have evolved numerous adaptive responses to cope with environmental stress. Recently, a plant-water-conservation strategy has been reported by Sinclair et al. (2005) to restrict TR under high evaporative conditions resulting in increased crop yields under water-limited conditions. In particular, water conservation by restricting TR under elevated VPD conditions can result in prolonged crop growth during lateseason drought. Genotypic differences for this trait have now been identified in soybean [Glycine max (L.) Merr.; Fletcher et al., 2007; Gilbert et al., 2011], peanut (Arachis hypogaea L.; Devi et al., 2010), pearl millet (Pennisetum glaucum L.; Kholova et al., 2010), sorghum (Sorghum bicolor L.; Gholipoor et al., 2010), and chickpea (Cicer arietinum L.; Zaman-Allah et al., 2011). Simulation analysis of the consequences of a maximum transpiration trait for nonirrigated conditions for sorghum in Australia (Sinclair et al., 2005) and soybean in the United States (Sinclair et al., 2010) demonstrated that the greatest benefits in yield occurred in dry, low-yielding growing seasons, which are critical for the economic survival of farmers.In those genotypes with restricted TR at high VPD, it has been hypothesized (Sinclair et al., 2008) that low hydraulic conductance in the plant limits the flow of water to guard cells so that there is a loss of guard cell turgor and a decrease in stomatal conductance. The ABSTRACT Limited transpiration rate (TR) under high vapor pressure deficit (VPD) conditions has been proposed as a desirable trait for crop yield improvement. The limited-TR trait has been identified in several single-cross maize hybrids, and among these hybrids, a range in the VPD breakpoint for limited TR was identified. It was hypothesized that the variation in the VPD breakpoint was due to differences in hydraulic conductance in their roots or leaves, or both. Therefore, the objective of this study was to compare relative hydraulic conductance in the roots and leaves across the maize hybrids expressing the VPD breakpoint. It was found that the VPD of the breakpoint was correlated with each of three indices of hydraulic conductance. That is, low VPD breakpoint was associated with low hydraulic conductance in both leaves and roots indicating a common, underlying limiting mechanism in these two tissues. It was hypothesized that expression of similar aquaporin populations influencing hydraulic flow across membranes in the roots and leaves may account for the consistency in results across the indices of hydraulic conductance.