Abstract
High intrinsic water-use efficiency (WUEi), the ratio of leaf photosynthesis to stomatal conductance, may be a useful trait in adapting crops to water-limited environments. In soybean, cultivar differences in stomatal response to vapor pressure deficit have not consistently translated into differences in WUEi in the field. In this study, six cultivars of soybeans previously shown to differ in WUEi in indoor experiments were grown in the field in Beltsville, Maryland, and tested for mid-day WUEi on nine clear days during the mid-seasons of two years. Measurement dates were chosen for diverse temperatures, and air temperatures ranged from 21 to 34 °C on the different dates. Air saturation deficits for water vapor ranged from 0.9 to 2.2 kPa. Corrected carbon isotope delta values for 13C (CID) were determined on mature, upper canopy leaves harvested during early pod filling each year. WUEi differed among cultivars in both years and the differences were consistent across measurement dates. Correlations between mean WUEi and CID were not significant in either year. It is concluded that consistent cultivar differences in WUEi exist in these soybean cultivars under field conditions, but that carbon isotope ratios may not be useful in identifying them because of cultivar differences in mesophyll conductance.
Highlights
With projected increased frequency of drought, and decreased availability or increased cost of water for agriculture, increasing the efficiency of water use in agriculture is an important goal [1]
Significant correlations between carbon isotope delta values for 13C (CID) and crop water-use efficiency (WUE) have been found in many crop species, such as wheat [3], peanut [4], tomato [5,6], cowpea [7], cotton [8], barley [9], and sugar beet [10]
It is highly likely that differences in gm among the cultivars disrupted the overall correlations between water-use efficiency (WUEi) and CID, this was tested only on the two dates each year when leaf temperatures were similar to those in which gm had been measured in prior indoor experiments
Summary
With projected increased frequency of drought, and decreased availability or increased cost of water for agriculture, increasing the efficiency of water use in agriculture is an important goal [1]. In spite of common correlations between CID and crop WUE, in some cases CID has not been correlated with leaf gas exchange measurements of Ci /Ca [5,13,14]. This type of result is of concern for the general usefulness of CID as a selection tool to change Ci /Ca. Warren et al [15] argued that mesophyll conductance (gm ) to CO2 movement from the sub-stomatal air space to the site of fixation. WUEi,of and carbon isotope delta (CID) values of five soybean cultivars i /C intrinsic leaf water-use efficiency Mature leaves harvested at early pod fill were analyzed for CID values for tests of correlations between CID and the mean leaf WUEi of the cultivars
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