Limited-transpiration rate at elevated vapor pressure deficit (VPD) can allow soil water conservation for use during late-season drought, but it can also result in decreased crop yields under well-watered conditions because of restricted crop gas exchange. Previous studies with sweet corn (Zea mays L.) have found the limitedtranspiration rate was quite common among commercial cultivars even though sweet corn is commonly grown under well-watered conditions. This study was undertaken to identify possible genetic sources of sweet corn that were not encumbered by the limited-transpiration trait. Additionally, data were obtained to compare the plant hydraulic conductance among the lines. Among the 26 sweet corn lines included in this study, only eight did not express the limited-transpiration trait. Four of the lines not expressing the limited-transpiration (IL395a, IL543c, P39, and SD245) had stomata vapor conductance values over the range of tested VPD similar to the values expressed by many of the limited-transpiration lines only at low VPD. The eight lines not expressing the limited-transpiration trait tended to have low plant hydraulic conductance. For those lines expressing the limited-transpiration trait, there was a correlation between the VPD at initiation of limited transpiration and plant hydraulic conductance. Expression of the limited-transpiration traits proved, however, to be temperature sensitive in 7 of 18 tested lines expressing the trait at 32 ˚C because they failed to express the trait at 38 ˚C. The genetic variation in expression of the limitedtranspiration trait and plant hydraulic conductance identified in this study offers specific candidate inbred lines that could be used as genetic resources for improving sweet corn growth and yield for well-watered environments.
INTRODUCTIONGas exchange by plants through stomata necessarily means that the ratio of plant growth and water loss are intimately linked. Therefore, water-deficit conditions that result in limited water loss from leaves also result in limited growth. However, to enhance crop seed yield from the available water, itAbbreviations: VPD, vapor pressure deficit.
Water deficit can have large impacts on plants, including likely alteration of root hydraulic conductance and root xylem vessel diameter, which can decrease crop productivity. No results, however, exist to assess possible linkages between these two variables as critical components contributing to plant water status. This linkage was investigated in three maize (Zea mays L.) cultivars. A stable water‐deficit treatment was established and maintained in pots by allowing soil drying to the point where transpiration rate was held constant at about 0.5 of well‐watered pots. Initially, the root hydraulic conductance of the water‐deficit plants was equivalent to that of well‐watered plants. Subsequently, however, hydraulic conductance decreased substantially. The results for xylem vessel diameter at 5 cm from the root tip exhibited a pattern similar to the decrease in root hydraulic conductance. A graph of root hydraulic conductance versus xylem vessel diameter at 5 cm showed a curvilinear response with lessening in the increase in hydraulic conductance with increasing xylem vessel diameter. The results indicate a possible link between root conductance and xylem diameter but the conductance is much less sensitive to vessel diameter than the fourth power of the radius predicted by Poiseuille's law. The association between conductance and xylem vessel diameter may reflect interaction of radial and axial water flux through the root system as indexed by vessel radius in the zone near the root tip.
Genotypes in crop species have been identified that initiate partial stomata closure at elevated atmospheric vapor pressure deficit (VPD), which results in conserved soil water for crop use during subsequent water‐deficit episodes and thereby allowing for possible yield increase. In sweet corn (Zea mays L), 17 genotypes have been previously identified with the VPD‐responsive trait, although the VPD value at the initiation of stomata closure varied among genotypes. A hypothesis to explain variation in transpiration response to VPD is that water flow capacity in the leaves differs among genotypes. To gauge water flow capacity in leaves, the rate of stomata opening was observed visually after stomata closure was induced by 3 kPa VPD. The stomata opening time was rapid and varied among genotypes from 90 to 179 s. However, there was no correlation between opening time and the VPD at which partial stomata closure was initiated in intact plants. An additional set of experiments was done to examine whether genotypic differences in a subpopulation of silver‐inhibited aquaporins might contribute to differences in leaf water flow. There was a correlation among genotypes between slow opening time of the stomata and greater inhibition of transpiration rate following feeding leaves with silver ion. However, the response to the silver treatment did not correlate with the VPD at which transpiration decrease of intact plants was initiated. These results indicate that the differences observed in the water flow capacity in sweet corn leaves were not major factors accounting for the genotypic differences in whole‐plant transpiration response to elevated VPD.
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