Zea diploperennis, a perennial grass closely related to corn also, exhibits agronomic traits associated with ability to withstand dessication stress. In a study of the linkage and inheritance of the gene for perennialism, W. C. Galinat (MNL 55:107, 1981) found that Z. diploperennis and its F1 and F2 hybrids with corn had traits associated with the capacity to withstand drought stress. In a 3-point test population of corn-diploperennis hybrids, P. C. Mangelsdorf and M. E. Dunn (MNL 58:54-55, 1984) demonstrated heterozygous progeny had robust root systems that may impart drought resistance. Such extensive root systems are also found in (corn X Tripsacum-diploperennis) plants that exhibit superior resistance to corn rootworm. The root biomass of these plants is approximately 50% greater than that of controls. This signals the possibility that the Tripsacum-Z. diploperennis genetic bridge might also be useful for transferring enhanced drought tolerance to corn.
A small-scale pilot study was conducted in a greenhouse at the Duke University Phytotron from March 8-16, 1999, to assess whether corn crossed with two different Tripsacum-Zea diploperennis hybrid lines exhibited enhanced resistance to drought when compared to corn. Plants were grown in Peterís professional potting soil in 10-inch diameter pots. Until the drought period, they were watered twice daily, and fertilized with one tablespoon Osmocote 14-14-14, plus they received liquid nutrient (modified Hoaglandís solution) three times a week.
The number of treatment plants included seven of 97-1; three of line E; three of (97-5 X 97-1); four of (97-1 X 97-3), and three of B73 corn. Tripsacorn and Sun Star, the two parent drought resistant hybrids in the above lineages, were also included.
Drought stress was induced when the plants began flowering. Treatment plants received no water for five days from March 9-13, during which time controls were watered twice daily according to the normal regimen.
The goal was to achieve 30% reduction in plant available water (% PAW), which was estimated at approximately 20% reduction in pot weight, and not to exceed 50% before the end of the drought treatment. This was monitored gravimetrically by weighing each pot at full saturation when the drought stress was initiated and recording pot weight daily until the end of the drought. The summary of per cent pot weight loss in Table 1 shows that water loss over all the lines tested ranged from 21.7% to 39.4%, indicating there was significant reduction in % PAW during the drought test.
Water use efficiency was monitored by measuring stomatal conductance and net photosynthesis using a Licor 6400 open photosynthesis system. The numbers are reported in Table 1. Leaf rolling and wilting were also observed. These drought symptoms were pronounced in the B73 corn plants, but virtually undetectable in the hybrid lines. At harvest, grain dry weight and shoot biomass dry weight were recorded to provide an index of drought intensity among lines as well as between treatment and control plants within lines.
From the data presented in Table 1, it can be seen that most of the treated hybrid plants had a reduction in stomatal conductance and photosynthesis, signaling that the drought stress induced stomatal closure and consequently reduced levels of carbon dioxide available for photosynthesis. In some cases the differences were dramatic, and in others the reduction in numbers was minimal. Evidence of surprising drought resistance was exhibited by increase of predrought stomatal conductance and photosynthesis measurements in four hybrid plants, numbers 150 and 153 in the 97-1 line and numbers 144 and 146 in the (97-1 X 97-3) line (see Table 1). Table 1 shows that grain yield in drought stressed hybrid plants was greater than the controls except for plant 151 in the 97-1 line and plant 147 in the 97-1 X 97-3 line. This is in striking contrast to the drought stressed W64A corn plants, all of which had significant reduction in grain yield compared to the control.
The results of the drought pilot study revealed evidence of genetic segregation for drought resistance among (Tripsacum-Zea diploperennis X corn) hybrid lines. The findings indicate there is good potential for superior drought resistance to be imparted to corn via a recurrent selection breeding program employing Tripsacum-Zea diploperennis in crosses with corn.
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