Masters Theses

Date of Award

8-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant Sciences

Major Professor

Robert M. Augé

Committee Members

Bonnie Ownley, Jaehoon Lee

Abstract

Arbuscular mycorrhizal (AM) symbiosis has been reported often to improve the abilities of host plants to tolerate drought stress. The physiological mechanism is uncertain, but one idea is that the effect might be linked to resistance to salt stress. Several studies have shown more growth in crop plants colonized with AM fungi than nonAM control plants under salt stress. Drought and salt stresses frequently occur together in nature and their initial symptoms in plants are similar. It may be interesting to scrutinize their physiological interaction in host plant as a function of AM fungi. Therefore, the objectives of my studies were to investigate if AM influence on plant response to drought is more evident in saline soils. I hypothesized that 1) AM and nonAM plants would have different values of water relation parameters with exposure to drought and 2) AM-induced drought tolerance would be greater when plants are subjected to salt stress during drought.

In two separate greenhouse experiments, sorghum was colonized with Glomus intraradices, Gigaspora margarita, or a mixture of AM species isolated from semiarid grasslands in Arizona (AZ). To induce drought stress to the host plants, watering was held after applications of soil solution. NaCl (40 mM and 80 mM) was applied to pot soil to initiate salt stress and macronutrient solution (-0.4 MPa and –0.8 MPa) was used for exposure of osmotic stress to host plants in experiment 2. To eliminate remaining salt ion in soil, a group of pots were leached heavily with distilled water in experiment 2. The pots receiving same amount of water as salt solution served as control plants. Several parameters in relation to leaf and soil water status were monitored to determine the effect of AM symbiosis under drought and salt stress.

Significant decline of stomatal conductance was often observed when salt solutions (NaCl or macronutrient solution) were applied to the pot soil. All the sorghum plants reached stomatal closure in 9 to 12 days in experiment 1 and 9 to 16 days in experiment 2. Higher stomatal conductance often was observed in mycorrhizal plants before and after application of salt solution relative to nonAM plants; mycorrhizal promotion of stomatal conductance was generally 10 to 20% and even 100% near the stomatal closure point. The days to reach stomatal closure and the lethal point (point at which most foliage had died) varied with soil treatments and mycorrhizal association. It took the most days to reach the lethal point when plants were subjected to the macronutrient solution.

The shortage of available soil water and increasing solute level generated by osmotic stress resulted in decreasing leaf and soil water potential. The salt-treated soil maintained higher water content and higher soil water potential than pot soil treated with just water or leached with distilled water, but lower leaf water potential was observed at the stage of stomatal closure and lethal point in plants in salt-treated soil. There were a few significant different values in leaf and soil water potential between AM and nonAM plants during experimental period. AM plants showed lower lethal leaf water potential than nonAM plants when colonized with Glomus intraradices under drought alone. Symbiosis with Glomus intraradices did not have any marked effect on the parameters when drought and salt stresses were combined. There were no mycorrhizal effects on most parameters when plants were colonized with Gigaspora margarita or AZ.

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