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Seed dormancy, smoke-stimulated germination and harvest timing of pili grass (heterpogon contortus), a native Hawaiian grass with potential for expanded re-vegetation use

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Item Summary

Title: Seed dormancy, smoke-stimulated germination and harvest timing of pili grass (heterpogon contortus), a native Hawaiian grass with potential for expanded re-vegetation use
Authors: Baldos, Orville Caldo
Keywords: Heteropogon contortus
Issue Date: Dec 2013
Publisher: [Honolulu] : [University of Hawaii at Manoa], [December 2013]
Abstract: Pili grass (Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult.) is a culturally and ecologically important native species in Hawaiʻi that has been used for re-vegetation of severely degraded land. Currently, it is being evaluated for expanded use on roadside and streambank stabilization, water efficient landscaping and agricultural buffer strip plantings. Despite efforts to increase its use on a variety of re-vegetation applications, planting materials such as seeds are still in limited supply. In order to develop large scale seed production protocols, studies on seed dormancy and seed harvest timing need to be conducted. The three main objectives of this dissertation were to: 1) elucidate the mechanisms involved in pili grass seed dormancy loss through the use of scarification and germination stimulants (i.e., gibberellic acid [GA], fluridone, food grade liquid smoke [FGLS], pili grass smoke infused water, xylose smoke infused water, karrikinolide and cyanide); 2) assess the effect of storage factors (i.e., seed moisture content and temperature) on pili grass seed dormancy loss and viability; and 3) determine an average cumulative growing degree unit (ACGDU) correlated indicator which has potential use for determining the optimum seed harvest timing.
Studies on seed scarification, germination with stimulants and dormancy loss under different storage conditions support the placement of pili grass under the non-deep physiological dormancy class of seeds. The observed partial germination with scarification (i.e., exposure of the embryo) suggests the presence of both physical and physiological dormancy controls.
Germination assays with the plant growth regulators, fluridone and GA, as well as with FGLS suggests that the abscisic acid (ABA):GA balance plays an important role in pili grass seed dormancy. Seed germination response to chemical stimulants across a gradient in dormancy relief supports the theory of increased effective levels of GA as storage duration increases.
Short term seed storage studies (i.e., 1 year or less) show that dormancy and seed viability loss is affected by storage temperature and equilibrium relative humidity (eRH) (i.e., seed moisture content). Storage at 10°C, regardless of eRH, maintained seed dormancy and viability for one year. To optimize dormancy loss while maintaining seed viability, fresh seeds must be stored at either 12% eRH (6% seed moisture content, dry weight basis) at 30°C for 12 months or at 50% eRH (11% seed moisture content, dry weight basis) at 30°C for 9 months. As storage temperature was increased from 20 to 30°C and storage relative humidity was increased from 50 to 75% eRH (11 to 14% seed moisture content, dry weight basis), loss in seed viability became more pronounced over time. The viability of pili grass seeds, relieved of dormancy, can be maintained for at least 6 months with storage at 12% eRH (6% seed moisture content, dry weight basis) and 5°C.
FGLS, pili grass smoke infused water and xylose smoke infused water were effective in stimulating germination of dormant pili grass seeds. FGLS and pili grass smoke infused water provided consistent germination stimulation in two batches of seed that represented two levels of dormancy relief. Xylose infused smoke water was less effective since its ability to stimulate germination was not consistent across the two seed batches. Assays which evaluated two bioactive compounds found in smoke (i.e., karrikinolide and cyanide) indicated that only cyanide stimulated germination (20 to 29% germination) of dormant pili grass seeds. Cyanide stimulated germination suggests the role of reactive oxygen species in dormancy loss of pili grass seeds under warm, dry conditions. The presence of cyanide was confirmed in pili grass smoke, but not in FGLS or xylose smoke infused water. Germination observed in these non-cyanide containing smoke sources indicates the presence of other bioactive compounds in smoke.
Germination assays conducted throughout this dissertation indicated differences in depth of dormancy with time of year harvest. March seeds exhibited the lowest levels of dormancy compared to seeds harvested in June, July and October. Seasonal dormancy in pili grass seeds can be attributed to differences in growing conditions during seed development.
Finally, harvest timing studies identified ACGDU, spike moisture content and the onset of seed head tangling as useful harvest timing indicators for pili grass seed production. Results indicate that maximum seed harvests can be obtained between 768 to 778 ACGDUs (79 to 82 days after cutting) under irrigated conditions. Spike moisture for optimum seed harvest timing was determined to be between 0.68 to 0.72 grams H2O per gram of dry weight. The onset of seed head tangling provided a visual cue which coincided with the optimum seed yield and range of spike moisture content. A decline in seed production over four harvest cycles (spanning two years) was recorded with possible causal factors such as stand age, cutting height and thatch accumulation.
Description: Ph.D. University of Hawaii at Manoa 2013.
Includes bibliographical references.
Appears in Collections:Ph.D. - Tropical Plant and Soil Sciences

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