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Accueil du site → Doctorat → Australie → 2011 → The development of grass establishment mats for use in revegetation : Implications of seed biology and water uptake mechanisms

University of Queensland (2011)

The development of grass establishment mats for use in revegetation : Implications of seed biology and water uptake mechanisms

Paterson Mary

Titre : The development of grass establishment mats for use in revegetation : Implications of seed biology and water uptake mechanisms

Auteur : Paterson Mary

Grade : Doctor of Philosophy (PhD) 2011

Université de soutenance : University of Queensland.

Résumé
Stabilisation of roadside and amenity areas by grass covers in dry environments depends on the physical stability of the surface and rapid germination of grass diaspores (seeds). Grass establishment mats, designed to minimise erosion and provide a suitable medium for germination were investigated between 1997 and 2000. Complementary studies of the relationships between grass seed structure, water uptake, temperature and germination identified necessary environmental conditions for the successful deployment of grass establishment mats. This research represents a new contribution to the seed biology of thirteen grass species and its application to revegetation. Germination and establishment of subtropical and tropical grasses are critically dependent on soil water potential and the hydraulic conductivities of both soil and seed coat. Species could be segregated according to rate of imbibition and tolerances of reduced water potential and cyclical wetting and drying, which influenced both their adaptability to stressful environments and their suitability for incorporation into plant establishment mat systems.
Germination at constant reduced water potentials identified three seed groups which could germinate only above -0.7, -1.2 and -1.7 MPa respectively. Cyclical wetting and drying regimes elicited three groups : (1) species (e.g. Lolium perenne and Bothriochloa bladhii) with rapid imbibition, able to germinate below -1.2 MPa, but intolerant of drying ; (2) species exhibiting hydropedesis (e.g. Dichanthium sericeum and Chloris gayana) with rapid imbibition and radicle emergence above -1.2 MPa, able to enter a resting phase on drying before germination is complete, followed by completion of germination upon rehydration : and (3) species (e.g. Themeda triandra) exhibiting slow imbibition, delayed germination and slow loss of germination capacity on dehydration. No single seed attribute accounted for the drought sensitivity or tolerance of a species. A model was developed to describe the postulated responses of the three groups of grass seeds to varying water availability.
The response of the seed to soil water is very closely related to seed coat structure. Important water uptake pathways in chaffy grasses (with hairs and a husk) are longitudinally through the callus and transversely through the junction of the glumes. Qualitative electron microscopy of apoplastic solute transport indicated that the embryo became hydrated and germination commenced well before the endosperm hydrated ; orientation of the seed with respect to moisture sources, and particularly contact with the callus, was important in some species ; hollow hairs and relatively unlignified tissues accumulated and possibly transported water and the glumes could retain absorbed water during a drying cycle. Germination of intact seeds was delayed or occurred within a smaller environmental range than for naked diaspores but intact seeds were more likely to germinate only when the environmental conditions were more likely to lead to successful seedling establishment. Pre-treatment of seeds with dry smoke or smoke solution frequently altered germination responses. Electron microscopy of Themeda triandra glumes showed apparently waxy plugs occluding many pores were disrupted by smoke treatments, presumably allowing moisture and gaseous exchange and leading to the breaking of seed dormancy in this species.
Critical characteristics of grass establishment mats include physical dimensions (thickness), mechanical (strength, flexibility and structure) and hydraulic properties (pore size, permeability, wettability, water holding capacity, water release characteristics and rate of drying). While the physical and mechanical properties all influence the practicability of a mat, this study showed that the hydraulic properties were crucial for plant growth. Fibre matting products were effective in establishing seedlings under dry conditions only when sufficient water was available to the seeds soon after installation. In most mat configurations, the germination of seeds in or under mats occurred 2 days later than for seeds sown on the soil surface because the mat water content had to be increased to a critical point before water became available for imbibition and germination. Thereafter, the rate, frequency and completeness of germination were dependent on the continuing availability of water within the matting system. Seeds sown onto a bare soil surface, while in immediate contact with soil water, were subject to greater variations in temperature and more rapid drying than seeds in or under a mat. Although no ideal matting design was identified, the study indicated that they could be beneficial in certain circumstances. The most successful matting system for use in dry environments contained outer layers of fine grade jute fibre, filled with bentonite clay and impregnated with grass seeds.
This thesis provides a perspective on the complex array of factors influencing the development of a commercial seed establishment mat. More importantly, it addresses the questions that must be resolved before large-scale investment in mat production or revegetation are undertaken and it demonstrates that knowledge of the biology of the target species increases the likelihood that revegetation will be successful.

Mots clés : hydropedesis ; water uptake ; seed morphology

Présentation

Page publiée le 20 mai 2012, mise à jour le 2 juillet 2017