Lateral development in roots and shoots of Pinus radiata seedlings

Abstract

Under natural conditions, short-roots of pines usually occur in intimate association with basidiomycetous fungi, forming symbiotic relationships known as ectomycorrthizas. Although mycorrhizas have been most extensively studied in regard to their nutritional contribution to host plants, there is also considerable literature dealing with their effect on root morphology and development. Pine ectomycorrhizas exhibit a characteristic morphology related to the ability of short-roots to undergo repeated dichotomous divisions. The result is often a dense cluster of profusely branched, swollen roots, derived entirely from a single lateral. The interesting, but not completely understood aspects of this phenomenon are the degree to which forking is an independent reaction of short-roots and the part played by mycorrhizal fungi in promoting its formation. Some workers have suggested that forking results from the inhibitory action of fungi, while others have proposed that it is an expression of enhanced root growth. Previous research has suggested that mycorrhizal fungi were capable of directing host root growth as a result of synthesising and exuding plant growth hormones. Subsequent work has provided further confirmation of, and exceptions to this idea are discussed in this thesis.

Responses of pine seedling shoots to root-applied growth substances were studied. These experiments, along with some interesting observations on the proliferation of short-shoots, and some preliminary results on rooting of cuttings and the ability of mycorrhizal fungi to produce a range of growth active substances are presented.

Pinus radiata was chosen because of its availability, its role in forestry and its rapid and hardy growth. Although much is known about its shoot growth and silviculture and rooting of cuttings, only a limited amount of work has been concerned with root growth and development.