Germinating the seeds of three species of Pimelea sect. Epallage (Thymelaeaceae)

Abstract

Abstract. Pimelea trichostachya Lindl., P. simplex F.Muell. and P. elongata Threlfall frequently cause pimelea poisoning of cattle. Fresh seeds of these species, belonging to sect. Epallage (Endl.) Benth. of Pimelea Gaertn. (Thymelaeaceae) are strongly dormant for years when in laboratory storage. Common methods of stimulating germination, such as scarification, dry heat and cold stratification, did not remove much of the dormancy. 'Smoke-water' stimulated some germination but its effect was unpredictable and many seedlings then grew aberrantly. Exposure of imbibed seeds to gibberellic acid greatly and reliably improved the germination of all three species. However, the manner of application and the concentration of gibberellic acid used had to be appropriate or many young seedlings grew abnormally or died suddenly, limiting successful plant establishment rates. The dormancy type involved is non-deep Type 2 physiological. Ten days of good moisture, in addition to gibberellic acid exposure, is required before appreciable laboratory germination occurs at optimal temperatures. Thus, the mechanism by which gibberellic acid stimulates good germination does not appear to be the same as that which primes seeds for the rapid and prolific germination often seen under natural conditions in arid Australia. Seeds of P. simplex subsp. continua (J.M.Black) Threlfall proved most difficult to germinate and those of P. elongata the easiest.

Poisonous Plants of the Genus Pimelea: A Menace for the Australian Livestock Industry

“…Significant features of the climate in areas where the Pimelea species are found include winter rain and frosts, high rainfall variability (leading to the possibility of long periods between suitable rainfall conditions for diaspore germination and plant growth) and temperature extremes (long, hot summers and cold winter nights). Mature diaspores shed from the plant in early summer do not germinate at the first rains because of a hard diaspore coat, possibly imposing a physical dormancy [ 22 ]. In general, Pimelea species germinate during autumn, flower in the spring and mature diaspores are dropped directly on to the ground during early summer leading to localised high density Pimelea stands.…”

“…According to Dadswell [ 9 ], a P. trichostachya population with an average plant density of six plants m −2 produced 1768 diaspores m −2 with each plant producing 288 diaspores [ 9 ]. Fresh diaspores are dormant and are hard to germinate even after scarification in laboratory conditions [ 22 ].…”

“…Pimelea elongata took less than 2 months weathering to germinate as its diaspores presumably had a weaker physical dormancy. Diaspores are known to remain viable for long periods of time in the soil, withstanding extremely dry conditions, fire, land remodeling by bulldozing, cultivation or following land disturbance by animal hooves [ 22 ].…”

“…However, for laboratory work, this is not desirable, and pretreatments are usually applied to achieve uniform germination. Chemical stimulation approaches such as the use of gibberellic acid (GA 3 ) and smoke water imbibition have provided variable germination success [ 22 ]. Similarly, physical dormancy breaking treatments such as scarification, seed coat removal, cold stratification and heat treatment have all provided some levels of germination success [ 22 ].…”

“…Chemical stimulation approaches such as the use of gibberellic acid (GA 3 ) and smoke water imbibition have provided variable germination success [ 22 ]. Similarly, physical dormancy breaking treatments such as scarification, seed coat removal, cold stratification and heat treatment have all provided some levels of germination success [ 22 ]. However, cutting the seed coat was extremely difficult and it could damage the embryo, so it was ruled out as an impractical method to produce seedlings for plant growth studies [ 22 ].…”