Variation in seed dormancy and germination among populations of Silybum marianum (Asteraceae)

Abstract

Milk thistle (Silybum marianum) is a medicinal plant; however, lack of consistency in past dormancy studies has hindered propagation of this species from seeds. We tested the germination responses of freshly harvested and after‐ripened (stored for 2 and 7 months; 25°C at 50% relative humidity) seeds from three populations (P1, P2 and P3) in Iran at varying constant or alternating temperatures, with or without GA3 and in light and continuous darkness. No germination occurred in freshly harvested seeds incubated at any condition without GA3 application, indicating that all the seeds were dormant. Seeds from P1 and P2, which developed under relatively dry, warm conditions, germinated over a wider range of temperatures after 2 months of dry storage, indicating type 6 of non‐deep physiological dormancy (PD). Seeds from P3, which developed under relatively wet, cool conditions, incubated at constant temperatures (especially on GA3), exhibited an increase in maximum temperature for germination, indicating type 1 of non‐deep PD. Light improved germination of after‐ripened seeds, and GA3 application substituted for the light requirement for germination. This is the first report that environmental conditions during seed development may be correlated with differences in the type of non‐deep PD. We conclude that milk thistle seeds are positively photoblastic and photodormant and the germination responses of after‐ripened seeds from different populations are different under darkness. Therefore, the impacts of genetic differences and maternal effects on the induction of dormancy during seed development should be considered in attempts to domesticate this medicinal plant.

Seed Dormancy and Seedling Ecophysiology Reveal the Ecological Amplitude of the Threatened Endemism Picris willkommii (Schultz Bip.) Nyman (Asteraceae)

“…In addition, the low water-use efficiency would hamper its spread into the surrounding sandy soils, as well as its ability to withstand increasingly frequent prolonged drought periods. The practically null emergence of plants during summer, even though some rainy episodes were recorded every studied year, would indicate that high temperatures (daily mean ≥ 24 °C) do not promote germination of this species [ 22 , 23 , 24 , 25 ], and/or that the seeds undergo some type of dormancy when they mature from which they are not released until late summer [ 10 , 26 , 27 , 28 , 29 ]. Similarly, once the mild temperatures of early autumn had passed (daily mean around 20 °C), the low temperatures (daily mean < 15 °C) of late autumn and winter considerably reduced germination, as occurs with many other species of the Asteraceae family [ 6 , 30 , 31 , 32 ].…”

“…The statistical significance of the Temperature × Seed type interaction revealed that the central seeds were much more sensitive to temperature than the peripheral ones, germinating in 80% when the temperature was close to the optimum. However, up to 20% of central seeds still remained dormant, whose germination control could be internally regulated [ 10 , 27 ]. This leads us to think that most of the central seeds are released from dormancy at the end of summer and go into a quiescent state, being able to activate germination as soon as the environmental conditions are favorable for it.…”

“…The positive response of germination of both types of seeds to gibberellins reveals physiological endogenous dormancy at the time of maturation [ 10 , 23 , 29 , 32 ]. Additionally, the favorable response to potassium nitrate (KNO 3 ) of peripheral seeds would indicate, apart from the release of physiological dormancy, a possible preference of the species for more fertile substrates for seed germination (at least with nitrates), and/or the presence of germination inhibitors that are eliminated by chemical oxidation by KNO 3 [ 22 , 23 ].…”

“…This dormancy can be released by means of a physiological stimulant. However, the significant increase in germination parameters, repeating the test 3–4 months later (October), suggests that a certain percentage of seeds naturally achieves dormancy alleviation at the end of summer, perhaps induced by the high temperatures of the warm season or by the washing of water-soluble inhibitors, thanks to the rains of early autumn [ 10 , 27 , 28 , 29 ]. The seeds that remain in the temporary aerial bank (fixed to the dry mother plant) possibly extend the dormancy for a longer time because they suffer less washing of germination inhibitors than the seeds that fall to the ground.…”

“…Dormancy can be defined as a blocked state in which the seed does not germinate, even though there are favorable environmental conditions for it, and it can be determined by the seed coat or by the embryo itself [ 1 , 4 , 8 ]. It has been estimated that 50–95% of species have seeds with some type of dormancy at maturity (physical, physiological, morphological, morpho–physiological or combinational dormancy), and among these species, 50–75% have physiological dormancy [ 9 , 10 ] induced by the accumulation of water-soluble germination inhibitors or abscisic acid [ 4 ]. This means that seeds have inhibition mechanisms that must be broken before germination, to release the dormant state, which will depend on the dormancy type.…”