Abstract
Salvia verbenaca (wild sage) is a commonly cultivated herbal medicine plant, which is native to the Mediterranean climate regions of Europe, Africa, Asia and the Middle East. However, it has become an invasive species in semi-arid and arid regions of southern Australia. Two varieties are present in this region, var. verbenaca and var. vernalis, each of which can be distinguished by differences in morphology and flowering period. Following trials to determine the optimum temperate regime for germination and response to light and dark, seeds of both varieties were tested for their response to variations in pH, moisture stress, salinity, and burial depth. The temperature and light trial was carried out using three different temperature regimes; 30/20°C, 25/15°C and 20/12°C, and two light regimes; 12 hours light/12 hours dark and 24 hours dark, with var. vernalis responding to relatively higher temperatures than var. verbenaca. The germination rate of neither species was significantly inhibited by complete darkness when compared to rates under periodic light exposure. Both varieties germinated at near optimum rates strongly to very strongly in all pH buffer solutions, from pH 5 to pH 10, but they responded most strongly at neutral pH. Var. vernalis showed slightly more tolerance to reduced moisture availability, moderate to strong salinity, and burial depth, compared to var. verbenaca. However, even a fairly shallow burial depth of 2 cm completely inhibited germination of both varieties. Thus, in circumstances where both varieties are present in a soil seedbank, var. vernalis could be expected to establish in more challenging conditions, where moisture is limited and salinity is ‘moderate to high’, implying that it is a more serious threat for invasive weed in conditions where crop plants are already challenged.
Highlights
There was no difference in the time to commence germination in var. verbenaca in either light regime (3.0 days for both), and only a slight delay for var. vernalis in the alternating light treatment (2.0 days) compared to complete darkness (2.8 days)
The effect of seed size on germination according to population source of seed within a species was investigated by Capon and Brecht (1970) [36], with a finding that a rough correlation exists between seed size and total percent germination. We suggest that this phenomenon be explored for S. verbenaca var. verbenaca and var. vernalis, in order to establish if there is any variation in seed size and if this can go some way towards explaining the different germination responses such as the rate of germination and burial depth response in these two varieties
Each variety did not contrast significantly in their response to light, var. vernalis appeared to be better adapted to higher temperatures, lower water availability and higher salinity than var. verbenaca, since it germinated more rapidly and at a slightly greater percentage, compared to var. verbenaca
Summary
As is common practice in several parts of the world for other Salvia species (e.g., S. officinalis, S. fruticosa, S. lavandulaefolia and S. sclarea), S. verbenaca is deliberately cultivated to (i) obtain dried leaves, which are used as raw material in herbal medicines and (ii) provide essential oils which are extracted from the plant [2]. It has inadvertently become an invasive environmental weed of semi-arid and arid regions of Australia [3]. As has been observed with other introduced weed species, both varieties may vary in their growth habitat and respond differently to different environments, which makes knowledge of their habits a priority for sensible management and control
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