We assessed inter-seasonal dynamics of seed banks, dormancy and seed germination in three endemic Chihuahuan Desert succulent species, under simulated soil warming conditions. Hexagonal open top-chambers (OTCs) were used to increase soil temperature. Seeds of Echinocactus platyacanthus (Cactaceae), Yucca filifera and Agave striata (Asparagaceae) were collected and buried within and outside OTCs. During the course of one year, at the end of each season, seed batches were exhumed to test viability and germination. Soil temperature in OTCs was higher than in control plots. Yucca filifera seeds always had high germination independently of warming treatment and season. Agave striata seeds from OTCs had higher germination than those from control plots. Agave striata exhibited low germination in fresh seeds, but high germination in spring. Seeds from this species lost viability throughout the experimental timeframe, and had no viable seeds remaining in the soil. Echinocactus platyacanthus showed high germination in fresh seeds and displayed dormancy cycling, leading to high germination in spring, low germination in summer and autumn, and high germination in winter. Germination of this species was also higher in seeds from OTCs than those from control plots. Echinocactus platyacanthus formed soil seed banks and its cycle of inter-seasonal dormancy/germination could be an efficient physiological mechanism in a climate change scenario. Under global warming projections, our results suggest that future temperatures may still fall within the three studied species’ thermal germination range. However, higher germination for A. striata and E. platyacanthus at warmer temperatures may reduce the number of seeds retained in the seed bank, and this could be interpreted as limiting their ability to spread risk over time. This is the first experimental study projecting an increase in soil temperature to assess population traits of succulent plants under a climate change scenario for American deserts.

This article briefly reviews how long weed seeds can live in the soil and what happens to them during burial. Freshly matured seeds of many weeds are water-permeable, but those of others are water-impermeable. Water-permeable seeds may have morphological, physiological, or morphophysiological dormancy, with physiological dormancy (PD) being the one most commonly found in buried weed seeds. Further, nondeep PD is very common in buried weed seeds, and many of them exhibit annual dormancy cycles in response to seasonal temperature changes. The time of year when seeds are nondormant varies with the species, i.e., autumn, spring, or spring to summer. A light requirement for germination plays an important role in preventing nondormant seeds from germinating in the soil. To germinate, soil disturbance that exposes seeds to light must occur at a time of year when seeds are nondormant. Buried seeds of some species come out of dormancy and remain nondormant regardless of seasonal changes in environmental conditions; however, a light requirement for germination prevents them from germinating in the soil. Water-impermeable seeds have either physical dormancy (PY) or a combination of PY and PD, with PY being the most common, e.g., in members of the Fabaceae and Malvaceae. Seeds with PY have a water gap in the seed coat that opens in response to an environmental signal, thereby allowing water to enter. When disturbance brings seeds to the soil surface, temperatures that are higher than those in the soil can cause the water gap to open. Consequently, the water gap indirectly serves as a depth sensor. A challenge for the future is to use information about buried weed seeds to better manage weeds in crops, and modeling is an important step in that direction.

Seed dormancy can prevent germination under unfavourable conditions that reduce the chances of seedling survival. Freshly harvested seeds often have strong primary dormancy that depends on the temperature experienced by the maternal plant and which is gradually released through afterripening. However, seeds can be induced into secondary dormancy if they experience conditions or cues of future unfavourable conditions. Whether this secondary dormancy induction is influenced by seed-maturation conditions and primary dormancy has not been explored in depth. In this study, we examined secondary dormancy induction in seeds of Arabidopsis thaliana matured under different temperatures and with different levels of afterripening. We found that low water potential and a range of temperatures, from 8°C to 35°C, induced secondary dormancy. Secondary dormancy induction was affected by the state of primary dormancy of the seeds. Specifically, afterripening had a non-monotonic effect on the ability to be induced into secondary dormancy by stratification; first increasing in sensitivity as afterripening proceeded, then declining in sensitivity after 5 months of afterripening, finally increasing again by 18 months of afterripening. Seed-maturation temperature sometimes had effects that were independent of expressed primary dormancy, such that seeds that had matured at low temperature, but which had comparable germination proportions as seeds matured at warmer temperatures, were more easily induced into secondary dormancy. Because seed-maturation temperature is a cue of when seeds were matured and dispersed, these results suggest that the interaction of seed-maturation temperature, afterripening and post-dispersal conditions all combine to regulate the time of year of seed germination.

Cycling of physically dormant (PY) seeds between states insensitive and sensitive to dormancy-breaking factors in the environment has recently been demonstrated in Fabaceae and Convolvulaceae, and it may be a common phenomenon in seeds with water-impermeable seed coats. In contrast to seeds of many species with physiological dormancy (PD), those with PY cannot cycle between dormancy and non-dormancy (ND). In this paper, we evaluate the role of sensitivity cycling in controlling the timing of germination of seeds with PY in nature, and show that sensitivity cycling in seeds with PY serves the same ecological role as dormancy cycling in seeds with PD. Thus, sensitivity cycling in seeds with PY ensures that germination in nature occurs only at (a) time(s) of the year when environmental conditions for growth are, and are likely to remain, suitable long enough for the plant to complete its life cycle or to form a perennating structure. Further, we describe the experimental procedures necessary to determine whether sensitivity cycling is occurring, and discuss briefly the possible relevance of sensitivity cycling to dormancy classification.

Seeds of the narrow-endemic Solidago shortii and of the geographically-widespread S. altissima and S. nemoralis collected and buried in 1992 and 1993 were incubated in light and in darkness at 15/6°, 20/10°, 25/15°, 30/15° or 35/20°C following various periods of burial in soil in a non-temperature-controlled glasshouse. At maturity in November, seeds of the three species germinated to 0–1% in light at 15/6°C and to 10–77% at 20/10°, 25/15°, 30/15° and 35/20°C. Seeds exhumed each April from 1993 to 1996 and incubated in light at 15/6° and 20/10°C germinated to ≥83% and ≥90%, respectively, whereas those exhumed each September of 1993–96 germinated to ≤2%and ≤40%, respectively. At 25/15°, 30/15° and 35/20°C in light, seeds of S. altissima and S. shortii germinated to ≥52% and those of S. nemoralis to ≥19/, regardless of when they were exhumed. Timson's index, which integrates percentages, rates and times for onset of germination, was higher at all temperature regimes for seeds exhumed in April 1995 than for those exhumed in September 1995. Freshly-matured seeds of the three species germinated to 0–11% in darkness. Furthermore, regardless of when they were exhumed, seeds of S. altissima and S. nemoralis incubated in darkness germinated mostly to only 0–9% over the range of temperature regimes. In contrast, ≤88% and ≤6% of the seeds of S. shortii exhumed and incubated in darkness each April and September of 1993–96, respectively, germinated, ≤1% of them germinating while buried in soil. Thus, although buried seeds of all three species exhibited an annual conditional dormancy/non-dormancy cycle, only those of S. shortii exhibited cyclic changes in their germination response in darkness.

Torilis japonica (Apiaceae) has a widespread distribution, extending from western Europe to eastern Asia. In Europe, it usually behaves as a spring-germinating biennial species. Ripe seeds of T. japonica have an underdeveloped embryo and can persist in the soil for several years. The aim of this research was to reveal the mechanisms regulating the seasonal emergence pattern of seedlings. Experiments in a natural environment were performed to study phenology of seedling emergence and embryo growth. Seasonal changes in the dormancy status of T. japonica seeds were examined by regularly exhuming buried seeds and incubating them in controlled conditions. The action of temperature and light in regulating dormancy, embryo growth and germination was studied in the laboratory. Results showed that seeds of T. japonica have non-deep, simple morphophysiological dormancy (MPD), whereby physiological dormancy is broken by moist chilling (5°C). Once MPD was broken, embryo growth and subsequent germination started in spring, when appropriate temperature and light conditions were present. Seeds buried at a depth where light could not reach them showed cyclic changes in their dormancy status; embryo growth in these seeds could not be initiated because of the lack of a light stimulus. As far as we know, this is the first extensive study of seasonal dormancy cycles in a spring-germinating species of the Apiaceae. T. japonica occurs in a temperate climate with cool winters and warm, moist summers. In this climate, spring is often the most favourable season for seedling establishment.

The most often used time-line for distinguishing a transient seed bank from a persistent seed bank is one calendar year. Thus, species whose seeds live in or on the soil for <1 year have a transient seed bank, whereas those whose seeds live for ≥1 year have a persistent seed bank. However, dormancy cycling of seeds buried in soil has not been given due consideration in these models. When dormancy cycling is considered, it is shown that seeds of both autumn-germinators and spring-germinators are in the dormant state when they are 1 year old. Thus, unless the seeds live until at least the second germination season (i.e. usually 16–18 months following dispersal), they are, in effect, part of a transient seed bank, having lived through only one germination season. We propose that for seeds of such species to be considered part of a short-term persistent seed bank, they should remain viable and germinable until at least the second germination season, and to be part of a long-term persistent seed bank, until at least the sixth germination season. Our definitions are applicable to seeds with physiological, physical or morphophysiological dormancy, which often require >1 year after maturity to come out of dormancy in nature. We discuss modifications of the seedling emergence method for detection of a soil seed bank, so that they correspond to our definitions of seed-bank strategies.

Dry plants of Mesembryanthemum nodiflorum L. (Aizoaceae), with mature seeds enclosed in their dry capsules, were collected in the Judean Desert near Jericho, Israel, in the summer of 1972, and have been stored dry under room conditions [15–30°C, 15–25% relative humidity (RH)] since then. More than 31 years after maturation, these unimbibed seeds retained an annual rhythm of germination. A significantly higher percentage of seeds originating from the terminal, central and basal parts of the capsules germinated in February or March compared to other months of the year. This phenomenon was repeated in all three groups of seeds in 2001, 2002 and 2003 for germination in darkness at 15°C, and in terminal seeds germinated at 25°C in darkness for 4 years from 2001 to 2004. The annual rhythm of M. nodiflorum germination may regulate the season and percentage of seeds that germinate at the proper time of the year, even after many years. But the open question still remains: how do dry seeds measure time, and what are the mechanisms involved?