Journal of Experimental Botany, Vol. 52, No. 358, pp. 1129-1133,
May 1, 2001
© 2001 Oxford University Press
Short Communication |
Water stress can induce quiescence in newly-germinated onion (Allium cepa L.) seedlings
1 Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK
2 Polish Academy of Sciences, Institute of Agrophysics, 20-290 Lublin 27, Poland
3 Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK
Received 7 September 2000; Accepted 26 January 2001
Abstract
The effect of water stress on the early seedling growth of onions was studied by placing newly-germinated seedlings in vermiculite equilibrated at different water potentials. Roots and shoots elongated more at -0.29 than at -0.64 MPa, but did not elongate at -1.66 MPa. However, roots and shoots of seedlings that had been incubated in vermiculite at -1.66 MPa for up to 35 d resumed elongation when subsequently placed on wet filter boards. This suggests that water stress can induce quiescence in newly-germinated seedlings.
Key words: Allium cepa L., growth curves, onion, recovery, water stress.
Introduction
In order to emerge, the shoot from a germinated seed has to be capable of reaching the soil surface, while continued root growth is required to gain access to water in drying seedbeds. Shoot and root growth are known to be sensitive to mechanical impedance and water stress in laboratory and field studies (Collis-George and Yoganathan, 1985
a, b; Finch-Savage et al., 1998; Townend et al., 1996; Whalley et al., 1999). Root and shoot elongation rate decrease directly with water potential in vermiculite (Sharp et al., 1988) but as soil dries it also tends to become stronger and mechanical impedance rather than water stress can become limiting (Weaich et al., 1992).
Most studies of physical stresses on seedlings have looked at the effects of a constant stress. In most seedbeds, however, the physical conditions fluctuate and the ability of seedlings to recover from stress may be important. In onion and carrot seedlings, increasing the length of time for which seedlings are exposed to mechanical impedance decreased the extent of subsequent growth (Whalley et al., 1999). Onion shoots showed a rapid recovery of elongation rate after removal of impedance, whereas carrot shoots showed delayed recovery. Seedling pea roots also showed a delayed recovery in elongation rate when allowed to grow from strong soil into weak soil (Bengough and Young, 1993). Little is known, however, about how well newly-germinated seedlings can recover following exposure to water stress. In this paper, the results of experiments in which newly-germinated onion seedlings were allowed to recover from different water potentials are reported. The data are interpreted in terms of the likely impact that water stress can have on crop emergence.
Materials and methods
Plant material and growth conditions
Experiments were performed on seedlings of onion (Allium cepa L. cv. Hysam), which were germinated on sloping filter boards (Gray and Steckel, 1983). All stages of the experiment were carried out at 20 °C in the dark, except when seedlings were exposed to light during measurements.
Experimental treatments
Thirty seeds were set to germinate for each experimental treatment. Each sloping board held six seeds and the five boards required for each treatment were randomized between ventilated incubator units. The seeds were inspected twice each day and at each inspection the seeds that had germinated (radicle emerged) were placed into experimental treatments. After 5 d, 99% of the seeds had germinated.
Germinated seeds were placed in moist vermiculite (fine grade, William Sinclair Horticulture Ltd., Lincoln, UK) equilibrated with water at 0.08, 0.15 or 0.25 g water g-1 dry vermiculite and then allowed to grow for 14, 25 or 35 d, giving nine treatments. For each water content, the vermiculite was mixed in one large batch and then divided between 15 plastic containers 130 mm in diameter and 160 mm deep, which were half-filled with vermiculite. The vermiculite was consolidated in the containers by applying a pressure of 3 kPa to the surface of the vermiculite with a loaded disc, which was removed after a few seconds. If the vermiculite was not consolidated, preliminary experiments showed that there was a tendency for root growth to push the seedling out of the vermiculite. In order to minimize disturbance of the seedlings, containers were planted sequentially as germinated seeds became available from the filter boards. Each container was planted with six germinated seeds at a depth of 10–12 mm, and sealed with parafilm after planting to minimize water loss. There were five containers per treatment. At the end of each time period (14, 25 or 35 d) the seedlings were harvested from the vermiculite and returned to fresh wet sloping filter boards. The water content of the vermiculite in each harvested container was measured by oven drying at this time. The length of the root and shoot was measured each day until elongation had stopped.
Thirty seeds were also set to germinate on sloping filter boards for each of two well-watered control treatments. Each treatment used a ventilated incubator unit containing five boards, with six seeds per board. For one control treatment, the germinated seeds were left in place on the boards and the length of the root and shoot was subsequently monitored until 21 d after germination. For the second control treatment, the seeds were inspected twice each day and at each inspection the seeds which had germinated were placed onto fresh sloping filter boards in plastic containers as used for the vermiculite treatments. Water was poured into the bottom of the containers to a depth of 20 mm and the containers sealed with parafilm. Root and shoot lengths were measured 21 d later.
Measurement of water potential
The relationship between water potential and water content for the vermiculite was measured using a Wescor HR-33T dew point psychrometer with a C52 sample holder (Wescor Inc., Logan, Utah, USA). The following calibration for water potential, 
, was obtained
 | (1) |
is the gravimetric water content. Equation 1 was fitted with Minitab® and had an r2 of 0.934 with P<0.001.
Statistical analyses
An analysis of variance using Genstat® 5 was performed on the vermiculite water content data to determine the effect of time of harvest on water content and whether this was affected by the initial water content.
The shoot and root recovery data in Fig. 1C and Fig. 2 were fitted with Genstat® 5 to the equation
 | (2) |
|
|
Results
Water potential of vermiculite
The initial water contents of the three vermiculite treatments (0.08, 0.15 and 0.25 g g-1) were equivalent to water potentials of -1.66, -0.64 and -0.29 MPa, respectively (Table 1). The vermiculite dried significantly (P=0.002) during the experiment, but the change in water potential was relatively small (Table 1). There was no significant interaction between initial water content and the extent of drying during the experiment (P>0.8). For simplicity, each treatment will be referred to by the initial water potential. The lowest potential (-1.66 MPa) was intended to be below the base water potential for expansive growth in onion, which is -1.1 MPa (Whalley et al., 1999).
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Response of seedling growth to water stress
When the newly-germinated seeds were planted in the vermiculite, there were no significant differences (P>0.05) in initial seedling length (root+shoot) for the different treatments. Seedling lengths were between 1 mm and 6 mm, with a mean of approximately 2 mm. Figure 1
shows recovery of seedling root and shoot elongation following exposure to water potentials of -0.29, -0.64 or -1.66 MPa for 14, 25 or 35 d. The shoot and root lengths achieved during growth in the vermiculite are indicated by the first data point for each data set. Growth during this time decreased with water potential. After 14 d, shoot lengths were 47 mm at -0.29 MPa and 26 mm at -0.64 MPa. Corresponding root lengths were 35 and 16 mm. During exposure to a water potential of -1.66 MPa there was no increase in seedling length in the vermiculite, even after 35 d (Fig. 1C). In the well-watered controls in the incubator unit (Fig. 2), the shoot length was 61 mm and the root length was 71 mm by 14 d after germination. In well-watered controls grown in containers sealed with parafilm, the maximum shoot length was 65 mm and the maximum root length was 56 mm (21 d after germination).
Recovery of shoot and root elongation following exposure to water stress
Seedlings exposed to water potentials of -0.29 MPa demonstrated little increase in shoot or root length when placed on wet sloping filter boards (Fig. 1A). This reflected the considerable root and shoot elongation that had occurred in the vermiculite, so that root and shoot lengths were already close to their maximum lengths for this water potential. Similarly, when seedlings exposed to -0.64 MPa for 25 or 35 d were placed on sloping filter boards, there was little increase in shoot length. After exposure to a water potential of -0.64 MPa, the only notable recovery in shoot elongation was for seedlings exposed to this treatment for 14 d, where they elongated from about 25 mm to just over 40 mm (Fig. 1B). However, there was no recovery in root elongation after exposure to -0.64 MPa, despite the root length on removal from the vermiculite being just under 20 mm.
The seedlings which had been exposed to a water potential of -1.66 MPa demonstrated strong recovery when placed on wet sloping filter boards (Fig. 1C). Maximum root and shoot lengths after recovery from stress were somewhat less than maximum root and shoot lengths in the well-watered controls. Increasing the time of exposure to -1.66 MPa from 14 d to 35 d showed no progressive effect on the final shoot length but a small effect on the final root length. The maximum slope of the curves can be estimated using the fitted parameters from equation 2 as cb/4. This shows that the maximum elongation rate after rewatering decreased with increasing length of the stress treatment (Table 2). However, the time after rewatering that this maximum rate was reached did not increase with length of the stress treatment.
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Discussion
Shoot and root elongation decreased with water potential so that at -1.66 MPa, there was no growth. Onion seedlings therefore appear to be more sensitive to low water potential than maize seedlings. Root elongation rates of 1 mm h-1 at a water potential of -1.8 MPa have been reported, although shoot elongation stopped at -0.8 MPa (Sharp et al., 1988). The water stress treatments in the present study used containers sealed with parafilm to minimize water loss. While sealing containers with parafilm can adversely affect plant growth (Sha et al., 1985), maximum elongation of the well-watered controls on filter boards was similar whether they were sealed with parafilm or grown in a ventilated incubator.
Onion seedlings exposed to -1.66 MPa were able to recover so that they achieved final shoot and root lengths at least as long as the seedlings exposed to -0.29 MPa. Final root lengths of seedlings exposed to -1.66 MPa were also longer than those of seedlings exposed to -0.64 MPa. The -0.64 MPa treatment therefore appeared to be more damaging than the -1.66 MPa treatment. Recovery from -1.66 MPa was only slightly affected by increasing the time of exposure to the low water potential for up to 35 d. Exposure of newly-germinated seedlings to a water potential sufficient to prevent expansive growth (–1.66 MPa) appeared to induce a state of quiescence. As such water potentials often occur in soil surface layers, this may be an important adaptation to growth in this environment. Severe desiccation of the seedling is a less likely event and would lead to injury and loss of viability. It is generally understood that seeds progressively lose tolerance to severe desiccation as germination and radicle emergence proceeds (Bewley and Black, 1994). A recent example of this is reported in wheat (Guedira et al., 1997).
The good recovery of shoot and root elongation after exposure to -1.66 MPa contrasts with how onion seedlings recover from exposure to mechanical impedance. In previous work (Whalley et al., 1999) onion seedlings were removed from strong wet sand after different lengths of time and placed on sloping filter boards. The longer seedlings were exposed to mechanical impedance, the less growth was able to recover, so that after 35 d in strong sand, the recovery in shoot and root elongation was less than 5 mm. The shape of the curves during recovery from stress is also different between water stress and mechanical impedance in onion. When onion seedlings recovered from water stress, the growth curve was sigmoidal and it took approximately 5–6 d before the elongation rate of the shoot or root reached a maximum (Fig. 1C). When onion shoots recovered from mechanical impedance, the initial elongation rate of the shoots was higher than the maximum elongation rate in onion seedlings which had not been impeded. These contrasts may suggest that the use of seedling resources is different when seedlings are exposed to water stress and mechanical impedance. Alternatively, prolonged exposure to mechanical impedance may affect cell wall extensibility.
The shoot growth responses suggest that a water potential of -0.29 or -0.64 MPa would not itself prevent emergence, as onion seeds are usually sown at 15 mm depth. Even at -0.64 MPa, the shoot length would be long enough for emergence to occur within 14 d. In most seedbeds, continued root growth at these water potentials would allow the root to grow down into wetter soil. In practice, soil drying is likely to be associated with an increase in mechanical impedance which may have a much greater impact on seedling development than the water potential (Weaich et al., 1992). Some soils can be quite strong at matric potentials as high as -0.02 MPa (Whalley et al., 1999). The decrease of root growth pressure with water potential is likely to exacerbate the effects of strong soil caused by drying. For example, in pea seedlings, the root growth pressure was approximately halved when the water potential of the growth environment was decreased from 0.00 to -0.45 MPa (Whalley et al., 1998). It is therefore likely that mechanical impedance, rather than low water potential itself, will prevent seedlings from emerging from soils at water potentials down to at least -0.6 MPa.
It has been suggested that delays to emergence can be largely attributed to slow germination, whereas seedling losses were more likely to occur in the post-germination phase (Finch-Savage et al., 1998). The data in Fig. 1 show that newly-germinated onion seedlings can recover from water potentials below the base potential for elongation. This suggests that if newly germinated seedlings in seedbeds were exposed to low water potentials which were later relieved by rain or irrigation, then emergence could be delayed without necessarily increasing seedling losses.
Conclusions
Newly germinated onion seedlings showed good recovery after exposure to low water potentials in the range -1.7 to -2.0 MPa for 35 d. This suggests that water stress can induce a quiescent state in these seedlings.
Acknowledgments
This work was funded by the Ministry of Agriculture, Fisheries and Food. Silsoe Research Institute is grant-aided by the Biotechnology and Biological Sciences Research Council. We thank Rodger White, Silsoe Research Institute, for statistical advice. We thank Katherine Dent, Horticulture Research International for measuring the water potential of the vermiculite. The constructive comments of two anonymous referees are gratefully acknowledged.
Notes
4 To whom correspondence should be addressed. Fax: +44 1525 860156. E-mail: richard.whalley{at}bbsrc.ac.uk. 
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