1SPII, Seed and Plant Improvement Institute, P.O. Box No. 31585-4119, Karaj, Iran, Email email@example.com
2ICARDA, International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria,
3CDC, Crop Development Center, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada,
Freezing stress is a major factor limiting barley survival in cold regions of Iran. The objective of this study was to determine how expression of freezing resistance (FR – temperature of 50% plant survival) is regulated by photoperiod and vernalization responses in barley cultivars acclimated under field conditions. Four barley cultivars with different vernalization response and photoperiod sensitivities were planted in the field at the Maragheh agricultural research station (37°15’N, 46°15’E; 1720m) in Iran during 2002-2003. Final leaf number (FLN) and growth of shoot apex were determined at intervals to measure vernalization saturation and phenological development, respectively. The FLN of ‘Rihane-03’ and ‘Dicktoo’ spring barley cultivars did not change, indicating that these cultivars do not have a vernalization requirement. In contrast, a significant decrease in FLN and delayed double ridge (DR) formation in ‘Dobrinya’ winter barley indicated that vernalization response delayed phenological development.
Dobrinya and ‘Kold’ winter barley reached their maximum FR as vernalization saturation occurred. The spring barley, Rihane-03, with no vernalization requirement, which had already entered the reproductive phase (DR formation), had only a limited ability to acclimate. However, the very short day (SD) sensitive Dicktoo barley delayed DR formation by 4 of December compared to the less SD sensitive Rihane-03, which formed DR very soon after planting on 27 of October. This delay in phenological development of SD sensitive Dicktoo barley was reflected in increased expression of FR. It is concluded that vegetative/reproductive transition is a critical switch in down-regulation of expression of FR in cereals grown under field conditions.
Cold tolerance, developmental control, Hordeum vulgare
Winter cereals have a vernalization requirement that maintains plants in the vegetative phase and prevents the transition to the reproductive phase until the freezing temperatures of winter have passed. In contrast, spring cereals (which do not have a vernalization requirement) generally develop rapidly toward their reproductive phase when grown under long days (Mahfoozi et al 2001). Satisfaction of the vernalization requirement has been associated with a decline in FR of over-wintering cereals, as measured by plant survival (Koch, 1973; Fowler et al., 1996). Day length is one of the most important environmental variables that influence the flowering of plants. Length of day affects apical morphogenesis, leaf production, and other developmental processes in cereals (Kirby and Appleyard, 1980). Long days accelerate floral initiation and heading by reducing the number of leaves in vernalized or spring plants (Mahfoozi et al 2000). Under SD regimes, reproductive transition is delayed in SD sensitive cultivars and the plants produce more leaves before a reproductive inflorescence (Holmes, 1973, Mahfoozi et al 2000).A close association between the vegetative/reproductive transition and the start of a decline in FR has demonstrated the regulatory influence that developmental genes have over low-temperature (LT)-induced genes in cereals under controlled conditions (Fowler et al 2001; Mahfoozi et al., 2001). However, the influence of vernalization and photoperiod genes on expression of FR has not been verified under field conditions. Consequently, the current experiments were initiated to study phenological development as indicated by main stem final leaf number and apical meristem changes and to determine the influence of developmental growth on expression of FR in winter and spring barley cultivars grown in cold region of dry land areas under field conditions.
Vernalization requirement, FR and phenological development were determined for ‘Dobrinya’ and ‘Kold’ winter barley, ‘Dicktoo’ and’ Rihane-03’ spring barley cultivars vernalized and acclimated under field conditions at the Maragheh research station station (37°15’N, 46°15’E; 1720 m) in Iran. These cultivars were planted on 7 of October in 2002. All cultivars were subjected to LT acclimation from 27 of October to 12 of February and sampled at 14 weekly intervals. FLN and LT50 (temperature at which 50% of the plants are killed by LT stress) were determined for each treatment. Two methods were used to determine the stage of phenological development: (1) dissection of the plant crown to reveal the shoot apex development, and (2) the FLN procedure described by Mahfoozi et al. (2001). Therefore, shoot apices of the plants were dissected and photographed for the establishment period and each acclimation period to determine when the double ridges (DR) stage occurred. In order to determine FLN number on the main shoot, at 0 d treatment (without exposing to low temperature) and at the end of each vernalization period, pots containing two plants grown in the field conditions were moved to 20°C until flag leaf emergence and leaves numbered on the main stem. Transition from the vegetative to the reproductive phase was considered complete when the FLN became constant (Delecolle et al., 1989).
For LT50 determination, plants were collected from the field at the end of each acclimation and vernalization periods started from 27 of October in 2002. The procedure outlined by Limin and Fowler (1988) was used to determine the LT50 of each cultivar at the end of each LT acclimation period for plants collected from field.
When exposed to LT acclimating temperatures, Dobrinya winter barley reduced its leaf number from 19 to 11 (Fig. 1), indicating that this cultivar is a winter habit cultivar with a vernalization requirement (Wang et al., 1995; Mahfoozi et al., 2001). Spring habit cultivars, Dicktoo and Rihane-03, did not reduce their FLN when grown at acclimation temperatures. Spring habit Rihane-03 reached its minimum leaf number very short time after planting indicating that this cultivar does not have a vernalization requirement (Fig.1). Dicktoo did not decrease its FLN when exposed to low temperature. It appeared that Dicktoo increased its FLN during acclimation rather than decreasing it (Fig.1); however, more replicates for FLN are required to determine the exact pattern of FLN production under field conditions. FLN measurements indicated that vernalization saturation was achieved between 4 to 11 of December for Dobrinya winter barley (Fig. 1). Double ridge formation is an earlier clear indication that transition to the reproductive phase has begun (McMaster, 1997). In Rihane-03 spring barley, DR was reached very soon after planting (about 27 of October), while in Dobrinya winter barley, DR was visible about 5 of December (Fig. 2). Also, in SD sensitive Dicktoo spring barley DR was visible very late (about 4 of December) indicating that short day sensitivity delayed vegetative/reproductive transition under field conditions (Fig.2).
Figure 1. Final leaf number (FLN) of Dobrinya winter barley, Rihane-03 spring barley with no vernalization requirement and Dicktoo very short day sensitive spring barley vernalized under field conditions of Maragheh agricultural research station from Oct 27 to Dec 25 in 2002-2003 (O=Oct, N=Nov, D=Dec).
Figure.2. Phenological development (apical development) of Dobrinya winter barley and a very short day sensitive Dicktoo (Nov28th, the picture in the left) and Rihane-03 spring barley (Nov3rd, the picture in the right) ,grown from 9 of Oct to end of March at the Maragheh agricultural research station, Iran. Comparative phenological advancement to double ridge formation is illustrated
Winter barley cultivars (Kold and Dobrinya) started to acclimate at a rapid rate. The rate of change in FR then gradually slowed until FR began to be lost (Fig. 3). Winter barley cultivars reached their maximum FR about the same time as vernalization saturation occurred. Rihane-03 spring barley had a limited ability to cold acclimated (Fig. 3) as it had already reached its reproductive phase and formed DRs around the 26 of November in 2002 (Fig. 2). However, the SD sensitive spring Dicktoo acclimated to a colder temperature and its LT50 was similar to Dobrinya and Kold winter barley cultivars (Fig.3). Higher level and longer duration of the expression of FR in Dicktoo appears to be the result of an extended vegetative period that delayed the transition to the reproductive phase as illustrated in Fig. 1 and 2.
Figure.3. Freezing resistance of Dobrinya and Kold winter barley, Rihane-03 spring barley and Dicktoo very short day sensitive spring barley acclimated under field conditions of Maragheh agricultural research station, Iran from Oct 27 to Feb 12 in 2002-2003 (O=Oct, N=Nov, D=Dec, J=Jan, F=Feb).
The results of this study show that photoperiod response and vernalization requirements interact with temperature to influence the rate of phenological development and the expression of FR genes. They also verify that vernalization requirement (Fowler et al., 1996, Mahfoozi et al., 2001) and SD photoperiod sensitivity (Mahfoozi et al., 2000) allow FR genes to be expressed for a longer period of time at temperatures in the LT acclimation range. These results demonstrate that factors that delay the transition from the vegetative to the reproductive phase can increase the duration of expression of FR genes. It is concluded that the transition from the vegetative to the reproductive phase corresponds to the loss of FR in barley grown under field conditions.
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