Journal of Experimental Botany, Vol. 51, No. 352, pp. 1861-1866,
November 1, 2000
© 2000 Oxford University Press
Original Papers |
The role of abscisic acid in controlling leaf water loss, survival and growth of micropropagated Tagetes erecta plants when transferred directly to the field
1 Centro de Investigación Científica de Yucatán, Unidad de Biotecnología. Calle 43, 130, Col. Chuburná de Hidalgo 97200, Mérida, Yucatán, México
2 Colegio de Postgraduados. Centro de Botánica. Km 36.5 Carretera México-Texcoco 56230, Montecillos, Edo.de México, México
Received 11 April 2000; Accepted 28 June 2000
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Abstract |
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Plants of Tagetes erecta L. (marigold) cultivated in vitro in ventilated containers exhibited greater control of leaf water loss and increased survival in the field than plants cultivated in sealed containers. Increased field survival of plants cultivated in ventilated containers was attributed to higher levels of endogenous abscisic acid (ABA). Therefore, ABA was supplied exogenously to plants in sealed or ventilated containers by adding ABA (10-6, 10-5, 10-4 M) to the in vitro culture media in order to evaluate control of leaf water loss, growth and field survival. The addition of 10-4 M ABA to the culture media in sealed containers produced plants that had similar control of leaf water loss and were morphologically similar to plants cultivated in ventilated containers without the addition of ABA. Field survival of 10-4 M ABA plants (75%) was increased compared to plants cultivated in sealed containers without ABA (31%), with survival being closer to that of plants cultivated in ventilated containers (90–100%). Plants cultivated with 10-4 M ABA (sealed and ventilated) also exhibited increased plant vigour and leaf area in the field compared to plants cultivated without ABA. The results suggest that the limited field survival and growth of plants cultured in vitro are related to the limited ABA concentrations they accumulate while in vitro. Consequently, conditions that increase the endogenous ABA concentrations of in vitro plants (like ventilation or ABA addition to the medium) would improve the control of leaf water loss, field survival and plant vigour.
Key words: Abscisic acid, field survival, in vitro, micropropagation, ventilation.
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Introduction |
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The special conditions under which plants are cultivated in vitro may lead to low survival when transplanted directly to the field (Sutter, 1988
). Plant water loss and desiccation are the principal reasons which lead to vitroplant death when transferred to the field (Brainerd and Fuchigami, 1982; Maene and Debergh, 1987; Short et al., 1987). The main problem in developing transpirational control of plants cultivated in vitro is poor functionality of the stomata (Brainerd and Fuchigami, 1982; Santamaría et al., 1993). This may be due to the limited capacity of the plants to produce abscisic acid (ABA). Numerous reports document the role that ABA performs in regulating stomatal closure and root growth in plants subjected to water stress (Davies et al., 1981; Hartung and Abou-Mandour, 1996; Jones and Mansfield, 1970; Zeevart and Creelman, 1988). However, the humidity in sealed in vitro containers is maintained close to 100%, thus it has been suggested that in vitro plants are never exposed to water deficits necessary to induce synthesis of ABA (Santamaría et al., 1993). Plants must also be subjected to a minimum amount of water stress before they can respond to the ABA produced (Raschke, 1987). Therefore, the use of ventilated containers that has been proposed to increase the exchange of gases can also facilitate the reduction in water vapour in the containers and may improve the vigour of plants in these containers (Dillen and Buysens, 1989). This may be related to an increase in endogenous ABA concentrations as the ventilated container medium loses more water that medium in sealed containers and so produces the slightly stressful conditions necessary to induce ABA synthesis (Santamaría et al., 1996).
The main objective of this research was to characterize the effect of ABA addition to the culture media of sealed and ventilated containers on the growth of plants in vitro and their subsequent survival and growth after transplanting directly in the field. This is based on results where plants from ventilated containers had better control of transpirational water loss and were more vigorous than plants from sealed containers (Aguilar, 1999). If increased ABA is a factor in this improved control of transpiration and increase in plant vigour, then the exposure of plants in sealed containers to exogenous ABA should produce plants that are morphologically and physiologically similar to plants cultivated in ventilated containers. In relation to this, it was decided to investigate if these plants could show improved transpirational control and survival in the field. This research will demonstrate that low survival of plants cultivated in sealed containers and transferred to the field is related to their low endogenous ABA levels and that the addition of ABA to the medium of sealed (conventional) containers improves stomatal control of water loss, but more importantly increases field survival of these plants. Furthermore, the addition of ABA to the in vitro culture medium improves plant growth in the field.
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Materials and methods |
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Plant material and culture conditions
Tagetes erecta L. (marigold) plants, obtained from meristematic shoots, were used for all experiments. Plants were cultivated on basic media containing macroelements and vitamins with 1.1% agar and 30 g l-1 sucrose but without growth regulators (Murashige and Skoog, 1962
). The media were supplemented with 18 mM KNO3 and 5 mM NH4NO3 (Robert et al., 1987). Different concentrations of ABA (0, 10-6, 10-5 or 10-4 M) were added to the culture media. The pH was adjusted to 5.75 and all media were autoclaved at 121 °C for 15 min. To enhance water and gas exchange in one-half of the culture containers (ventilated containers), holes (133 mm2) were perforated on two opposing sides of the containers (Magenta GA-7) and covered with cellulose fibre paper (Whatman no. 1) which was permeable to water vapour. Water losses from sealed (without holes) containers without plants and from ventilated containers without plants were determined by weighing them every 7 d throughout an in vitro culture period of 21 d. Water losses were 0.14 and 0.70 g d-1 from sealed and ventilated containers, respectively. Medium water potentials were measured with a hygrometer (Wescor HR 33T). Initial medium water potentials were -0.12 MPa.
Plant survival, growth analysis and harvest
Fifteen plants from each treatment were harvested after 21 d in vitro for analysis. Another 10 plants from each treatment were transplanted directly to the field nursery. Plants were planted in raised beds without shading (midday PPF
2000 µmol m-2 s-1, T0 °C, RH40%) and irrigated every 2 d. Plant survival, height and number of nodes were determined every 7 d and plants were harvested 28 d after transplanting into the field. Harvested plants were divided into roots, stems and leaves. Leaves harvested 28 d after transplanting were further subdivided into leaves produced in vitro and leaves produced ex vitro or after transplanting. Leaf area was measured with AgVision (Decagon Devices Inc., Pullman, WA, USA), a computerized video imaging system. Root, stem and leaf material were frozen at -84 °C and lyophilized to determine dry weight.
Abscisic acid extraction and analysis
Leaf tissue, collected from plants at the end of the in vitro period (21 d) and lyophilized, was prepared for ABA extraction by grinding the tissue to a fine powder. Powdered tissue was mixed with degassed distilled water (1 : 50 w/v) and agitating at 2 °C for 12 h. The solution was centrifuged at 8800 g for 1 min and the supernatant collected.
ABA concentrations were determined by radioimmunoassay (according to Quarrie et al., 1988). The monoclonal antibody specific for (+)-ABA (AFR MAC 62) was obtained from Dr SA Quarrie (Institute of Plant Science Research, Norwich, UK).
Leaf water loss
Control of leaf water loss (LWL) of in vitro plants was determined by allowing detached leaves to desiccate at room temperature (25±2 °C) and humidity (48%) and weighing every 5 min for 1 h. Leaf water loss rate was calculated as the weight loss during the first 25 min.
Statistical analyses
Experimental data were evaluated by ANOVA and regression analysis. The relationship between LWL rate and field survival was analysed using the linear plateau model (Anderson and Nelson, 1975).
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Results |
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Leaf ABA, in vitro development and control of leaf water loss
Medium water potentials were -0.50 and -0.88 MPa in sealed and ventilated containers, respectively, after 21 d of culture. Leaf ABA concentrations were higher in plants from ventilated containers compared to plants from sealed containers (Fig. 1A
). The concentrations of ABA in leaves of plants from sealed and ventilated containers also increased as the concentration of ABA added to the medium increased.
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) containers with different concentrations of ABA in the culture media. Data points represent the mean ±SE of 30 replicates.Plants cultivated in ventilated containers and sealed containers with 10-4 M ABA had leaves that were darker green, stems with more red pigmentation and shorter internode length, and roots that were shorter and more compact in comparison to plants in containers without ABA, however, there were no differences in root dry weights. In all concentrations of ABA in both sealed and ventilated containers, plants had short symmetrical root systems at the base of the stem with fewer adventitious roots above the medium surface.
Stem length was reduced in all ventilated containers compared to sealed containers. Furthermore, stem length decreased as the ABA concentration added to the container medium increased.
Leaf area was always greater in plants in ventilated containers compared to plants from sealed containers (Fig. 2A). Leaf area was similar between plants in sealed containers regardless of ABA treatment. In ventilated containers, plant leaf area increased as the concentration of ABA added to the medium increased.
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Leaf dry weight in ventilated containers with 10-4 M ABA was 44% greater than leaf dry weight in sealed containers with 10-4 M ABA (data not shown). However, total dry weight was similar for plants from sealed and ventilated containers regardless of ABA treatment (Fig. 2B
).
The rate of LWL was reduced in plants from ventilated containers compared to sealed containers (Fig. 1B). Furthermore, LWL rate decreased as the concentration of ABA added to the medium increased. The rate of LWL of plants treated with 10-4 M ABA in sealed containers was similar to plants from ventilated containers without ABA. The lower LWL rate in plants exposed to ABA in the medium may indicate that stomata closed sooner than in plants not treated with ABA. Although LWL rate was reduced in both sealed and ventilated plants exposed to 10-4 M ABA (0.42 and 0.26 µmol s-1, respectively) compared to plants not treated with ABA, water loss from their leaves was still greater than from leaves from field plants (0.03 µmol s-1).
Field survival and growth
Only 31% of the plants from sealed containers without ABA survived after 28 d in the field compared to 100% of the plants from ventilated containers without ABA (Fig. 1C). Field survival of plants from ventilated containers with ABA was 90–100%. Field survival of plants from sealed containers increased as the ABA concentration added to the medium increased and leaf ABA concentrations increased (Figs 1C, 3A).
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Stem length after 28 d in the nursery was similar among all treatments. However, the number of leaf nodes on plants from ventilated containers (5.1) was always greater than on plants from sealed containers (1.9) indicating shorter internodes length on plants from ventilated containers.
Leaf area was approximately four times greater on plants from ventilated containers compared to plants from sealed containers (Fig. 2C). Plants also exhibited increasing in vitro leaf area (old leaves retained from in vitro), ex vitro leaf area (new leaves), and total leaf area with increasing media ABA concentrations in both sealed and ventilated containers. Leaf area was two to four times greater in the 10-4 M ABA treatment than in the 0 or 10-6 M ABA treatments.
Plant dry weights in ventilated containers were greater than plant dry weights in sealed containers (Fig. 2D). However, plant dry weights were similar among ABA treatments within sealed or ventilated containers.
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Discussion |
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Are the beneficial effects of ventilation on the control of LWL, plant development, field survival, and growth related to increased leaf ABA levels?
Ventilated plants of T. erecta had greater leaf ABA concentrations, lower LWL rates, and greater field survival than sealed plants (Fig. 1A
, B, C). Greater water loss from the media in ventilated containers resulted in decreased media water potential. The decrease in media water potential in ventilated containers may have induced the increase in leaf ABA concentrations in ventilated plants (Fig. 1A), which in turn may have led to improved stomatal function and increased control of LWL (Fig. 1B) and field survival (Fig. 1C) compared to sealed plants.
Plants cultivated in ventilated containers were shorter with more leaf area (Fig. 2A) and thicker stems, while sealed plants appeared less vigorous and exhibited increased lodging. After transplanting, ventilated plants retained a greater number of their leaves produced in vitro than sealed plants (41% versus 23%) and exhibited greater overall growth after 28 d in the field (Fig. 2C, D).
These results suggest that ventilation of the in vitro container leads to decreased media water potential, which induces increased leaf ABA that can lead to increased control of LWL and acclimatization to the field. To test this hypothesis further, different concentrations of ABA were added to the in vitro culture media.
Effects during the in vitro stage:
It could be argued that ABA concentrations added in the medium were high and could have been metabolized by the plant, however, leaf ABA concentrations increased exponentially in plants with exponentially increasing medium ABA concentrations (Fig. 1A). Ventilated plants also generally exhibited greater concentrations of leaf ABA than sealed plants possibly due to decreased media water potential and increased transpiration that could have promoted increased accumulation of leaf ABA.
The improved morphological features of plants cultivated in ventilated containers suggest an effect of ABA since sealed plants exposed to 10-4 M ABA exhibited a plant morphology similar to ventilated control plants which contained twice as much endogenous ABA as sealed control plants (Fig. 1A). Also, inhibition of adventitious root growth, evident in plants in sealed containers with 10-4 M ABA and in ventilated containers, is probably related to ABA. Roots of different species exposed to ABA exhibited reduced numbers of lateral roots and root primordia, while roots cultivated with fluridone, an inhibitor of endogenous ABA synthesis, exhibited large numbers of lateral roots (Harvey et al., 1994; Hooker and Thorpe, 1988; Pilet and Barlow, 1987). The reduced lateral root growth in ventilated containers is in accordance with their increased endogenous ABA levels, but the proliferation of adventitious roots in sealed control plants suggests they contain minimal levels of ABA.
There were no differences in leaf area in sealed plants in vitro, but in ventilated plants, leaf area increased as ABA concentrations in the media increased (Fig. 2C). Although ABA is known as a growth retardant, some studies indicate that ABA initially inhibits growth but after a short latency period, the growth rate of leaves increases (Hall and McWha, 1981). With T. erecta, plants exposed to ABA exhibited reduced growth (leaves, stem, roots) during the first 10 d but then exhibited increased growth in leaves and stem diameter.
Effects after being transferred to the field:
Field survival was high in ventilated plants regardless of ABA treatment (Fig. 1C). This could be related to higher leaf ABA concentrations in ventilated plants compared to sealed plants (Fig. 1A). Field survival of sealed plants increased as the concentration of ABA added to the media increased and leaf ABA concentrations increased (Figs 1C, 3A). This appears to be due to the increased control of LWL as leaf ABA increased. Leaf water loss rate of both ventilated and sealed plants decreased as leaf ABA concentrations increased and field survival increased as LWL rate decreased (Fig. 3A, B). In tobacco, ABA applied to the ex vitro substrate similarly reduced stomatal conductance and LWL and increased plant acclimatization (Pospisilova et al., 1998).
The relationship between field survival and LWL rate (Fig. 3B) suggests that the capacity of in vitro plants to survive in the field (irrespective of whether cultured in ventilated or sealed containers), depends strongly on the capacity of their leaves produced in culture to control excessive water loss. Thus, plants grown in ventilated vessels exhibited high survival because of tight control of LWL. On the other hand, plants grown in sealed containers showed high field survival only when the ABA treatment improved their control of LWL, otherwise, they exhibited low survival field rates associated with a limited control of LWL. Thus, field survival of in vitro plants could be increased if their capacity to control LWL could be improved, for instance with the addition of ABA to the medium.
However, increased leaf ABA concentrations at transplanting do not explain all the differences in LWL rate and field survival. Sealed plants in the 10-4 M ABA treatment had higher levels of leaf ABA but greater LWL and lower survival rates than ventilated plants in the 10-6 M ABA treatment. This indicates that other factors are also contributing to LWL and field survival.
Ventilation appears to give an additional increase in both control of LWL and field survival (Figs 1B, C, 3B). Ventilated plants exhibited greater control of LWL than sealed plants across the range of ABA treatments. Likewise, field survival of ventilated plants was always greater than sealed plants even with the addition of ABA. Ventilation of the culture container changes gas (CO2, O2, ethylene) concentrations inside the container, in addition to probably lowering the humidity which may improve plant acclimatization.
Ventilated plants also exhibited an increased capacity to produce ABA. In additional experiments, leaf ABA concentrations increased over 7-fold in detached leaves of ventilated plants when left to desiccate for 80 min, while ABA concentrations in detached leaves from sealed plants increased only 3-fold. Ventilated plants also exhibited increased sensitivity to ABA. In another experiment, stomatal aperture of leaf discs from ventilated plants decreased from 5.9 µm to 2.3 µm when incubated in 10-4 M ABA for 3 h, while leaf discs from sealed plants decreased from 8.2 µm to only 7.0 µm when incubated similarly. Thus, leaf ABA levels at transplanting appears to be important for the initial control of LWL, but increased field survival may also be related to increased production of ABA after transplanting and increased sensitivity to ABA such as that exhibited in ventilated plants.
Ventilation or addition of ABA to the medium increased leaf area and growth in the field (Fig. 2C, D). The increase in total leaf area after exposure to increasing ABA levels in both sealed and ventilated plants resulted from both improved retention of in vitro produced leaves as well as the production of new leaves. Additionally, ABA may have increased the accumulation of starch in the leaves during the in vitro stage (Smart and Trewavas, 1983). Increased accumulation of starch reserves and their remobilization from old leaves for new growth accompanied by reduced LWL rates could be factors facilitating the improved growth and survival of plants in the field (Colón-Guasp et al., 1996).
Thus, field survival in sealed plants increased as the concentration of ABA added to the medium increased (Fig. 1C) and control of LWL appears to be highly related to survival (Fig. 3B), however, ABA effects on leaf and root growth may also have influenced plant survival.
Can exogenous ABA substitute for ventilation of containers and vice versa?
The addition of 10-4 M ABA to the container medium or the ventilation of the in vitro container resulted in improved plant morphology and increased field survival. Ventilation of the in vitro container increased the endogenous ABA concentrations (Fig. 1A) and reduced LWL 20% in T. erecta plants indicating improved stomatal control in ventilated plants which may aid in increasing survival in the field. Thus, ventilation of containers may substitute for the addition of 10-4 M ABA to the container medium of sealed containers or vice versa, the addition of 10-4 M ABA to the medium of sealed containers may substitute for ventilation of the container.
However, ventilation of the container and the addition of ABA to the container medium appear to have a synergistic effect in ex vitro plant growth and development. Ventilated plants from containers with 10-4 M ABA added to the medium had greater control of LWL and greater plant dry weight and leaf area than any of the other treatments. If ventilation and ABA increase sugar uptake (Colón-Guasp et al., 1996), starch accumulation (Smart and Trewavas, 1983), and leaf retention, then carbohydrate reserves may also be important factors in determining ex vitro acclimatization and growth.
In conclusion, the addition of 10-4 M ABA to the medium of sealed containers resulted in plants with morphological characteristics similar to plants cultivated in ventilated containers without ABA. The increase in leaf area, reduction in stem length, and reduction in adventitious roots above the culture medium in plants exposed to 10-4 M ABA were similar to plants cultivated in ventilated containers without ABA. Furthermore, the addition of ABA to the media increased control of LWL and plant survival in the field. These results suggest that the low field survival rates of plants cultivated in sealed in vitro containers is due to the lack of minimum levels of water stress needed to induce ABA accumulation necessary for normal stomatal functioning and regulation of LWL when plants are transferred to the field.
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Acknowledgments |
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This work was supported in part by Conacyt grant 2219P-B. MLA gratefully acknowledges economic support from Conacyt, Mexico (113757).
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Notes |
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3 To whom correspondence should be addressed. Fax: +52 99 813900. E-mail: jorgesm{at}cicy.mx
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Abbreviations |
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ABA abscisic acid; LWL leaf water loss.
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References |
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