Identification and Measurement of Summer Dormancy ...

SYMPOSIUM

Identification and Measurement of Summer Dormancy in Temperate Perennial Grasses Mark R. Norton,* F. Volaire, F. Lelièvre, and S. Fukai

ABSTRACT There is an apparent increase in frequency of prolonged droughts in regions with Mediterranean climates. This has rekindled interest in the summer dormancy trait for improving drought resistance of temperate perennial pasture grasses. In orchardgrass (Dactylis glomerata L.), tall fescue [Lolium arundinaceum (Schreb.) Darbysh.], and phalaris (Phalaris aquatica L.), identification of the three common drought resistance traits of vascular plants, dehydration avoidance (or delay), dehydration tolerance, and summer dormancy, is often confused. Summer dormancy, the least common of these traits, is found in cool-season grasses from semiarid and arid Mediterranean climates and provides an enhanced level of drought resistance. It is best measured in field-grown plants previously exposed to low winter temperatures and short photoperiods. Those perennial grasses not exhibiting summer dormancy survive drought by avoiding and/or tolerating dehydration and express this as a response to water deficit irrespective of the season. Here we review three summer water-supply environments, full irrigation, midsummer storm, and summer drought, for their precision and repeatability in measuring dormancy. Full irrigation and midsummer storm can be recommended, but measurement of dormancy under prolonged drought caused inconsistencies.

M.R. Norton, Industry and Investment NSW, GPO Box 1600, Canberra ACT 2601, Australia, and Future Farm Industries Cooperative Research Centre, The Univ. of Western Australia, 35 Stirling Hwy., Crawley, WA 6009, Australia; F. Volaire and F. Lelièvre, Institut National de Recherche Agronomique (INRA), UMR SYSTEM, 2 place Viala, 34060 Montpellier, France; S. Fukai, Univ. of Queensland, School of Land, Crop and Food Sciences, Brisbane, QLD 4072, Australia. Received 13 June 2009. *Corresponding author ([email protected]). Abbreviations: S/Sndc, summer yield of the cultivar compared with summer yield of a nondormant control cultivar; S/Sp, summer yield of a cultivar compared with the spring yield of the same cultivar; VSENd, visual rating of senescence under drought; VSENi, visual rating of senescence under irrigation; VSENs, visual rating of senescence after storm.

T

he majority of research addressing the effects of drought on plant growth and crop yield has focused on species of annual habit, and most work has been aimed at maintaining yield under moderate water deficits (Turner, 1997) rather than ensuring survival of perennial species in the face of life-threatening water deficiencies. Recent farming systems research has identified the positive contribution that perennial forage species make to the sustainability of cropping systems and rekindled interest in using these species (Ridley et al., 1997). However, it has also served to highlight the knowledge gaps in the role of drought in the mortality of perennial plants particularly since the droughts of the last decade. Nevertheless, understanding of the mechanisms used by perennial forage plants to survive drought has advanced significantly in the last two decades. These mechanisms include dehydration avoidance, dehydration tolerance (Levitt, 1980; Turner, 1986), and summer dormancy (Volaire and Norton, 2006). Whether a plant employs a dehydration avoidance or tolerance strategy is determined by the Published in Crop Sci. 49:2347–2352 (2009). doi: 10.2135/cropsci2009.06.0319 © Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

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extent to which it regulates internal water potential as soil water deficit increases (Turner, 1986), so that a dehydrationavoiding plant must maintain high tissue water potential, whereas one that employs dehydration tolerance is able to withstand lower water potentials (Levitt, 1980). Both strategies may be exhibited in the same genotypes of perennial forage plants at different stages of the drought, with avoidance responses generally expressed earlier, while tolerance responses become apparent in the latter, more intense phases of the drought (Volaire and Thomas, 1995; Volaire et al., 1998). Summer dormancy is another drought resistance trait found in some of the temperate perennial grasses mainly originating in semiarid and arid Mediterranean climates where summer drought typically lasts 4 mo or more (Cooper, 1963). Summer dormancy is defined as an endogenously controlled and coupled series of processes comprising the cessation or reduction of leaf growth, the complete or partial senescence of herbage, and in some cases the endogenous dehydration of meristems. These responses are expressed consistently under the environmentally inductive conditions typical of Mediterranean summers and also under nonlimiting water conditions (Volaire and Norton, 2006; Norton et al., 2008). In contrast to summer dormancy, dehydration avoidance and dehydration tolerance are expressed only in response to water deficit, but unlike summer dormancy they can be expressed in any season. Their responses should be tested under short winter photoperiods where temperature is nonlimiting to growth. This approach will avoid possible confounding with summer dormancy which would be expressed, if present in the test plant, were the testing to occur in summer (Volaire et al., 2001). The level of drought resistance of a perennial grass is therefore a combination of varying levels of expression of dehydration avoidance and dehydration tolerance and possibly summer dormancy and is directly associated with the duration of survival of the fully enclosed apical meristems of the youngest vegetative tillers. These are the last living tissues that the grass must maintain for survival, and the duration of survival of this tissue varies markedly between and within species (Volaire et al., 1998). The relative contribution of these traits to drought resistance within orchardgrass (Dactylis glomerata L.) was well illustrated by an experiment in which a nondormant cultivar with high dehydration avoidance and tolerance suffered 22% mortality after a severe drought, whereas a cultivar with complete summer dormancy had 100% survival (Norton et al., 2006a, 2006b).

MEASUREMENT OF THE SUMMER DORMANCY TRAIT A number of approaches to the identification and measurement of summer dormancy expression in perennial grasses are cited in the literature. These are based on (i) the level of herbage production under full, continuous irrigation over the summer (Laude, 1953); (ii) assessment of herbage production 2348

or “greening-up” after a midsummer storm (or midsummer irrigation) occurring during an extended summer drought (Oram, 1983); and (iii) measurement of herbage senescence after an extended summer drought (Silsbury, 1961; Lorenzetti et al., 1981; Ceccarelli and Somaroo, 1983). Recent research to compare these measurement methods studied two cultivars contrasting in expression of summer dormancy from each of three species, orchardgrass, tall fescue [Lolium arundinaceum (Schreb.) Darbysh.], and phalaris (Phalaris aquatica L.) (Norton et al., 2008). The cultivars were (i) ‘Medly’ orchardgrass, a nondormant, summer-active, early-flowering variety of Mediterranean origin (Volaire, 2002), and ‘Kasbah’, a completely summer-dormant, early-flowering population from southern Morocco (Oram, 1990; Norton et al., 2006a); (ii) ‘Demeter’ tall fescue, a coarse-leaved, nondormant, summer-active cultivar (Reed et al., 2004), and ‘Flecha’, a fi ne-leaved population of Tunisian parentage with incomplete summer dormancy (Miller, 2000; Norton et al., 2006b; Volaire and Norton, 2006); and (iii) ‘Australian’ phalaris, a nondormant or incompletely dormant population (Oram, 1990), and ‘Atlas PG’, described as highly summer dormant by Oram (1990) and in the class of incomplete dormancy of Volaire and Norton (2006). This research studied the same cultivars in each of three trials differentiated by the above summer water regimes. The goal was to identify the advantages and disadvantages of each method. The robustness of the methods of dormancy measurement was judged according to (i) the consistency of scores for the same cultivar according to the different methods and (ii) a sufficient magnitude of the range of scores produced by a method.

Dormancy Measurement Based on Herbage Production under Irrigated Conditions Laude (1953) was one of the first to note that the degree of expression of summer dormancy was associated with the level of herbage production under summer conditions of nonlimiting moisture. Consequently, the summer dormancy index of a cultivar, S/Sndc, is derived thus: summer yield of the cultivar/summer yield of nondormant control cultivar. The following formula transforms the value from an index of summer activity to one of dormancy and then to within the range 0 to 10: S/Sndc = {100 − [(summer yield of x/summer yield of nondormant control cultivar) × 100]}/10.

Summer here is defined as the 3-mo period from the summer solstice to the autumn equinox. Note that this index will usually have a large range, as the least dormant, highest yielding control cultivar will always be assigned the value 0. An alternative index, S/Sp, consists of the ratio of the summer yield of a cultivar to the yield of the same cultivar from another season. In this case spring was chosen because

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typically it is the season of peak yield in a Mediterranean climate. However, cumulative yields from autumn can also be used (C.P. West, personal communication, 2009). Spring was defined as the 3-mo period from the spring equinox to the summer solstice. This index, in contrast to S/Sndc, only uses data from the cultivar in question. The summer yield of a cultivar is compared with the spring yield of the same cultivar and transformed to focus on dormancy, as follows: S/Sp = {100 − [(summer yield/spring yield) × 100]}/10

Forage production data used to develop scores for this method (Table 1) were the result of manual harvesting of herbage from the plots at monthly intervals over the summer. Plots were cut to 4 cm and fertilized at a level sufficient to overcome any nutrient deficiencies.

Dormancy Measurement Based on Herbage Senescence under Irrigation, Midsummer Storm, or Drought Conditions The measurement of senescence assumes that dormancy level is equivalent to the percentage of herbage that is senescent. The measurements, estimated visually, may be performed under full summer irrigation, termed here visual rating of senescence under irrigation (VSENi), after a midsummer storm or irrigation, termed here visual rating of senescence after storm (VSENs), or during an extended summer drought, entitled visual rating of senescence under drought (VSENd). In the trials reviewed here, VSENi was assessed approximately 30 d after the summer solstice while VSENs was undertaken 12 d after a midsummer storm within a prolonged drought (here simulated by a midsummer irrigation), occurring approximately 40 d after the summer solstice. Measurement using VSENd took place on the 47th day of an unbroken summer drought commencing approximately 10 d before the summer solstice (Table 2). To improve the precision of VSENs, old herbage should be cut and removed immediately before the storm as this will improve the estimation of production response to the storm. A midsummer irrigation can substitute for a storm although a significant amount of water should be applied, for example, 40 mm. Several modifications of the measurement of response to the storm exist (Oram, 1983) and the measurement of the ratio of newly emerged tillers to each associated old reproductive tiller is commonly used in phalaris (Culvenor and Boschma, 2005).

IDENTIFYING SUMMER DORMANCY Among the perennial cool-season grasses, it is difficult to separate the responses by which plants avoid dehydration from those associated with the expression of summer dormancy because these traits have similar responses under summer water deficit, viz. cessation of growth, senescence, and shedding of top growth. This apparent similarity is well CROP SCIENCE, VOL. 49, NOVEMBER– DECEMBER 2009

depicted by comparing the response to varying levels of water over summer by two cultivars of orchardgrass known for their contrasting expression of summer dormancy (Table 2). Dormancy scores of Medly in the irrigated and storm trials were all quite low (range 0–2.4) and were indicative of a summer-active grass able to produce new biomass when water was nonlimiting (Norton et al., 2008). In contrast, the same cultivar had a dormancy score of 6.2 when the assessment was made 47 d after the commencement of a summer drought (Trial 3), and this score normally indicates a moderately high level of dormancy. However, while Medly is a plant with a moderately high level of drought resistance (Volaire et al., 1998), much of this resistance is due to dehydration avoidance, expressed by the shedding of foliage leading to reduced transpiration and conservation of soil water, while dehydration tolerance in its meristems increases as the drought intensifies (Volaire and Lelièvre, 2001). The vigorous growth response of Medly to nonlimiting summer moisture in the irrigated and summer-storm trials demonstrates clearly that it is not summer dormant. In contrast, Kasbah showed minimal growth response to nonlimiting summer water supply across all the watering environments and therefore expressed a consistently high level of dormancy (range of 8.7–9.9).

COMPARISON OF THE DIFFERENT METHODS OF SUMMER DORMANCY MEASUREMENT Summer Dormancy Expression Across all three trials and three species, Kasbah orchardgrass was consistently scored as the most dormant cultivar, irrespective of method, and was always more dormant than Medly (Table 2). Similarly, of the two tall fescues, Flecha was always rated as more dormant than Demeter, even though the differences were not as great as between the two orchardgrasses. Atlas PG was more summer dormant than Australian phalaris when rated by the methods S/ Sndc, VSENi, and VSENs, although there was no significant difference when using S/Sp and VSENd. This is an interesting result because Atlas PG is considered to be more dormant than Australian (Oram, 1999). Four of the six cultivars had quite consistent dormancy scores irrespective of the measurement index used (Table 2). The exceptions to this were Medly and Australian, which had low to moderate dormancy when assessed either under full irrigation (S/Sndc, S/Sp, VSENi) or after a storm following a drought (VSENs) but had moderately high dormancy if scored after extended drought (using VSENd, both cultivars had scores of 6.2). One consequence was that the lowest correlation coefficients of one index with another were obtained between VSENd and S/Sp and S/Sndc (0.61) because of the lack of consistency of scores between VSENd and S/Sndc and S/Sp (Table 3).

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Table 1. Total seasonal herbage yield (Trial 1) of fully summer-irrigated autumn-sown cultivars of orchardgrass, tall fescue, and phalaris from winter, spring, summer, and autumn 2002, and summer 2002 production under either midsummer storm (Trial 2) or prolonged summer drought (Trial 3). Source: Norton et al. (2008), with permission by CSIRO Publishing. Species

Cultivar Winter 2002

Trial 1 Full summer irrigation Spring 2002 Summer 2002

Autumn 2002

Trial 2 Storm Summer 2002

Trial 3 Drought Summer 2002

—————————————————————————————— g m−2 —————————————————————————————— Orchardgrass Tall fescue Phalaris LSD0.05

Kasbah Medly Flecha Demeter Atlas PG Australian

145 117 133 144 156 147 NS

311 487 465 517 628 695 62

The results show that the key factor determining the difference in dormancy scores was the water regime under which measurement occurred, that is, full irrigation or simulated midsummer storm in contrast to continuous drought. Caution is needed because perennial grasses of Mediterranean origin that are able to withstand summer droughts of at least 100 d, such as Medly and Australian, might give the appearance of being dormant under summer drought because they cease growth rapidly and shed a high proportion of their herbage. However, these cultivars stop growing and shed their herbage as part of a dehydration avoidance strategy. This strategy conserves soil water (Ludlow, 1989) but it is not dormancy, even though it is sometimes incorrectly referred to as facultative dormancy. As shown here and elsewhere (Piano et al., 2005), when summer water deficit is relieved or in the absence of summer water deficit, these cultivars can continue to grow or immediately recommence production of new herbage. These inconsistencies highlight why it is inadvisable to measure summer dormancy under continuous drought, that is, use of method VSENd. Following the logic that seed dormancy is never tested without the addition of water ( Junttila, 1988), these results dispute the claim that it is possible to consistently measure the

37 435 189 384 187 369 74

197 346 394 346 262 343 26

5 27 36 81 64 75 45

1 7 12 18 48 56 22

expression of summer dormancy in mature grasses without also supplying water.

Type of Water Application in Summer—Full Irrigation or Simulated Midsummer Storm

Full irrigation and simulated midsummer storm, the two dormancy-screening environments that include water, gave highly correlated dormancy scores. We suggest that these methods assess related but slightly different aspects of dormancy. As the full-irrigation environment makes a stable, non-water-limiting environment, this method presents an integrated view of the intensity of dormancy expression over the entire summer. In Kasbah the monthly harvesting regime showed the rapid increase in dormancy intensity of the early summer as seen in the complete absence of herbage production in the first month of summer and the concurrent decline in plant water status (Norton et al., 2006a). After this, regrowth of Kasbah recommenced slowly in the second summer month, which was probably associated with the partial increase in water potential observed in response to the storm. Thereafter, an increasing level of production was noted in late summer, as dormancy levels in the plant steadily declined to nonexistent by early autumn. The key principles guiding the measurement of summer dormancy in the field with this method Table 2. Summer dormancy index scores (0 = minimum dormancy, 10 = max- should be precision and repeatability according imum) from Trials 1, 2, and 3 determined over 2002 by five methods,† in two to the following principles: (i) autumn sowing cultivars of each of three species, orchardgrass, tall fescue, and phalaris. to ensure optimal induction conditions, (ii) Source: Norton et al. (2008), with permission by CSIRO Publishing. irrigation to replace evapotranspiration in all Trial 1 Trial 2 Trial 3 seasons and in particular over summer, and (iii) Species Cultivar Irrigated Storm Drought measurement of summer biomass to assess the S/Sndc S/Sp VSENi VSENs VSENd potential for herbage production with nutriOrchardgrass Kasbah 9.1a‡ 8.7a 8.9a 9.9a 9.7a ent replacement fertilization. An index based Medly 0c 1.0d 2.0d 2.4d 6.2b on comparing summer herbage yield of any Tall fescue Flecha 5.5b 5.8b 3.2c 4.0c 4.5c cultivar with that of the summer yield of the Demeter 1.3c 2.7cd 2.0d 1.9d 2.8d typically nondormant control cultivar (the Phalaris Atlas PG 5.5b 6.9ab 7.0b 7.3b 7.2b most productive) at the site provides a reliable Australian 1.6c 4.5bc 4.2c 5.0c 6.2b numerical score of dormancy intensity with † S/Sndc, summer yield of the cultivar compared with summer yield of a nondormant control cultivar; S/ Sp, summer yield of a cultivar compared with the spring yield of the same cultivar; VSENd, visual rat- cultivars of comparable productivity. However, ing of senescence under drought; VSENi, visual rating of senescence under irrigation; VSENs, visual while this index is suitable when comparing rating of senescence after storm. cultivars of comparable production potential, ‡ Significant differences (P = 0.05) are expressed with different lowercase letters.

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when assessing dormancy within germplasm of much lower productivity, it may be necessary to index their summer production against their spring biomass potential. Scoring foliage senescence at monthly intervals over summer (using VSENi) can complete the characterization of the sward under summer irrigation. In contrast, the midsummer storm (or storm simulation with irrigation) within a drought provides an instantaneous view of the intensity of dormancy. The view is unique to the specific observation time because before the storm herbage growth is stopped by water deficit. A concern about this method is that it may be difficult to consistently obtain a similar prestorm drought and storm necessary for the poststorm observations. Moreover, rewatering plants in the middle of the summer is a technique difficult to standardize because the duration of the dormancy period can vary greatly. For example, the dormancy ranged from 30 to 127 d within ecotypes of phalaris grown in California (Sankary et al., 1969). Nevertheless, a highly significant correlation coefficient (r = 0.79, P < 0.001) between the simulated midsummer storm index (VSENs) and that developed using herbage yields under full summer irrigation (S/Sndc) was obtained (Table 3). The results suggest that the midsummer storm technique has considerable merit, warranting further research to refine the technique including validation with a broader range of species and cultivars. VSENs might also have an advantage over S/Sndc when growing the plants on soils of slow internal drainage. Malinowski et al. (2008) reported that orchardgrass populations died on such soils under the full-summer irrigation regime necessary for S/Sndc. A method that only requires one measurement after a single storm may be much less likely to cause waterlogging and plant death, so this should facilitate the measurement of dormancy in mature plants on these soils. In the Australian phalaris breeding program, assessment of response to a storm (VSENs) has been the primary method used in dormancy identification and measurement (Oram, 1983; Culvenor and Boschma, 2005). Moreover, in recent work with tall fescue, it was the only summer water regime of the three tested where dormancy was unambiguously detected in Flecha after spring sowing (Norton et al., 2006b). This result indicated that the drought preceding the storm had a role in inducing dormancy. Indeed, there are situations where knowledge of the dormancy behavior of younger spring-sown plants could be necessary, and the summer storm in the above work always elicited some degree of summer dormancy in contrast to full summer irrigation. The two methods, S/Sndc and VSENs, can complement one another. For example, using a small group of cultivars representative of the extent of dormancy expression within a species, S/Sndc could be used to describe the kinetics of dormancy expression and identify the optimal time to apply a simulated storm. Subsequently, a plant CROP SCIENCE, VOL. 49, NOVEMBER– DECEMBER 2009

Table 3. Correlation coefficients (r) between dormancy scores derived using five different methods† using a group comprising two cultivars each of orchardgrass, tall fescue, and phalaris. Source: Norton et al. (2008), with permission by CSIRO Publishing. Summer S/Sndc dormancy index

S/Sp

S/Sndc

0.96***

–

S/Sp

–

VSENi VSENs VSENd

VSENi

VSENs

VSENd

0.83***

0.79***

0.61**

0.89***

0.82***

0.61**

0.97***

0.80***

–

–

0.85*** –

**Significant at 0.01 probability level. ***Significant at 0.001 probability level. †

S/Sndc, summer yield of the cultivar compared with summer yield of a nondormant control cultivar; S/Sp, summer yield of a cultivar compared with the spring yield of the same cultivar; VSENd, visual rating of senescence under drought; VSENi, visual rating of senescence under irrigation; VSENs, visual rating of senescence after storm.

breeding program could measure dormancy across much larger numbers of populations using VSENs, which would be advantageous as it is a simpler, quicker, and cheaper method than full summer irrigation using S/Sndc. The methodology proposed here parallels the quantification of fall dormancy in alfalfa (Medicago sativa L.), which is based on plant growth potential in autumn (Teuber et al., 1998). In line with the approach on alfalfa, the measurement of summer dormancy could be expected to be more robust with repeated observations across several locations, over >1 yr and with plants of a range of ages. Indeed, caution needs to be exercised because the comparative studies reviewed here were all undertaken over only one field-year, so extra validation is necessary. Dormancy measurement should be conducted in typical Mediterranean climates (between 30 and 45° latitude) experiencing cool-season rainfall and typical hot-summer droughts. Only this will ensure that grasses are subjected to the range of temperatures and photoperiods able to induce and initiate the adaptive endo-dormancy responses described above. Measurement needs to follow a standardized protocol that incorporates the use of a set of acknowledged control cultivars of high, moderate, and low dormancy. The common use of a measurement protocol will also allow the evaluation of the degree of genotype by environment interaction for this trait which has not been studied. The use of a common method to measure summer dormancy in a research network to study the trait should achieve rapid progress in extending the uses of these grasses.

CONCLUSIONS 1. Summer dormancy is inversely proportional to the level of plant production occurring during a period of nonlimiting water supply over summer. This nonlimiting summer water environment can either be produced by a regime of full summer irrigation

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or a midsummer storm (or midsummer irrigation) after a drought. 2. Attempts should not be made either to identify or measure summer dormancy under prolonged summer drought. The strategy to avoid/delay dehydration has a similar appearance to summer dormancy under drought, and the two traits are often confused. 3. The level of expression of the summer dormancy trait must be quantified to advance our understanding of the trait as well as our ability to exploit it. The scoring methods performed under either full summer irrigation (S/Sndc) or after a midsummer storm (VSENs) will assist breeders to produce improved summerdormant cultivars and agronomists to develop stable and compatible mixtures of pasture cultivars. Acknowledgments We are particularly indebted to the Samuel Roberts Noble Foundation for hosting “The First International Workshop on Summer Dormancy in Grasses—Coping with Increasing Aridity and Heat under Climate Change”. This Workshop paper was partially supported by International Science Linkages–Science Academies Program, part of the Australian Government Innovation Statement, Backing Australia’s Ability, administered by the Australian Academy of Technological Sciences and Engineering.

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