M. R. Norton

Summer dormancy in Festuca arundinacea Schreb.; the influence of season of sowing and a simulated mid-summer storm on two contrasting cultivars

Due to the shortage of information on summer dormancy in tall fescue (Festuca arundinacea, syn. Lolium arundinaceum), we tested the response of 2 cultivars of differing dormancy expression and growth stage to a range of summer moisture conditions, including full irrigation, drought, and a simulated mid-summer storm and analysed whether traits associated with summer dormancy conferred better survival under severe field drought. Autumn-sown reproductive and younger, spring-sown plants of 2 cultivars, claimed to exhibit contrasting summer dormancy, were established and then tested in summer 2002 under either long drought, drought+ simulated mid-summer storm, or full irrigation. The autumn-sown reproductive plants of cv. Flecha exhibited traits that can be associated with partial summer dormancy since under summer irrigation they reduced aerial growth significantly and exhibited earlier herbage senescence. Moreover, cv. Flecha used 35% less soil water over the first summer. However, the water status of leaf bases of young vegetative tillers of both cultivars was similar under irrigation and also throughout most of the drought (leaf potential and water content maintained over -4MPa and at approx. 1 g H2O/g DM, respectively). The summer-active cv. Demeter did not stop leaf elongation even in drought and produced twice as much biomass as Flecha under irrigation. Cultivar Demeter responded to the simulated storm with a decline in dehydrin expression in leaf bases, whereas no decline occurred in Flecha, presumably because it remained partially dormant. The younger, spring-sown swards of both cultivars had similar biomass production under summer irrigation but whereas Demeter regrew in response to the simulated storm, cv. Flecha did not, indicating that dormancy, although only partially expressed, was reinforced by summer drought. In all trials, cv. Flecha out-yielded Demeter in autumn regrowth. In particular, the severe drought in 2003 caused a 25% loss of the basal cover in cv. Demeter, whereas Flecha fully maintained its sward allowing it to produce a higher post-drought autumn yield. This work links summer dormancy with higher persistence over long, dry summers.

Measurement of summer dormancy in temperate perennial pasture grasses

The search to improve drought survival in temperate perennial grasses has led to a renewed interest in summer dormancy and how to quantify it. This endogenously controlled trait, found in some temperate perennial grasses, is associated with drought that normally occurs in summer. While cessation of leaf growth and senescence of herbage occurs in all grasses in response to drought, it is under summer irrigation that these same responses are observed only in summer-dormant germplasm and hence the trait can be identified in germplasm. Across the spectrum from completely summer-dormant to non-dormant, there is a range of expression. Our objective here is to highlight differences in characteristics of indices which measure summer dormancy and to identify aspects for incorporation into a superior index for use in measuring this trait. The experimental program comprised three field trials that compared 6 cultivars and a fourth that assessed a larger group of 12 cultivars of the same three species, cocksfoot (Dactylis glomerata L.), tall fescue (Festuca arundinacea Schreb.), and phalaris (Phalaris aquatica L.). Seasonal herbage yield and foliage senescence were measured under three summer watering regimes: complete drought, mid-summer storm, and full irrigation at Mauguio, France. Different indices were calculated to compare against the approach which evaluates senescence under drought. The key outcomes are as follows. (1) The assessment of summer dormancy needs to be viewed as the plant response to a period of non-limiting water supply over summer. It makes little difference whether this is produced by full summer irrigation or a mid-summer simulated storm after a drought. Assessment of this trait under conditions of unbroken drought is discouraged because it can result in false scores. (2) The determination of summer dormancy intensity under full summer irrigation is most appropriate for the intensive study of the dynamics of dormancy expression over the entire summer. A simulated mid-summer storm within a drought gives an instantaneous view of dormancy intensity at a specific observation date and may be well adapted to the requirements of plant breeding. These methods are complementary. (3) Summer dormancy intensity can be assessed either by measuring herbage production or by a visual assessment of the level of herbage senescence. (4) An index of summer dormancy based on comparing irrigated summer herbage yield of any cultivar with that of a high, summer-yielding, non-dormant control cultivar was able to provide a reliable score of dormancy intensity. This index functions across a range of cultivars and species of perennial grasses. Further refinement of the index is needed to identify ‘standard’ high and low summer-dormant populations.

Plant drought survival under climate change and strategies to improve perennial grasses. A review

The three cool-season perennial forage grasses cocksfoot/orchardgrass, Dactylis glomerata L., tall fescue, Festuca arundinacea Schreb. syn. Lolium arundinaceum (Schreb.) Darbysh., and phalaris/harding grass, Phalaris aquatica L., are of major economic and ecological importance in regions with summer-dry environments. This review considers the constraints that these species are likely to experience under current and predicted increase of droughts due to climate change scenarios in south-eastern Australia, the southern Great Plains of USA and the Western Mediterranean Basin. The review identifies research required to maximise the development and use of C3 cool-season grasses with enhanced resilience to drought while considering the concern of some regulators that these grasses may be potential weeds. The state of knowledge of factors influencing plant drought survival and therefore recovery after stress and long-term persistence is discussed in the light of adaptive strategies. The major research needs identified to enhance traits conferring drought survival include (1) increasing the depth and density of grass root systems to strengthen dehydration avoidance; (2) exploring the biochemical, molecular and hydraulic bases of dehydration tolerance and improving techniques to measure this trait; (3) breaking the trade-off between summer dormancy and forage yield potential and improving understanding of environmental, biochemical and genetic controls over summer dormancy; (4) identifying non-toxic endophyte strains compatible with summer-dormant cultivars of tall fescue to enhance drought survival; and (5) enhancing seed production capability of new cultivars as well as the development of agronomic management packages for promoting stable mixtures combining perennial grasses and legumes. The weed potential of newly introduced summer-dormant cultivars is concluded to be minor. The research directions proposed here should improve pasture grass resilience and forage crop sustainability in Mediterranean and temperate summer-dry environments under the future drier and warmer conditions associated with climate change.

Measuring dehydration tolerance in pasture grasses to improve drought survival

Abstract. Cool-season grasses, both annual and perennial, typically employ the strategies of dehydration avoidance and dehydration tolerance to help them to survive extended periods of low soil moisture. Summer dormancy is an extra trait employed by perennial grasses particularly adapted to regions experiencing extended hot, dry summers. Of the three strategies, it appears that least is known about dehydration tolerance. Using and extending a methodology developed for cocksfoot (Dactylis glomerata L.), this study compared a range of cultivars of cocksfoot, tall fescue and phalaris differing in expression of summer dormancy. Both inter- and intra-specific variation in dehydration tolerance was observed, with cocksfoot expressing the trait strongly, whereas it was least evident in phalaris. The trait was more strongly evident in cultivars originating in drier environments, and the ability to express dehydration tolerance appeared to be independent of summer dormancy. It has been confirmed that dehydration tolerance is a powerful drought-survival trait, one that warrants increasing attention in plant breeding programs for drying environments.