The role of summer dormancy in improving survival of temperate perennial pasture grasses in drought-prone environments

Norton, Mark Roger (2007). The role of summer dormancy in improving survival of temperate perennial pasture grasses in drought-prone environments PhD Thesis, School of Land, Crop and Food Sciences, University of Queensland.

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Author Norton, Mark Roger
Thesis Title The role of summer dormancy in improving survival of temperate perennial pasture grasses in drought-prone environments
School, Centre or Institute School of Land, Crop and Food Sciences
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Professor Shu Fukai
Abstract/Summary The poor survival of perennial pasture grasses is a major constraint to agricultural systems where summers are characterised by long, severe droughts and high temperatures. The search for grasses with superior tolerance to these conditions has resulted in a re-awakening of interest in the summer dormancy trait leading to the recent release of several summer dormant cultivars. This has led to renewed questioning about the nature of summer dormancy particularly as the definition of the trait is unclear. Therefore, this research has aimed to strengthen understanding of summer dormancy in the pasture grasses Dactylis glomerata L. (cocksfoot), Festuca arundinacea Schreb. (tall fescue) and Phalaris aquatica L. (phalaris) by studying two cultivars contrasting in the expression of summer dormancy from each species. A range of field trials in the south of France aimed to identify key physiological mechanisms whereby summer dormancy enhances survival over long, dry summers by concentrating on the effects of different levels of summer water supply and plant maturity. Field trials in that region also compared and evaluated several methods to measure the intensity of summer dormancy and developed a new method based on herbage production from irrigated summer culture. Growth chamber trials with small pots studied the relationship between temperature and the expression of summer dormancy by comparing the growth response to a range of temperatures of two cohorts of plants contrasting in exposure to conditions aimed to induce summer dormancy. Finally, winter research conducted in pots in the glasshouse subjected plants to drying cycles to study dehydration tolerance and avoidance and related these to the expression of the summer dormancy which had been observed in the summer field studies. The component mechanisms of summer dormancy were clearly expressed under full summer irrigation by reproductive plants which had been vernalised and although present in younger, spring-sown plants they were approx. 40% less intense while the tall fescue cv only expressed dormancy if subjected to prior drought. Expression began in late spring/early summer and included growth cessation and herbage senescence resulting in substantially less summer evapotranspiration in the dormant tall fescue and cocksfoot swards compared to their summer active counterparts. The dormant cocksfoot under drought dehydrated to -6.7 MPa and even under irrigation dehydration to -3 MPa occurred . In contrast, under irrigation the dormant tall fescue and phalaris cvs remained hydrated. Dehydrin expression increased in all cvs as water deficit intensified but while expression declined after a storm in the non-dormant cvs (in response to increased water status), levels remained uniformly high in the three dormant cvs. The summer dormant tall fescue maintained full sward cover after summer droughts of 116 and 144 days whereas cover in the summer active cv fell by approx. 25%. Similar sward loss (22%) occurred in a summer active cocksfoot, in contrast to the summer dormant cv after a 144 day drought. No survival advantage for the summer dormant cv was measured in phalaris although this seemed to be due to unique field trial site characteristics. Five methods for summer dormancy measurement varying according to summer water availability, i.e., full irrigation, mid-summer storm and complete drought, and assessment method, viz., herbage production measurement or visual estimation of herbage senescence, were compared and evaluated. A method using the herbage harvested from irrigated summer culture was developed. Summer dormancy is best measured under non-limiting moisture, because assessing the trait under complete drought can result in false scores, e.g. Medly cocksfoot was moderately dormant under drought (6/10), but non-dormant under full summer irrigation or after a storm (1-2/10). Dormancy was scored equally well by measuring herbage production or by visually assessing the level of herbage senescence. The examination of the effect of temperature on summer dormancy questioned whether dormancy is independent of temperature or is simply a decline in growth in response to high summer temperatures. In cocksfoot the decline associated with summer dormancy (approx. 100%) was independent from any which occurred as a result of high temperatures. In tall fescue, despite indications that dormancy was a separate effect it was not possible to definitively separate the effects of rising temperature from prior dormancy induction suggesting the need for further research. Inconsistent and inconclusive results occurred in phalaris suggesting that small pot culture is inappropriate for studying dormancy in this species. Subjecting plants to drying cycles in a winter glasshouse environment demonstrated that dehydration tolerance, dehydration avoidance and summer dormancy are separate responses able to be independently expressed. In cocksfoot and tall fescue two highly dehydration-avoiding cvs also expressed high levels of dormancy in summer as reproductive plants although a summer active cocksfoot was also highly dehydration-avoiding. In contrast, the highly summer dormant phalaris cvs showed no greater levels of dehydration avoidance/tolerance than much less dormant cvs. The research has identified key mechanisms of drought survival as being component, defining characteristics of summer dormancy and has also demonstrated robust screening methods to extend utilisation of the trait in plant improvement.

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Created: Fri, 21 Nov 2008, 16:09:12 EST