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Title: Effect of varying temperature regime on phyllochron in four warm-season pasture grasses

item Bartholomew, Paul

Submitted to: Agricultural Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2014
Publication Date: 9/25/2014
Citation: Bartholomew, P.W. 2014. Effect of varying temperature regime on phyllochron in four warm-season pasture grasses. Agricultural Sciences.

Interpretive Summary: Appropriate timing of harvests, fertilizer or herbicide application is essential for effective and productive grassland management. Since growing conditions can vary from year to year, timing of management activities by calendar date will provide very inconsistent results because of differences in crop growth stage at the same calendar date in different growing seasons. Management of pastures according to stage of grass growth is likely to be more effective, but determination of growth stage may be difficult, time-consuming and not feasible in practice. To facilitate pasture management simple alternatives to direct observation of grass development are needed. Mean daily temperature and its cumulative effect over time has a fundamental influence on leaf appearance and hence stage of development, in grasses. Accumulated temperature, also described as growing degree days or thermal time, is simply calculated as the mean of daily maximum and minimum air temperatures where this value falls between minimum and maximum temperatures for growth that are plant species-specific. If the phyllochron, or accumulated temperature requirement for each additional leaf on a grass plant, is constant for a given species of grass it should be possible to predict the stage of development of a plant if the temperature conditions under which it is grown and if the minimum temperature for growth (base temperature) of each species are known. The base temperatures for each of four warm-season forage grasses common to the southern Great Plains (Big bluestem, little bluestem, Indiangrass and Sideoats grama) were measured and consistency of phyllochron values for these same species was evaluated under a range of temperature conditions. An average minimum temperature for growth was estimated at 8.1 °C and this appeared to be applicable to the four species tested. For each species and under each temperature regime tested there was a close straight line relation between leaf appearance and accumulated temperature. The phyllochron did not remain constant under the different temperature regimes. In all cases phyllochron increased with increase in mean daily temperature, but the change occurred consistently at 6.4 °C of accumulated temperature above 8.1 °C for each degree C increase in daily mean temperature. Within the range of mean daily temperatures observed from early to late spring it should therefore be possible to adjust the phyllochron value according to average daily temperature, so that accumulated temperature values can be used throughout the Spring growing season (up to around mid-June) to predict warm-season grass development stage.

Technical Abstract: Using accumulated temperature measures to predict plant development may provide guidance on timing of management practices to minimize competition between warm and cool-season components of mixed pastures. However, temperature and plant development relationships for warm-season pasture grasses common in the southern Great Plains of the USA have not been extensively studied. Under controlled environment conditions, base temperature (Tbase) values were determined for Big bluestem (Andropogon gerardii Vitman), Indiangrass (Sorghastrum nutans, (L.) Nash), Little bluestem (Schizachyrium scoparium (Michx) Nash) and, Sideoats Grama (Bouteloua curtipendula (Michx) Torr). Measures of the accumulated temperature requirement for the phyllochron (leaf appearance interval) were made under a range of temperature regimes for these same species. Mean Tbase was 8.1°C and differences among species were not significant (P>0.05). Within temperature regimes mainstem leaf appearance was closely and linearly related to accumulated temperature above Tbase. Increase of 7.5 °C in night temperature increased phyllochron by a mean of 43%, but similar increase in day temperature only increased phyllochron by 16%. Phyllochron increased by 6.4 °C leaf-1 for each 1 °C increase in daily mean temperature within the range of 15.0 to 22.5 °C. If accumulated temperature measures are to monitor reliably the development of warm-season grasses, allowance must be made for changes in phyllochron as the growing season progresses.