The phenology of D. flexuosa was followed through a growing season in both a grassland and a woodland habitat. A flowering tussock form in the open habitat contrasted with a non-flowering mat in shade. A major environmental difference between the habitats was in photosynthetically active radiation (PAR). Vegetative growth of tillers from the two habitats, grown under the same conditions in a glasshouse, was similar while showing a marked response to PAR. When nitrogen (N) and PAR were decreased, plants grown under summer glasshouse conditions produced less shoot dry weight through reduction in tiller numbers. Decreased PAR increased tiller dry weight, length, total leaf length, leaf weight, leaf number, leaf length ratio (= SLA) and leaf weight percentage. Decreased N increased tiller weight, leaf weight and leaf number. Deschampsia flexuosa required marked vernalisation for flowering. Plants grown at low PAR produced juvenile tillers which were unable to flower irrespective of vernalisation. In open grassland, tussocks flowered intermittently and this was probably determined by the level of PAR penetrating the tussock. Massed flower culms from previous seasons were a major barrier to PAR and when removed (naturally or artificially) the tussocks flowered again. Deschampsia flexuosa has a well-known tolerance of deep shade under woodland on dry acidic and nutrient-poor soil but it is also widespread on similar soils in open grasslands. In grassland, but not woodland, it can successfully flower and set seed. Deschampsia flexuosa should be seen ecologically as a species primarily adapted to open rather than shaded habitats.

Bermudagrass (<i>Cynodon</i> spp.), when cultured at the northern limit of adaptation for semitropical grasses, is exposed seasonally to temperatures low enough to limit growth and turf quality. Research was conducted to investigate the influence of foliar iron and cytokinin applications on bermudagrass growth during fall and spring. The relationship of photosynthesis, respiration, and nonstructural carbohydrate composition to chilling temperatures was also studied. Foliar applications of Fe in late-summer and fall extended bermudagrass performance during low temperature periods of fall. Frequent Fe applications aided the retention of green bermudagrass turf during prolonged exposure to chilling temperatures. Iron applied the previous season stimulated post-dormancy recovery. Benzyladenine (BA) applied alone was not as effective as Fe for promoting green bermudagrass color retention during fall and BA had few effects on spring growth when applied the previous season. Applications of BA in conjunction with Fe were beneficial for retention of green bermudagrass color during late fall when clear plastic covers were used to prevent frost injury. A 6- to 8-week longer bermudagrass growing season occurred when clear plastic covers were used to prevent frost injury. Iron and BA did not significantly affect the total nonstructural carbohydrate (TNC) levels in Midiron bermudagrass rhizomes and stolons at the onset of dormancy in field studies. Midiron bermudagrass had higher photosynthetic and respiration rates than Tifgreen bermudagrass after 4 days exposure to chilling (10/7°C day/night) temperatures in controlled environment studies. Midiron recovered higher photosynthetic rates than Tifgreen when returned to a warm (30°C) environment after exposure to chilling temperatures. The TNC in leaves of Midiron and Tifgreen increased 88 and 160%, respectively, during 5 days at chilling temperatures. The inability to transport carbohydrate from and the subsequent accumulation of high photoassimilate levels in leaves was associated with the inability of bermudagrass to fully recover high photosynthetic rates following chilling. Reduced respiratory activity was apparently responsible for the accumulation of high TNC levels in leaves. In contrast to photosynthesis, respiration was reversibly inhibited by short term exposure of bermudagrass to chilling temperatures. Rapid recovery of high respiratory activity may be important for maintenance of aesthetically pleasing bermudagrass turf following chilling. Foliar applied Fe or BA generally caused darker green Midiron and Tifgreen turf after exposure to chilling temperatures in a controlled environment, although the enhancement of physiological activity differed with chemical applied and cultivar. Iron stimulated recovery of photosynthetic and respiratory activity in both cultivars after exposure to chilling temperatures. However, during chilling Midiron CO₂ exchange was more responsive to Fe applications. Benzyladenine increased photosynthesis in Tifgreen but not in Midiron and did not significantly affect respiration in either cultivar. Neither Fe nor BA had a consistent effect on TNC levels in bermudagrass leaves, rhizomes, or stolons. These investigations indicate that cultivar selection may play a major role in determining turf quality at chilling temperatures. Iron may modify bermudagrass physiology and enhance performance of bermudagrass exposed to chilling temperatures. / Ph. D. / incomplete_metadata

Doctor of Philosophy / Department of Agronomy / P.V. Vara Prasad / The pathway for the movement of water through plants, from the soil matrix to the atmosphere, constitutes the hydraulic architecture of a plant. The linkage between the hydraulic architecture of woody plants and drought tolerance has received considerable attention, but much less work has been done on grasses. I investigated the linkage between the hydraulic architecture of grasses to physiological patterns of water use across a range of species and conditions. The rate of stomatal conductance (g[subscript]s) and photosynthesis (A) increased acropetally along the leaves of 5 grass species, which is a unique feature of this growth form. The internal structure of leaves also changed acropetally in order to minimize the pressure gradient across the mesophyll that would otherwise occur as a result of increasing g[subscript]s. The resistance to water movement through the mesophyll represented 80-90% of leaf resistance in six genotypes of Sorghum bicolor L. (Moench). This resistance was most important in controlling g[subscript]s and A when water was readily available, but as soil-moisture decreased it was the efficient transport of water through the xylem that was most important in maintaining plant function. I also investigated the relationship between hydraulic architecture and stomatal responses of grasses to increasing Vapor Pressure Deficit (D). Grasses with a larger proportion of their hydraulic resistance within the xylem were less sensitive to increasing D and plants with high root conductance maintained higher rates of gas exchange D increased. Finally, I investigated the tolerance of grasses to extreme drought events to test if there was a trade-off between drought tolerance and growth in grasses. Plants with drought tolerant leaf traits typically sacrificed the ability to move water efficiently through their leaves. Having drought tolerant leaves did not limit the plants ability to have high rates of gas exchange, and, in fact, the most drought tolerant plants had the high rates of g[subscript]s when expressed on a mass basis. Leaf-level drought tolerance did contribute to species’ occurrence, as the drought intolerant species I studied are not commonly found in low precipitation systems. The results presented here highlight the importance of studying the hydraulic architecture of plants to provide a better understanding of what controls plant function across a range of environmental conditions.