The effect of altered assimilate allocation and partitioning due to PcGA-oxidase overexpression on the growth and performance of creeping bentgrass (Agrostis stolonifera L.) in full sun and reduced light

Creeping bentgrass (Agrostis stolonifera L.) is a species commonly used in high maintenance turf areas where it is often exposed to shade. Shade stress induces harmful physiological and morphological changes in turf plants which, under intense management conditions, ultimately lead to loss of turfgrass coverage. Adaptive responses to light stimuli are mediated, in part, by phytohormones, gibberellins. Suppression of gibberellin levels by overexpression of genes involved in their catabolism has been proven to successfully control plant stature. Further, it was shown to alter plant morphological and photosynthetic characteristics.;The main objective of this study was to determine the effect of runner bean (Phaseolus coccineus) GA2-oxidase gene (PcGA2ox ) overexpression on creeping bentgrass quality and performance in light limited environments, in the perspective of potential changes in assimilate partitioning and allocation caused by the transformation. Studies were performed at The Ohio State University, Columbus, Ohio, USA from September 1 2008 to July 31 2010. Two genetically modified lines, Ax6548 and Ax6549, transformed with CP4 EPSPS and the PcGA2ox genes and nontransformed control were included in the study.;The purpose of the first experiment was to evaluate overall visual quality of plants grown under different light conditions and low mowing height. Two greenhouse studies were conducted from September 1 to October 31, in both 2008 and 2009. Lines were grown under full sun, reduced red to far red light ratio (R:FR), neutral shade (reduced photosynthetic photon flux (PPF), and canopy shade (reduced PPF and R:FR) conditions. Turf was evaluated visually for color and percent of coverage. GA2ox overexpression resulted in superior quality of Ax6549 under all shade treatments by delaying the decline of its color and coverage.;The aim of the second study was photosynthetic characterization of the lines. The greenhouse studies were performed in September-October 2009 and April-May 2010. Plants were subjected to 4 light treatments as described above. Obtained data showed increased (P=0.05) net CO2 uptake rates and higher maximum quantum yield of photosynthesis in leaves of Ax6549 across all light treatments. Increased CO2 uptake rates in Ax6549 were most likely due to lower specific leaf area and higher stomatal conductance. Lines did not differ in regards to dark respiration rates and light compensation points despite the light treatment. In conclusion, higher photosynthate supply per leaf area unit and enhanced low light use efficiency were suggested as potential factors associated with the superior quality of Ax6549 under reduced light conditions;The purpose of the last study was to investigate potential differences in assimilate partitioning of the plants. Morphological and growth analysis were performed under controlled environment conditions in July 2010. Plants were subjected to high and reduced irradiance treatments and evaluated every 10 days. PcGA2ox overexpression resulted in reduced biomass accumulation in both transgenic plants. Further, Ax6549 plants were characterized by lower growth efficiency and leaf biomass production efficiency. Although this was in contrary to data obtained during photosynthetic characterization, it could be explained by increased self shading in Ax6549 due to greater leaf width and more horizontal leaf orientation. Under high irradiance conditions, Ax6549 invested more assimilate into roots while Ax6548 into leaves. Under reduced irradiance condition, all plants partitioned more biomass into leaves at the expense of root biomass. Transgenic plants invested more assimilate into leaves compared to nontransformed control plants. Morphological data did not help clarify the methods of improved performance of Ax6549 under shaded conditions. Whole canopy photosynthetic measurements performed at different cutting heights are likely needed to further explain the incongruities between photosynthetic and growth analysis data.