Abstract:

Timothy （Phleum pratense） is widely cultivated globally because of its numerous superior traits such as good herbage quality and abundant natural oils and fats. This grass is beneficial for maintaining racehorse body shape， improving their respiratory system， and enhancing their endurance. It also stimulates rumination in cows， thereby prolonging the peak milk production period. However， the germplasm resources of timothy in China are relatively scarce， with only two species introduced so far. To investigate the photosynthetic capacity of timothy germplasm and screen for varieties with highly efficient photosynthesis， we conducted a comparative analysis of 21 timothy germplasm lines across four growth stages （tillering stage， culm elongation stage， heading stage， and reproductive maturity stage）. We determined their chlorophyll pigment contents， photosynthetic parameters， and chlorophyll fluorescence parameters. Among the 16 photosynthetic traits of the tested timothy germplasm lines， 13 showed an initial increase， followed by a decrease， and then an increase to reach maximum values at the heading stage. These 13 traits were chlorophyll a content， chlorophyll b content， carotenoid content， total chlorophyll content， transpiration rate， stomatal conductance， net photosynthetic rate， intercellular carbon dioxide concentration， maximum photosynthetic efficiency of photosystem Ⅱ （PSⅡ）， potential photochemical efficiency of PSⅡ， photochemical quenching coefficient， apparent electron transfer efficiency， and PSⅡ photochemical quantum yield. A principal component analysis was conducted using the mean values of various traits from four growth stages. It was found on principal component analysis that these 16 traits yielded 5 principal components with eigenvalues greater than 1 and cumulatively explaining 78.76% of data variance. These five principal components were weighted according to their eigenvalues and added to obtain a discriminant score （D） for each germplasm line. A stepwise regression analysis identified eight traits that could be used to identify germplasm lines with a high D score， assumed by the authors to indicate high light-use efficiency. These eight traits were potential photochemical efficiency of PSⅡ， photochemical quantum yield of PSⅡ， total chlorophyll content， net photosynthetic rate， carotenoid content， stomatal limiting value， apparent electron transfer efficiency， and non-photochemical quenching coefficient. Using cluster analysis， the tested timothy germplasm lines were classified into four groups on the basis of their overall D scores. Germplasm lines designated 9451， 10676， and 9657 belonged to group Ⅰ with high multivariate scores. These lines have potential uses as excellent parental materials for breeding new timothy varieties with high light-use efficiency and improved photosynthetic traits.

Key words: timothy, photosynthetic characteristics, chlorophyll fluorescence, high light-use efficiency screening