Abstract: [Objective] Under the influence of global climate change, the Northern China is experiencing more and more severe volatility in soil water contents, which may significantly affect photosynthesis. The objective was to elucidate the threshold effects of photosynthesis in leaves of Forsythia suspense to soil moisture, clarify the threshold range of photosynthetic productivity, and define the regional water adaptability, by investing the photosynthetic responses of F. suspense to different soil water conditions in spring and summer.[Method] A potted experiment was carried out in a greenhouse with two-year-old F. suspense seedlings. During the experiment, 14 levels of soil water regimes in spring and 10 levels in summer were applied with the artificial water supply and the natural water consumption by plants. At each soil water level in both spring and summer, light response of photosynthesis of F. suspensa was measured by using a Li-6400 portable photosynthesis system. By defining the net photosynthetic rate (P_n) and water use efficiency (WUE) of photosynthesis as the index of plant productivity and soil water efficiency, respectively, key thresholds and the grading of soil water in summer and spring were investigated.[Result] The results showed that when the relative soil water content (RSWC) decreased to 37.5% in spring and to 46.2% in summer, both P_n and WUE showed an obvious decrease with the continuous decreasing of RSWC. At the soil water point, the stomatal limitation changed into non-stomatal limitation in both seasons and the relationship between P_n and stomatal conductance (G_s) was also changed from linear relationship into non-linear relationship as RSWC decreased. All these indicated that when soil water decreased to the turning water point of stomatal mechanism of P_n (RSWC_SL→nSL), P_n begun to decrease seriously as the non-stomatal limitation dominated the decrease of photosynthesis. Thus, we defined RSWC_SL→nSL as the lower limit of soil water maintaining high productivity and efficiency in which P_n or WUE would be above the average level. As a result, the soil water point (RSWC_SL→nSL) was also one of the key thresholds of soil water. Meanwhile, the processes of P_n and WUE responding to RSWC in spring and summer were both able to be better fitted by polynomial, by which the other soil water thresholds were determined, including the water compensation point of P_n (RSWC_Pn=0), the water saturation point of P_n (RSWC_Pn=max), the water point of average P_n (RSWC_Pn=ave), the water saturation point of WUE (RSWC_WUE=max), and the water point of average WUE (RSWC_WUE=ave). RSWC_Pn=0, RSWC_SL→nSL, and RSWC_Pn=ave(or RSWC_WUE=ave) were defined as the lower limit of the low productivity or efficiency water, of the middle productivity or efficiency water, and of the high productivity or efficiency water, respectively, we graded the soil water of F. suspensa in both spring and summer into non-productivity and non-efficiency water (NPNEW), low productivity and low efficiency water (LPLEW), middle productivity and low efficiency water (MPMEW), high productivity and middle efficiency water (HPMEW), and high productivity and high efficiency water (HPHEW) using the coordinate graphic figures. In addition, there were differences in the key soil water thresholds or soil water ranges between the two seasons. The RSWC_Pn=max, RSWC_Pn=ave, RSWC_WUE=max, RSWC_WUE=ave, and RSWC_SL→nSL were all higher in summer than in spring. The upper and lower limits of available soil water to plants including LPLEW, MPMEW, HPMEW, HPHEW, were all higher in summer than in spring, indicating that the available soil water mass was different in different periods of plant growth.[Conclusion] Thus, the field soil water should be maintained in the range of HPHEW which is from 39.5% to 80.0% in spring and from 49.2% to 83.7% in summer, in which both P_n and WUE are higher than the average level so that F. suspensa can obtain higher productivity and efficiency. Meanwhile, the soil water thresholds, the water availability ranges, and the periods of plants growth and development should be considered in field water management of plants.

Key words: soil water thresholds, net photosynthetic rate (P_n), water use efficiency(WUE), photosynthetic productivity grading, season