Effects of different phosphorus levels on root physiological and leaf photosynthetic characteristics of flue-cured tobacco

【Objectives】 Root is the vital organ for absorbing water and nutrients. Its physiological intensity plays a very important role in the growth of aerial parts. In this research, the effects of different phosphorus levels on root physiological and leaf photosynthetic characteristics of flue-cured tobacco at different growth stages were analyzed to clarify the physiological effect of phosphorus on the growth of flue-cured tobacco, to illustrate the formation mechanism of its superior quality and provide some references for reasonably application of phosphorus in tobacco production.【Methods】 Four phosphate levels were designed in a pot experiment in Henan Agricultural University in 2014. The four levels were P2O5: 0 (CK), 3.5 (T1), 7.0 (T2) and 10.5 g/plant (T3). In each pot, 3.5 g N and 10.5 g K2O were applied. Root physiological traits and leaf morphological and physiological indexes were investigated at different growth stages of flue-cured tobacco.【Results】 Higher root activity, total root absorption area, root active absorption area and the net photosynthesis rate of middle leaf in the whole growth period were achieved in T2 treatment than in other ones. With increasing of the phosphorus supply levels, all the root physiological traits, middle leaf areas and the contents of chlorophyll increased at the knee-high stage and the increase in T3 treatment was higher than CK significantly. The leaf chlorophyll content declined faster in T3 treatment than in other ones at the mature stage. For all treatments, the net photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration of tobacco leaf were firstly increasing and then decreasing along with the elongation of growth period and all the indexes above reached a maximum at 60 days after transplanting. The leaf net photosynthetic rate, transpiration rate and stomatal conductance of T2 were all the highest in the whole growth period and the differences with those in CK were significant at 60 days after transplanting. With the elongation of growth period, the root to shoot ratio of tobacco fluctuated but showed an upward trend. During the whole growth period, the maximum root to shoot ratio was in T3 treatment, followed by CK, those in T1 and T2 were relatively small. 【Conclusions】 The rapid growth of plants at the knee-high stage and decreasing of the chlorophyll content at the mature stage in T3 treatment suggested that supplying adequate phosphorus within a certain range contributed to the development of tobacco root and the expansion of leaf at the early growth stage and the maturity of tobacco leaves in due time. The affection of phosphorus on flue-cured tobacco root to shoot ratio proved that phosphorus had positive effect on leaf photosynthetic characteristics based on the structure and function of tobacco root and shoot on a coordinated basis. Once that coordination was broken, the net photosynthesis rate of its leaves would reduce. The growth and development of T2 was superior overall, while T1 could meet the needs of the normal growth of tobacco and reduce production costs at the same time. It would provide some references to phosphate fertilizer application rate in tobacco production.