At the grain-filling stage, net photosynthetic rate (P N ), stomatal conductance (g s ), and ribulose-1,5-bisphosphate carboxylation efficiency (CE) were correlated in order to find the determinant of photosynthetic capacity in rice leaves. For a flag leaf, P N in leaf middle region was higher than in its upper region, and leaf basal region had the lowest P N value. The differences in g s and CE were similar. P N , g s , and CE gradually declined from upper to basal leaves, showing a leaf position gradient. The correlation coefficient between P N and CE was much higher than that between P N and g s in both cases, and P N was negatively correlated with intercellular CO 2 concentration (C i ). Hence the carboxylation activity or activated amount of ribulose-1,5-bisphosphate carboxylase/oxygenase rather than g s was the determinant of the photosynthetic capacity in rice leaves. In addition, in flag leaves of different tillers P N was positively correlated with g s , but negatively correlated with C i . Thus g s is not the determinant of the photosynthetic capacity in rice leaves.Additional key words: carboxylation efficiency; flag leaf; intercellular CO 2 concentration; leaf position; net photosynthetic rate; Oryza; stomatal conductance. ______Net photosynthetic rate (P N ) in plant leaf is often influenced by environment factors such as irradiance, temperature, and water supply, and also by leaf age, leaf position, and developmental stage. The findings on the changes in P N , g s , and ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBPCO) content during leaf development are summarized in Šesták (1985) and Čatský and Šesták (1997). However, the question remains what is the determinant of the differences in P N between leaves of different age, different position, and different regions of a leaf. We tried to solve these questions using a correlation analysis.Rice (Oryza sativa L. hybrid combination GD-1S/ RB207) plants were grown in the experimental field of China National Hybrid Rice Research and Development Center (Changsha, Mapoling). They were managed using common culture techniques for the region. P N was measured when rice plants were at the filling stage. During 10:00-16:00 (Beijing time) P N in attached flag leaves was measured in situ at saturating irradiance, air CO 2 concentration of 350 µmol mol -1 , and air temperature of 30 °C using a portable infrared gas analysis system LI-6400 (LiCor, USA). The irradiation source provided by LI-6400 was used to supply irradiance saturating photosynthesis, about 1 200 µmol(photon) m -2 s -1 . CE was measured using a set of CO 2 concentrations from 250 to 50 µmol mol -1 , decreasing by 50 µmol mol -1 . About two minutes were required to stay at each CO 2 concentration before P N value was recorded. The Sigma Plot 6.0 software was used to analyze experiment data and make correlation analysis.The measured flag leaves were about 60 cm long and their basal, middle, and top regions were about 10, 30, and 50 cm from their leaf sheath, respectively. P N in different regi...

Predictive models for the accumulation of available phosphorus (Olsen-P, extracted with 0·5 mol/l sodium bicarbonate (NaHCO 3 ) at pH 8·5) in the north-western arid areas of China, especially in Xinjiang, are essential for the improved management of phosphorus (P) fertilizers. In the present study, an accumulation model for Olsen-P in grey desert soil (Calcaric Cambisol) was developed using the data for initial Olsen-P in soil, P fertilizer application rate (organic and inorganic P), crop yields, and soil pH from a 22-year long-term experiment (1990-2011) with 3-year rotation of wheat (Triticum aestivum L.), maize (Zea mays L.) and cotton (Gossypium spp.). The model was also validated independently using previously published data from the literature. The results indicated an average net accumulation of Olsen-P in the plough layer (0-200 mm) of 0·36 mg/kg/year (from 0·083 to 0·47 mg/kg/year) when P fertilizer was applied, while an average net Olsen-P loss of 0·12 mg/kg/year (from 0·067 to 0·26 mg/kg/ year) was observed without P fertilization in the soil. For target yields of wheat, maize and cotton at 5, 6 and 6 tonne/ha (t/ha), respectively, in soil with pH 8, the rates of Olsen-P increase in the soil as estimated by the model were 0·11, 0·24, 0·36, 0·49 and 0·61 mg/kg/year when P application rates were 60, 70, 80, 90 and 100 kg P/ha per 3-year period, respectively. For every 100 kg/ha of P surplus, Olsen-P increased by 1·1 mg/kg in Xinjiang grey desert soil. This Olsen-P accumulation model was valuable for the management of soil P in agricultural production and environmental protection in north-western China and other arid areas planted with a yearly rotation of wheat, maize or cotton.