Research on leaf photosynthesis and chlorophyll fluorescence parameters of two C_4 crops at different leaf position

Both maize and sorghum are C4 crops while their abilities of photosynthesis in the tasseling stage are different.The study was carried out to investigate the relative chlorophyll content,photosynthesis and chlorophyll fluorescence parameters of leaves at three different positions of the two crops.The results indicated that for the upper leaf,the relative chlorophyll content(SPAD) of maize in this period was significantly(P0.05) higher than those of sorghum for the upper leaf,while for the middle and lower leaves the measuring result of SPAD was not significantly different(P0.05).For leaves at three different positions,net photosynthetic rate(Pn),water use efficiency(WUE),and light use efficiency(LUE) in maize were significantly higher than sorghum.The mean CO2 saturation point(Cisat) and carboxylation rate(CE) of maize are 13.29% and 29.33% higher than sorghum at the three different leaf positions,respectively;indicating better light response capability of maize than that of sorghum.For leaves at three different positions,the actual photochemical coefficient(ΦPSⅡ) and photochemical quenching(qP) of maize were significantly higher than sorghum,while the difference in initial energy conversion coefficient(Fv/Fm) was not significant(P0.05).It was therefore concluded that maize had a better photosynthetic adaptability than sorghum;the photosynthetic system of maize had much more advantages in using the photosynthetically active radiation in the field.