Study on diurnal photosynthetic characteristics in different alfalfa leaf layers in Loess Plateau.

In order to study the spatial differences in assimilation capacity and their impact factors and provide the scientific basis for variety improvement and field management in the Loess Plateau,the diurnal dynamics of photosynthetic characteristics in different alfalfa leaf layers were monitored using portable LI-6400 photosynthesis system in 2006.The results show that the curves of diurnal variation in net photosynthetic rate(Pn) and transpiration rate(Tr) of different leaf layers had three peaks,and an obvious midday depression appeared at 12:00 o'clock.There was a significant difference in Pn as the upper leaf layerthe middle leaf layerthe lower leaf layer;a distinct difference in intercellular CO2 concentration(Ci) as the lower leaf layerthe middle leaf layerthe upper leaf layer;an opposite trend for stomal limit value(Ls) to Ci.;no distinct difference in Tr among different leaf layers.Based on the tendency of changes in Pn,Ci,and Ls,it was speculated that Pn depression was controlled by stomatal factors before the highest PAR,and then by non-stomatal factors.Alfalfa had different photosynthetic characteristics for leaf layers: the upper was a high photosynthetic layer;the lower was a limit both for photosynthesis and light energy capture;the middle layer between the upper and the lower layers.