Photosynthetic characteristics of spring ephemerals in the desert ecosystem of Dzungaria Basin, northwest China

Spring ephemerals have the highest photosynthetic rates in higher plants. Gas exchanges and related environmental factors of four typical spring ephemerals, Eremopyrum orientale, Malcolmia scorpioides, Tetracme quadricormis and Arnebia decumbens were measured in their natural environments in the Gurbantunggut Desert of the Dzungaria Basin, northwestern China. Diurnal variations in net photosynthetic rate (P-N), transpiration rate (E), stomatal conductance (gs), and water use efficiency (WUE) were analyzed at individual leaf level for these species. P-N of E. orientale and M. scorpioides showed a bimodal diurnal pattern, whereas that of T. quadricormis and A. decumbens showed a unimodal pattern, with the maximum value occurred at noon (around 12: 00). E of all four species showed a unimodal pattern. Higher WUE in the morning was related to higher P-N and lower E. The maximum value of gs appeared in early morning, and then decreased gradually during the daytime. The highest photosynthetically active radiation (PAR) occurred at 14: 00, but there was a 2 h lag between the highest temperatures of air and leaf and the highest PAR. The maximum net photosynthetic rate (P(N)max) of the four spring ephemerals were 18.44, 32.03, 33.17, and 20.04 mu molCO(2) m(-2) s(-1), respectively. The present study revealed that the midday depression of photosynthesis of E. orientale and M. scorpioides was mainly due to non-stomatal limitation, whereas that of T. quadricormis and A. decumbens was mainly due to stomatal limitation during 14:00-16:00 followed by non-stomatal limitation after 16:00. Our results illustrated that the change of E was consistent with the diurnal fluctuation of air temperature, but the diurnal change of P-N was affected by the whole microclimate, i.e. temperature, relative humidity (RH), PAR, etc.