Stomatal or non-stomatal limitation of photosynthesis of spring wheat flag leaf at late growth stages under natural conditions in semiarid rainfed regions

Rainfed agriculture is a critical production mode in arid and semiarid regions, which accounts for 30% of the earth's surface. Spring wheat is one of the main crops in the semiarid region of the Loess Plateau in China. It is therefore important to investigate the photosynthetic characteristics of spring wheat (Triticum aestivum L.) in the semiarid region of the Loess Plateau. Previous studies have mainly focused on the responses of photosynthesis to environmental factors, with few researches on photosynthetic limitations under natural conditions in the semiarid regions of Northwest China. Exploration of photosynthetic limitations of flag leaves of spring wheat under natural conditions could contribute to current understanding of the principles of stomatal regulation in semiarid regions. It can also provide a theoretical basis for water use efficiency at the leaf-scale. In order to investigate stomatal and non-stomatal limitations of photosynthetic rates of flag leaves of spring wheat at different growth stages under natural conditions in semiarid areas, a field experiment was conducted in 2012 during the heading and filling stages of spring wheat. Physiological char-acteristics such as photosynthetic rate (P_n), stomatal conductance (g_s) and intercellular CO₂ concentration (C_i) were measured by the LI-6400 portable photosynthesis analyzer. Then environmental factors such as photosynthetically active radiation (PAR), vapor pres-sure deficit (VPD), air temperature (T_a) and air relative humidity (RH) were simultaneously recorded. The study firstly analyzed di-urnal variations in environmental factors and photosynthetic characteristics of flag leaves of spring wheat. Analyzed next was the response of photosynthetic characteristics to environmental factors, finally followed by discussion on stomatal and non-stomatal limitations at heading and filling growth stages. Results showed that PAR, T_a, and VPD had unimodal type of diurnal curve at both growth stages, while RH had uni-trough type of diurnal curve. The peak values of PAR and VPD occurred at about 11:00 am, with forenoon values obviously larger than afternoon ones. The peak value of Ta and RH occurred at about 15:00 pm. PAR and VPD were markedly larger at filling stage than at heading stage. The P_n tracked a unimodal type of diurnal curve at heading and bimodal type at filling stage with similar peak values of about 18.5 ?mol(CO₂)·m^-2·s^-1. gs followed a diurnal curve that was similar to that of P_n. C_i decreased in the forenoon and slowly increased in the afternoon. This was attributed not only to photosynthetic consumption and stomatal conductance limitation, but also to mesophyll conductance limitation. P_n and g_s were highly correlated at both growth stages, with correlation coefficients (R²) of 0.916 and 0.945, respectively. gs generally limited Pn at both heading and filling stages, with stomatal limitation indexes of 0.64 and 0.63, respectively. At heading stage, gs was highly sensitive to VPD. For this reason, there was obvious stomatal limitation in the afternoon due to induced water deficit by high transpiration rate. At filling stage, midday de-pression was mainly caused by stomatal closure due to strong radiation and high VPD. Besides this, there was somewhat non-stomatal limitation during midday depression. Due to declining sensitivity of gs to VPD and midday depression strategy, stomatal limitation decreased from heading stage to filling stage. This contributed to higher P_n of flag leaves of spring wheat in semiarid regions and thereby guaranteed good harvest.