Abstract:

The experiment was conducted using the typical wheat cultivars with different phosphorus (P) efficiencies with normal P (P⁺) and P deficiency (P－) two treatments to study the photosynthetic characteristics and their physiological mechanism. With the leaf growth the CO₂ conductance parameters, such as stomatal conductance (G_s), mesophyll conductance (G_m) and carbonic anhydrase (CA) activity in flag leaves (Table 2), the chlorophyll content (Chl) and soluble protein content (SP) in the fourth spring leaves and flag leaves (Fig.1, Fig.5), as well as the inorganic phosphorus (P_i) content in the chloroplast (Fig.3), Mg²⁺-ATPase activity in the chloroplast and the photosynthetic rate (P_n) (Fig.2, Fig.4) were all decreased. Meanwhile, the key enzyme in the dark reaction of photosynthesis, RuBPCase, showed the highest activities at 15 d after leaf expansion, the middle at the early growth stage (0 d after leaf expansion), and the lowest at 30 d after leaf expansion (Fig.6). Under P-deficient condition, all the physiological parameters measured in the whole experiment in the fourth spring leaves and flag leaves of cultivars with high absorption efficiency (Ha) and high utilization efficiency (Hu) were higher than those with low P efficiency (L), implicating that the relative improvement in photosynthesis of Ha and Hu under P-deficient condition were coordinately caused by the increase of light energy capture, the decrease of the stomatal limitation and non-stomatal limitation of CO₂ conductance, and the enhancement of dark reaction of photosynthesis. The higher Mg²⁺-ATPase activities of chloroplast in leaves of cultivars with high P efficiency were possibly related to the more Pi content of the chloroplast, RuBPCase activities and RuBPCase specific activities at 15 d after leaf expansion were higher than those at the early growth stage (0 d after leaf expansion), implying that some metabolic enzymes in plants, such as RuBPCase, had some self-regulation and protection mechanisms to maintain plant function under the conditions of leaf aging and P-deficiency.

Key words: Wheat (Triticum aestivum), Phosphorus efficiency, Photosynthesis, Physiological mechanism