Relationship between Leaf Structure and Gas Exchange in Wheat Leaves at Different Insertion Levels

Abstract

Net photosynthesis rate (P_n), stomatal conductance to CO₂ and residual conductance to CO₂ were measured in the last six leaves (the sixth or flag leaf and the preceding five leaves) of Triticum aestivum L. cv. Kolibri plants grown in Mediterranean conditions. Recently fully expanded leaves of well-watered plants were always used. Measurements were made at saturating photosynthetic photon flux density, and at ambient CO₂ and O₂ levels. The specific leaf area, total organic nitrogen content, some anatomical characteristics, and other parameters, were measured on the same leaves used for gas exchange experiments. A progressive xeromorphic adaptation in the leaf structure was observed with increasing leaf insertion levels. Furthermore, mesophyll cell volume per unit leaf area (V_mes/A) decreased by 52·6% from the first leaf to the flag leaf. Mesophyll cell area per unit leaf area also decreased, but only by 24·5%. However, nitrogen content per unit mesophyll cell volume increased by 50·6% from the first leaf to the flag leaf. This increase could be associated to an observed higher number of chloroplast cross-sections per mm² of mesophyll cell cross-sectional area in the flag leaf: values of 23000 in the first leaf and 48000 in the flag leaf were obtained. P_n per unit leaf area remained fairly constant at the different insertion levels: values of 33·83±0·93 mg dm⁻² h⁻¹ and 32·32±1·61 mg dm⁻² h⁻¹ were obtained for the first leaf and the flag leaf, respectively. Residual conductance, however, decreased by 18·2% from the first leaf to the flag leaf. Stomatal conductance increased by 41·7%. The steadiness in P_n per unit leaf area across the leaf insertion levels could be mainly accounted for by an opposing effect between a decrease in V_mes/A and a more closely packed arrangement of photosynthetic apparatus. Adaptative significance of structural changes with increasing leaf insertion levels and the steadiness in P_n per unit leaf area was studied.