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Tendencies towards a C4 Leaf: Quantitative Studies on Leaf Anatomy of Selected C3 and C4 Grasses

Hiruni Nuwanthika Weerasooriya, Anil Jayasekera and Iroja Caldera

Grass family (Poaceae) is one of the largest families that represent monocots; containing species like Oryza sativa, Triticum aestivum, Zea mays, Eleusine coracana, Panicum miliaceum, Panicum sumatrense and Sorghum bicolor which are important to humans since they, directly or indirectly provide more than 3/4 of our food. C4 photosynthesis is a complex physiological adaptation, that is believed to have resulted from a series of anatomical and biochemical modifications to the ancestral C3 pathway. It is believed that C4 photosynthesis to have evolved in a stepwise manner nearly 22-24 times creating intermediates with different combinations of C4 -like components in grasses. C4 photosynthesis confers greater productivity than the C3 photosynthetic type in environments under high irradiance and high temperature. Accompanying higher photosynthetic rate C4 plants possess a vascular system in leaves with higher export rates which permit efficient translocation of photosynthates.

In the current study, selected 11 grass species were subjected to analysis of leaf vascular system in relation to their C3 and C4 photosynthesis. The leaf vascular network in both C3 and C4 grasses consist of LLV, SLV and TV that are connected to longitudinal veins. Analysis carried out using the mature leaves of selected C3 and C4 grasses have shown that there are significant differences in anatomical features such as venation, stomata and bundle sheaths. The comparison of DLLV, DSLV and DTV of C3 and C4 grasses demonstrate that C4 grasses have a denser vascular pattern compared to C3 species (respective distances of C4 grasses were 71.74 μm, 11.85 μm and 81.4 μm). It has been found that these reduced distances between veins have led to significant low stomatal density among C4 grasses. Further, characteristic large bundle sheath structure of C4 species has been clearly seen from significantly high values obtained for BSD, ISD and OSD (respectively 10.4 μm, 4.32 μm and 7.15 μm).

Furthermore, this screening has found Oryza nivara (a C3 species) having C4 like anatomical characters in leaf venation and bundle sheath structure. This species showed significantly low mean values for the distance between transverse veins (41.75 μm) resulting in significantly high total vein length per unit leaf area (81.77μm^-1). The C4 tendency of its leaf anatomy was further supported by significantly large bundle sheaths, inner sheath and outer sheath distances (13.1 μm, 5.22 μm and 8.22 μm respectively).

At the end of the analysis we were able to demonstrate significant differences of anatomical characters like venation, stomata and bundle sheaths in C3 and C4 plants. In addition, observations suggest characteristic leaf anatomy that is compatible with C4 anatomy may have evolved in Oryza nivara resulting it to be considered as C3- C4 intermediate.