Diversity analysis of leaf vein density of selected Sri Lankan rice varieties: an indicator of C₃ to C₄ photosynthesis pathway transition

Abstract

Out of the commonly cultivated cereals in the family Poaceae, maize and sorghum follow a C₄ photosynthetic pathway, while rice and wheat follow a C₃pathway.Hence, carbon assimilation of C₃ is lower compared to C₄plants. Studies revealing the evolution of these photosynthetic pathways indicate that C₄plants evolved from C₃ plants. The transition is demarked by increased leaf vein density and subsequent cellular compartmentalization that facilitate efficient carbon fixation. In a leaf, photosynthates from nearby cells are collected by small longitudinal veins, transverse veins and large longitudinal veins, and are translocated out of the leaf via the phloem. Hence, with a denser leaf vein density, the translocation process is expected to be more efficient. In the current study, longitudinal leaf vein (LV) density of 12 selected rice varieties was measured to assess their diversity with respect to collective assessment of both small and large LV density. The total LV length per unit area (TLV) and the average distance between LV (ADLV) were measured in a one-cm leaf sample fixed in a permanent slide, made from the widest middle portion of the first leaf of the main culm. The sections were imaged and the distances were measured using DinoCapturev2.0. The TLV and ADLV of the selected varieties wereanalysed using a one-way ANOVA with PROC GLM, and mean separation was carried out using Duncun’s multiple range test in SAS v9.1.3. A significant negative correlation was observed between TLV and ADLV (r= 0.969, p<0.05). Among the selected rice varieties, Dewareddhiri and Madayalvee have the highest TLV (5.7±0.1 and 5.6±0.1 mm/mm² respectively) and the lowest ADLV (159.6±1.2 and 160.8±3.3 μm, respectively). The variety Moroberekan shares the lowest TLV with Pachchaperumal, and Sivappukuruvikkar (4.8±0.1, 4.8±0.1 and 4.6±0.1 mm/mm² , respectively) and Moroberekan has the highest ADLV (195.3±4.0 μm). The high TVL and low ADLV denote a high leaf vein density. Thus, the high vein densities of the varieties Dewareddhiri and Madayalvee could be a possible indicator of transition from C₃ to C₄ photosynthetic pathway. Further, analysis of these varieties with respect to leaf anatomy is underway.