Chapter 1: THE ENHANCEMENT OF FUNDAMENTAL PHOTOSYNTHETIC PROPERTIES, GROWTH AND YIELD IN MR219 AND MR263 RICE VARIETIES VIA EARLY-STAGE CO2 ENRICHMENT TO THE SEEDLINGS BEFORE TRANSPLANTING (THESIS)

THE ENHANCEMENT OF FUNDAMENTAL PHOTOSYNTHETIC PROPERTIES, GROWTH AND YIELD IN MR219 AND MR263 RICE VARIETIES VIA EARLY-STAGE CO2 ENRICHMENT TO THE SEEDLINGS BEFORE TRANSPLANTING

By

AZZAMI ADAM MUHAMAD MUJAB

September 2022

Chairman      :   Muhammad Nazmin bin Yaapar, PhD

Faculty         :   Agriculture

Controlled environment systems such as glasshouses regularly utilise elevated CO₂ (eCO₂) to boost yield and quality in the production of high-value crops. Although this approach is quite commonly practised in commercial horticulture, its implementation on major crops such as rice is technically not feasible, especially elevating CO₂ throughout the production field for the entire life cycle of the crop. During the early stage of rice plant development, the structure of the leaf is sensitive to environmental factors, including responses to CO₂ levels. In this project, the response of rice seedlings exposed to eCO₂ only during the initial nursery phase before field transplant can have a lasting impact until the harvest period was investigated. The study aims include understanding the effects of early-stage eCO₂ treatment on rice growth, leaf stomatal properties, photosynthetic performance and yield components at the rice seedling stage and mature stage. This experiment used two local rice varieties, namely MR219 and MR263. Rice plants were grown in a two-stage procedure. First, seedlings were grown in DIY ambient CO₂ (400 ppm) and elevated CO₂ (~800 ppm) chambers for 24 days and then transplanted to a rain shelter structure where the plants were grown to harvest. The eCO₂ source came from the fermentation of a mixture of sugar, distilled water, and baker’s yeast (Saccharomyces cerevisiae) granules. The first experiment showed that eCO₂ priming had significantly increased the seedling height (38-42%), the number of leaves (26-30%), leaf thickness (22-38%), leaf length (8-32%) and dry weight (58-69%) for MR219 and MR263 varieties. In general, eCO₂ treatment resulted in a larger stomatal complex (14-46%) and stomatal pore area dimensions (62-64%) with reduced stomatal density (11-19%) than aCO₂-grown leaves also in both varieties. Moreover, the intrinsic water use efficiency (iWUE) of eCO₂ leaves was also 38-68% higher in both MR219 and MR263. In terms of photosynthetic performance, the maximum assimilation rate (A_max), maximum Rubisco carboxylation rate (V_cmax), maximum electron transport rate (J_max), the quantum yield of PSII (ΦPSII), and quantum yield of CO₂ assimilation (ΦCO₂) were significantly higher for eCO₂ rice seedlings than aCO₂ for both rice varieties. In the second experiment, significant photosynthetic parametres enhancement (A_max, J_max, ΦPSII, ETR, ΦCO₂) were quantified in eCO₂ MR263 flag leaves but not in MR219. Interestingly, both rice varieties' seedlings when exposed to eCO₂ maintained a significantly higher V_cmax (> 10%) during the mature phase of plant development than plants grown continuously under aCO₂. In terms of yield components, both varieties exposed to early-stage eCO₂ treatment showed a significantly 14-27% higher filled spikelet number per panicle, 3% higher 1000-grain weight, 11.5-12.5% increase in tillers and 10-12% panicles numbers per plant with significantly 5-6% lower plant height. The yield potential shows an increase of 4-7% for MR219 and MR263 eCO₂-treated seedlings compared to aCO₂. In conclusion, brief and targeted eCO₂ enhancement during the seedlings phase demonstrates a promising way of improving plant growth development, photosynthetic properties and rice yield performance. CO₂ priming has been suggested as a potential strategy for improving the productivity of rice crops, especially in regions where maintaining elevated CO₂ levels throughout the entire crop life cycle is not feasible or practical. By exposing rice plants to elevated CO₂ levels during their early growth stages, farmers may be able to take advantage of the benefits of CO₂ priming without having to maintain elevated CO₂ levels throughout the entire crop life cycle and, in turn, increase farmers' income. This study can contribute to the development of more sustainable and efficient agricultural practices that can meet the growing demand for food.

INTRODUCTION

1.1 Background

Rice (Oryza sativa L.) is indeed an important cereal for its role as the staple food for almost half of the world's population (Sekhar, 2018). GRiSP (2013) reported that over half of the world's population relies on rice for at least 20% of their daily caloric intake. However, rice production is not keeping pace with the demand stemming from the ever-rising global human population. As a result, agricultural productivity is in the need to improve remarkably to cater for the hungry people who are going to reach 9.9 billion by 2050, an increase of more than 25% from the current population of around 7.8 billion (PRB, 2021) .

CO₂ is one of the substrates that limits photosynthesis, particularly in the C3 plant system living at the current CO₂ level. Thus increasing CO₂ levels in crop production has been employed as a way to enhance the carboxylation rate of photosynthesis while minimizing photorespiration through oxygenation reaction suppression of Rubisco (Bhagat et al., 2014). This simple method increases photosynthesis efficiency which generally results in better plant performance that leads to a higher yield (Sakai et al., 2019; Usui et al., 2016). In the Free-Air Carbon dioxide Enrichment (FACE) experiment, elevated CO₂ (eCO₂) will promote the net photosynthetic of the plant and thus plant productivity, according to Long et al., 2004 and Leakey et al., 2009. eCO₂ also has been used widely for many years as a CO₂ gas fertilizer to increase photosynthetic performance and yield in vegetables and high-value crops grown in greenhouses (Bisbis et al., 2018).

1.2 Problem Statement

The CO₂ enrichment method is relatively common in commercial horticultural crop production, due to the massive areas involved in major food crops such as rice, it is technically not feasible to elevate CO₂ throughout the whole rice crop life cycle. In addition, large-scale manipulation of atmospheric CO₂ levels is not practical or economically feasible. Furthermore, rice is a unique crop that is typically grown in flooded paddies, which complicates attempts to manipulate CO₂ levels. The flooded fields make it difficult to regulate CO₂ levels within the rice canopy, as the water covering the fields can trap and release CO₂ at different rates, making it difficult to maintain consistent CO₂ levels. Given these limitations, CO₂ priming is suggested as a potential alternative to continuous CO₂ elevation for rice crops.

Early-stage CO₂ enrichment or CO₂ priming is a technique that involves exposing crops to elevated levels of carbon dioxide (CO₂) for a short period during their early growth stages. The idea behind this technique is that by subjecting plants to higher concentrations of CO₂ early on, they can become more efficient at utilizing CO₂ throughout their life cycle, even when grown under normal CO₂ concentrations. This technique can help to increase rice yields and improve crop productivity more practically and sustainably.

1.3 Objectives

In this study, we aim to investigate the effects of early-stage CO₂ enrichment on fundamental photosynthetic properties, growth, and yield in two rice varieties, MR219 and MR263. The objective of this study is:

1. To evaluate how eCO₂ influences rice seedling establishment before they could be transplanted into the field for MR219 and MR263 rice varieties.

2. To assess the efficacy of growing rice in eCO₂ during the seedling stage in improving rice harvest components in both MR219 and MR263 rice varieties.

The results of this study could have significant implications for rice production and food security in the face of climate change. By identifying a potential method for enhancing the efficiency of photosynthesis and improving growth and yield in rice plants, this study may contribute to the development of more sustainable and efficient agricultural practices that can meet the growing demand for food.