Plant material and growth conditions
Two experiments were conducted to investigate the diversity of local C source–sink ratio and its impact on rice photosynthetic and yield response to e-CO2. Exp. 1 was conducted in growth chambers, while Exp. 2 was a more detailed trial conducted in a climate-controlled greenhouse. The trials used different but partially overlapping sets of genotypes (Table 1). They were selected to represent diversity in local C source–sink ratio (LSSR), based on previous field phenomics data provided by CIAT in Colombia (Rebolledo et al. , 2016). The selection of genotypes also attempted to limit other sources of variation, such as degree-days to flowering, tiller and panicle numbers per plant, plant height and spikelet fertility.
Genotypes were germinated on wet filter paper and transplanted into 6-L pots filled with Jiffy substrate, pH4.5 (Jiffy Products International BV) . This pot size is sufficient for rice to avoid reductions in photosynthesis or biomass accumulation along plant cycle (Poorter et al., 2012; Sage, 1994). Basal fertilizer was applied before transplanting using a mixture of Basacot 6 M high (Compo Expert, France), 13%N–5%P–18%K and Siforga (MeMon BV), 5%N–3%P–8%K at 2 g l-1. A second application at 2 g l-1 was performed just before heading stage to avoid post-floral nitrogen deficiency.
Exp. 1: Plants were grown from March to July 2021 at the Agronomical Research and International Center for Development (CIRAD, Montpellier, France) in two adjacent fully climate controlled walk-in growth chamber, ARALAB FITOCLIMA 25.000HP located in CIRAD’sAbioPhenplatform for climate change studies. They were grown under artificial light provided by Philips full spectrum ceramic metal halide (CDM-TMW 315W/930 1CT) providing on average photosynthetic irradiance of 800 μmol m-2 s-1 at the top of the canopy level during a 12-h photoperiod. Air temperature was set to 28° C (day) and 22° C (night). Air relative humidity was set to 65% (day) and 80% (night). The two chambers were differentiated by the atmospheric CO2 level applied from transplanting to maturity: 400 μmol mol-1 (ambient) versus 700 μmol mol-1 (e-CO2).
Exp. 2 : Plants were grown from March to July 2022 in the same building as EXP1 (AbioPhen complex) in two adjacent, climate-controlled greenhouse compartments. They were grown under natural daylight with supplemental lighting maintaining a 12-h photoperiod using horticultural red-blue LED projectors (Alpheus Radiometrix 15M1006) providing R/FR ratio of about 1.2. Microclimate was monitored using data loggers (CR1000 Campbell Scientific) installed in each compartment. Air temperature averaged 27°C (day) and 21°C (night) as measured with a PT1000 probe under fan-aspirated shield. Air relative humidity averaged 65% (day) and 75% (night), measured by HMP45 (Vaisala, Helsinki, Finland), and photosynthetic photon flux density (PPFD) was measured with a SKP215 (Skye Instrument quantum sensor, Powys, UK) providing on average photosynthetic irradiance of 800 μmol m-2 s-1 at the top of the canopy level during the daytime. The mean photosynthetically active radiation received by the plants during their life cycle was 7.58 MJ m-2 d-1. The two compartments were differentiated by the atmospheric CO2 level applied from transplanting to maturity: 400 μmol mol-1 versus 650 μmol mol-1.
In both experiments, all the pots (including a row of border plants) were arranged at 20 cm spacing among plants in a randomized design with four replications per cultivar and per CO2 treatment on movable tables. Pots were kept watered at field capacity while maintaining the perforated pot bottoms in 5 cm of standing water. To minimize border effects on each table, border plants on the tables were not used for measurements. The tables were moved weekly to avoid the effects of spatial heterogeneity.
For each CO2 treatments, plants were characterized for growth and development traits along the cycle, photosynthesis and biochemical measurements at 15 days after heading, final biomass and grain production as described hereafter, whereas EXP1 has focused only on the photosynthesis measurement at 15 days after heading.