Mitochondrial respiration in the dark (R dark) is a critical plant physiological process, and hence a reliable, efficient and high-throughput method of measuring variation in rates of R dark is essential for agronomic and ecological studies. However, currently methods used to measure R dark in plant tissues are typically low throughput. We assessed a high-throughput automated fluorophore system of detecting multiple O2 consumption rates. The fluorophore technique was compared with O2-electrodes, infrared gas analysers (IRGA), and membrane inlet mass spectrometry, to determine accuracy and speed of detecting respiratory fluxes. The high-throughput fluorophore system provided stable measurements of R dark in detached leaf and root tissues over many hours. High-throughput potential was evident in that the fluorophore system was 10 to 26-fold faster per sample measurement than other conventional methods. The versatility of the technique was evident in its enabling: (1) rapid screening of R dark in 138 genotypes of wheat; and, (2) quantification of rarely-assessed whole-plant R dark through dissection and simultaneous measurements of above- and below-ground organs. Variation in absolute R dark was observed between techniques, likely due to variation in sample conditions (i.e. liquid vs. gas-phase, open vs. closed systems), indicating that comparisons between studies using different measuring apparatus may not be feasible. However, the high-throughput protocol we present provided similar values of R dark to the most commonly used IRGA instrument currently employed by plant scientists. Together with the greater than tenfold increase in sample processing speed, we conclude that the high-throughput protocol enables reliable, stable and reproducible measurements of R dark on multiple samples simultaneously, irrespective of plant or tissue type.

Heterosis or hybrid vigour has been used widely for more than a decade in Brassica napus (Canola) production. Canola hybrids show heterosis in a variety of traits compared to parents, including increased biomass at early stages of seedling establishment, which is a critical developmental step that impacts future plant growth and seed yield. In this study, we examined transcriptomes of two parental lines, Garnet (Gar) and NX0052 (0052), and their reciprocal hybrids, Gar/0052, at 4 and 8 days after sowing (DAS). In hybrids, early seedling biomass heterosis is correlated with earlier expression of genes in photosynthesis pathways relative to parents. The hybrids also showed early expression of genes in the auxin biosynthesis pathway, consistent with the higher auxin concentrations detected in hybrid seedlings at 4 DAS. Auxin is a key phyto-hormone that regulates plant development promoting cell expansion and cell proliferation. Consistent with the increased levels of auxin, hybrids have larger and more palisade cells than the parents at the same time point. We propose a possible mechanism of early biomass heterosis through early establishment of photosynthesis and auxin biosynthesis, providing insights of how transcriptional changes in hybrids are translated into phenotypical heterosis. This finding could be utilized in future Canola breeding to identify hybrid combinations with superior early seedling establishment and strong levels of hybrid vigour in later plant development.

In crop improvement programs, hybrid vigour (heterosis) is an important breeding strategy but the molecular mechanisms of hybrid vigour are still unclear. Grain yield declines after F1 generation due to phenotypic segregation. We found that, at the early seedling stage in hybrids derived from the temperate japonica rice varieties ‘Doongara’ and ‘Reiziq’, hybrid vigour was approximately 40% greater than in the better parents. Inbred high-yielding lines (Hybrid Mimics) were developed from the ‘Doongara’ × ‘Reiziq’ F1 by selfing and recurrent selection for F1-like plants in the F2 through to the F5 generation. Grain yields are stable over subsequent generations in the Hybrid Mimic lines. The importance of photosynthesis in early seedling development was demonstrated. Photosynthesis-related genes were expressed in the hybrid earlier than in the parents; physiological evidence using gas exchange indicated the early commencement of photosynthesis. Dark germination experiments supported the requirement for photosynthesis for early vigour in hybrids.