Rice (Oryza sativa) is one of the most important cereal crops worldwide. To meet the increasing demand for grain as the world’s population increases, rice productivity must be increased by ~ 50% relative to the current level by 20501,2. The rice yield increases during the “green revolution” depended largely on the development of semi-dwarf cultivars with greater harvest index and on greatly increased N fertilizer application3,4. This strategy is reaching its limits, however, because harvest index is reaching its theoretical maximum and excess application of N fertilizer causes environmental pollution5–7. Further enhancement of grain yield must be achieved through increases of total biomass accumulation via improved radiation use efficiency without increased nutrient inputs8. Single-leaf photosynthesis has long been considered a target trait for increasing radiation use efficiency6,9,10. Recent studies have shown the importance of enhancing single-leaf photosynthesis and crop productivity in the field11; for example, the promoted recovery from photoprotection increased biomass production in tobacco (Nicotiana tabacum)12, and overproduction of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) increased grain yield in rice13.
Using natural genetic resources could be a useful approach for improving photosynthesis14–16. Wide intraspecific variation in net CO2 assimilation rate per leaf area (A) has been found in several crop species, including rice17–19 and wheat (Triticum aestivum)20,21. The underlying genetic variations can be used in quantitative genetic analyses to identify genomic regions relating to leaf photosynthesis, facilitating DNA marker-assisted selection14,16. An important question in such an approach is whether the enhanced A effectively increases total biomass production and grain yield22. Positive close correlations of A with plant (or crop) growth rate, biomass production, and final yield through large-scale surveys of diverse sets of accessions have been reported in rice19,23,24, wheat25,26 and soybean (Glycine max)27. Simulation analyses showed that a 25% increase in single-leaf photosynthesis based on rice genetic resources could enhance biomass production by 22–29%28. Furthermore, newer rice cultivars developed in Japan with high yield capacity have higher A than older cultivars, especially after heading29,30. These studies underpin the potential for enhanced productivity by improved photosynthesis achieved through the use of natural genetic resources.
In contrast, there are conflicting results on the photosynthesis–productivity relationship. Poor correlations between A and biomass accumulation have been reported in rice18,31, wheat21 and maize (Zea mays)32. Evans (1993) questioned the effects of the genetic improvement of single-leaf photosynthesis for better crop yields33. In fact, crop breeding has often selected increased leaf area production at the expense of photosynthetic capacity, as occurred in wheat34. The inconsistencies between studies could reduce the potential value of natural genetic resources for improving leaf photosynthesis and delay the enhancement of crop productivity.
The value of A changes across the growing season owing to the progression of plant age and leaf senescence35–37. However, most studies of the photosynthesis–productivity relationship selected only one or two growth stages for evaluation of photosynthesis18,19,26,30. Such a “snapshot” analysis can reveal only limited aspects of crop production and potentially cause inconsistent results. The need for comprehensive evaluation is supported by the fact that the total CO2 uptake per tobacco plant, calculated from multiple measurements of leaves at several positions throughout the day and the growing season, agreed well with actual dry weight increase38. Therefore, multiple photosynthetic measurements are necessary when we examine natural genetic resources across their growing season.
Conventional open gas exchange systems require several to tens of minutes to acclimatize a leaf to the leaf chamber, limiting the number of samples to be examined39. To overcome this limitation, we recently created a new closed gas exchange system (MIC-100; Masa International Corporation, Kyoto, Japan), which takes 15–20 s per measurement, ~ 90% less than conventional open gas exchange systems. We hypothesize that with the new measurement system, tracing photosynthetic dynamics of multiple rice accessions across their growing season will tell us which photosynthetic dynamics can maximize productivity and which developmental stage should be targeted in breeding for photosynthesis.
In previous studies, we determined that the indica cultivar Takanari, which has one of the highest grain yields among Japanese rice cultivars, accumulated more biomass than Nipponbare and Koshihikari, standard japonica cultivars40,41. Since then, Takanari has been widely used to analyse the physiological and molecular mechanisms of biomass accumulation42–48 and their effects on grain yield49–52. Although the higher biomass accumulation in Takanari is characterized by a higher net assimilation rate around the full heading stage, which could be partly explained by the higher leaf photosynthetic capacity, only rough analysis of gas exchange during growth has been conducted41. Here, we aimed at collecting the data on temporal changes in canopy photosynthesis of Koshihikari and Takanari over the entire growing season by using the MIC-100 to analyse its association with crop growth rate (CGR) and total biomass accumulation. We assumed that photosynthesis in the uppermost fully expanded leaf is representative of canopy photosynthesis, since it has the highest photosynthetic capacity and receives the strongest radiation in the canopy41,44,53. We also observed ontogenic changes of chlorophyll content (SPAD value) and single leaf area (single LA). To analyse the phenotypic variation caused by introgressions between the cultivars, we used reciprocal sets of chromosome segment substitution lines (reciprocal CSSLs) derived from a Koshihikari/Takanari cross50,54. Each CSSL carries a single genomic segment from the donor cultivar (either Koshihikari or Takanari) in the genetic background of the other cultivar, and the full set of substituted segments covers the entire genome50,55. The variation in flowering date is much smaller in CSSLs than in other populations such as recombinant inbred lines, which is advantageous in examining whether changes in photosynthesis affect biomass accumulation. From this study, we propose that maintaining a high rate of photosynthesis after heading, rather than having a high maximum photosynthetic rate, can increase total biomass accumulation.