Monthly variability in the photosynthetic capacities, leaf mass per area and leaf nitrogen contents of rice (Oryza sativa L.) plants and their correlations

The monthly variability in the photosynthetic capacities, leaf mass per area (LMA) and leaf nitrogen (N) contents of rice (Oryza sativa L.) plants is important for determining the growth conditions, estimating the biomass and yield, and developing a rice growth model. Moreover, the correlations among LMA, leaf N content, and photosynthetic capacity parameters, such as the maximum rate of carboxylation (Vsub(cmax)) and the maximum rate of electron transport (Jsub(max)), have not been fully investigated. The objectives of this study were (1) to analyze the monthly variability in Vsub(cmax), Jsub(max), LMA, and leaf N content; and (2) to investigate the correlations among Vsub(cmax), Jsub(max), LMA, leaf N content, and available N content in the soil. We measured the monthly changes in Vsub(cmax) and Jsub(max) in 120 leaves of two rice cultivars, Koshihikari and Akitakomachi, subjected to standard-N and non-N treatments using gas-exchange measurements under open-air field conditions. Vsub(cmax) and Jsub(max) values adjusted to a 25℃ leaf temperature were significantly greater after the standard-N treatment compared with the non-N treatment in both cultivars, and the values of the photosynthetic capacities tended to decrease over time. In particular, these values significantly decreased at the ripening stage compared with the previous growth stages. The correlations among Vsub(cmax) and Jsub(max) at a 25℃ leaf temperature, leaf N content based on mass and area, and SPAD readings were significant for both cultivars but were not correlated with LMA. As indicators of photosynthetic performance, the monthly variability in Vsub(cmax) and Jsub(max) for the fully expanded top or flag leaves were expressed easily as functions of the SPAD readings, rather than the leaf N contents based on mass and area, which are relatively expensive and labor intensive to measure. This research provides useful information for developing a universal rice growth model.