Allometric relationship between the size and number of shoots as a determinant of adaptations in rice to water-saving aerobic culture

For efficient canopy expansion, field crops must choose between producing more organs or building larger organs. The objective of this study was to characterize the effects of soil water condition (flooded vs. aerobic) on the allometric relationship between leaf area on the main stem and tiller number (i.e., shoot size vs. shoot number) in nine rice cultivars representing contrasting plant types (indica, temperate japonica, and tropical japonica). We also evaluated individual leaf area and plant leaf area in the middle of the vegetative stage for 91 cultivars. In aerobic fields, soil water potential at a 20-cm depth averaged between −15 and −30 kPa, but frequently reached −60 kPa. The allometric slope (i.e., the ratio of shoot size to shoot number) was lower in aerobic than in flooded culture, and was further reduced by increased soil water deficit. Among the nine cultivars, a slight change in the allometric slope was associated with larger variation in leaf area in aerobic culture than in flooded culture. Consequently, cultivars with larger leaves (shoots), mediated by a higher allometric slope, were advantaged compared to those with smaller leaves with respect to early vigor under limited soil moisture. A significant relationship between leaf size and early vigor was also observed for the 91 cultivars grown in aerobic culture. The study confirms the existing perception that tropical japonica cultivars are useful genetic sources for yield stability because of their ability to maintain large shoot size despite fluctuations in hydrological conditions.

► The shoot size:shoot number ratio was related to early plant vigor in rice.
► It was lower in water-saving aerobic culture than conventional flooded culture.
► High shoot size:shoot number ratio appears an important trait for aerobic adaptation.
► Tropical japonica cultivars were suggested as valuable donors in aerobic rice improvement.