Low Cell‐Wall Extensibility Can Limit Maximum Leaf Growth Rates in Rice

The aim of this research was to clarify physiological mechanisms underlying slower leaf growth rates in rice( Oryza sativa L.) as compared with maize ( Zea mays L.) and barley ( Hordeum vulgare L). Seedlings were grown under controlled‐environment conditions with adequate supplies of water and mineral nutrients. The growth rates of emerging first leaves in both paddy and upland rice were low by comparison with maize and barley genotypes. The possibility that differences in mature cell size, comparative cell production rates, osmotic potentials, or extensibility characteristics of growing cell walls were involved in regulating these species differences were investigated. The lower growth rate of the rice leaves was not associated with comparatively lower rates of epidermal cell production but was associated with the production of smaller mature cells. The smaller size of the rice leaf cells was not associated with less negative osmotic potentials in the cell‐expansion zone. However, the comparative extensibilities of the growing leaf tissues and cell walls decreased in the order barley > maize > rice and gave good linear correlations with leaf relative‐growth rates ( r = 0.87** and 0.97**, respectively). The data suggest that exceptionally low cell‐wall extensibility is involved in limiting maximum leaf growth rates in rice. Future introduction of increased wall extensibility in paddy rice might lead to increases in leaf growth potential and conceivably to improved yields.