The regulation of leaf thickness in rice (Oryza sativa L.)

Abstract The regulation of leaf thickness in rice (Oryza sativa L.) S. Narawatthana One of the most important targets to improve crop yield is leaf photosynthetic capacity. Leaf thickness is one parameter closely associated with photosynthetic function and is strongly influenced by the level of irradiance. Generally, high light grown leaves are thicker, have higher light-saturated rates of photosynthesis, higher amounts of Rubisco and a higher chlorophyll a:b ratio than shade grown leaves. However, the developmental stage at which leaf thickness is set and how it is set are unclear. In this thesis I investigate the outcome on leaf thickness of changing irradiance level at specific points in the development of leaf 5 of rice plants via a series of transfer experiments from high light (HL) to low light (LL) at specific stages of leaf development. The results from these experiments show that the P2- to P4-stage of rice leaf development represents a developmental window during which final thickness can be altered via light regime. Analysis of photosynthetic capacity and gas exchange of the leaves from the transfer experiments indicated some correlation of leaf thickness with biochemical/physiological adaptation to the prevailing irradiance level. Interestingly, whilst HL induced the development of thicker rice leaves with a visibly larger mesophyll cell size, transferral of the leaves to LL conditions at any developmental stage led to a LL-acclimated photosynthetic response. To identify lead genes potentially involved in the growth response of young leaves to the prevailing light environment, I performed a microarray analysis of leaf primordia at P3-stage undergoing a leaf-thickness response to altered irradiance level. A number of lead genes were identified and a selection process based on independent expression analyses was performed to narrow the number of candidates for future functional analysis. An initial analysis of some of these genes is reported.