Leaf appearance and leaf blade meristematic activity in rice (Oryza sativa L.) with varying growth duration and soil water regime

Leaf appearance as the peacemaker for the emergence of the successive leaves for the formation of the whole shoot system is central to the development events in rice. The reduction in the rate of leaf appearance approximately at the time of emergence of Leaf 12 is affected by the environment, and anatomical and physiological changes in plant. This study determined the association between the rate of leaf appearance and the leaf blade parameters as affected by sowing time, growth duration, development events and water regime. Field experiments were conducted at IRRI [International Rice Research Institute] from January to December 2015 during the dry, wet and late wet cropping seasons, IR72, a medium-duration rice genotype was used throughout the study and compared to a long duration genotype, SAIM 29. The leaf meristem parameters were determined using tissue clearing method for the plant microtechnique preparations. The leaf appearance of IR72 fitted in a trilinear model in all sowing dates and in both flooded and aerobic systems of production. For SIAM 29 with leaf number beyond 20, another breaking point was observed and a quadrilinear model was used in this variety. The kinetics of leaf appearance of both IR72 and SIAM 29 were similar until the appearance of the flag leaf in IR72, while additional break point was added for SIAM 29. Reduction of the leaf appearance rate and PI occurred synchronously or either earlier or after the breaking point in IR72. While in SIAM 29, PI did not yet occur, hence was not associated with the decrease in leaf appearance. The onset of stem elongation shortly proceeded the appearance of the flag leaf and happened after the occurrence of the breaking point, and was neither related to the decline of the leaf appearance rate. The length of the division zone and the total cell number per file increased linearly with leaf rank until flag leaf emergence regardless of the sowing date, crop duration and growing condition. The number of dividing cells increased simultaneously with the total number of cells in the division zone. Hence, the kinetics of leaf appearance and the observed leaf meristem parameters were not correlated. Conversely, regardless of variety, sowing date and cropping system, the length of the leaf blade was consistently correlated with the length of the division zone per leaf rank. The consistent observation from the three experiments of the non-association between the rate of leaf appearance and the profiles of the leaf blade meristem, suggests that the apical meristem is the only determinant of the leaf appearance kinetics. The relationship, between the apical meristem activity and the rate of leaf appearance needs further investigation.