Temperature effects on systemic endoreduplication in orchid during floral development

Endoreduplication, the replication of nuclear DNA without subsequent mitosis and cell division is an important component of organ development and is often related to plant cell size. Little is known, however, about the effect of sub-optimal temperatures on the endoreduplication and floral development. We analyzed ploidy levels, cell weight and cell number during flower development in Phalaenopsis aphrodite and Oncidium varicosum grown at 25 and 15 °C. Low temperature decreased the growth rate and the endoreduplication transition rate. A longer growth period compensated for the decreased growth rate so that the distribution of ploidy levels similar at both growth temperatures. Low temperature decreased growth rates and endoreduplication transition rates for both orchid species. These results are consistent with previous studies that endoreduplication cycle may use the same molecular machinery for their DNA replication as that used during the S-phase of the mitotic cell cycle. In both species, the average C value is positively correlated with the average cell fresh weight. However, 15 °C increased the ratio of average C value to average cell fresh weight in P. aphrodite but decreased the ratio in O. varicosum. Although ploidy level plays a role in final cell size, growth temperature modulates the effect ploidy level on final cell weight.