Submitted to: Seed Science Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 5, 2011
Publication Date: January 30, 2012
Citation: Perez, H.E., Hill, L.M., Walters, C.T. 2012. An analysis of embryo development in palm: interactions between dry matter accumulation and water relations in Pritchardia remota (Arecaceae). Seed Science Research. 22:97-111. Interpretive Summary: Palms produce among the largest seeds of all plants. The large seed size suggests that palms are extremely proficient at sequestering food reserves to developing embryos. Dry matter accumulation into cells is the basis for a new mechanical model we developed to explain strategies to protect cells from lethal shrinkage during drying. One might expect from this model, that palm seeds would be able to survive drying. We tested this hypothesis on maturing embryos of Pritchardia remota, an endangered species that is endemic to Hawaii. We demonstrate that P. remota embryos are increasingly able to survive water loss as they accumulate food into their cells; but they never achieve the same tolerance as most seeds because embryos stop growing midway through gestation when food is only deposited in the endosperm. We believe that the new model provides the first quantitative explanation for physiological differences among seeds of diverse species. This understanding is critical to our abilities to preserve genetic resources in seed banks. This work has enabled us to develop appropriate storage protocols to preserve genetic diversity for palms, which are economically and ecologically very important to tropical and subtropical regions.
Technical Abstract: Despite their global importance, the genetic diversity of many palm taxa is threatened. Ex situ conservation of palm germplasm requires survival of seeds in genebanks. Seeds from Arecaceae exhibit a broad range of responses to the low temperature and moisture conditions used by genebanks, and assessments of palm seed storage physiology are often inconclusive. This paper reports on the embryogenic program of the Nihoa fan palm (Lou’lu), Pritchardia remota to address the interrelationships between mechanical strain during low temperature and moisture stress, dry matter reserve accumulation, cell structure and water relations. P. remota seeds developed over a 400 d gestation period, with embryo histo-differentiation and final maturation occurring in the dry season of successive years. Embryos reached maximum dry mass at about 250 d post-anthesis (DPA), but endosperm and fruits continued to accumulate dry matter until about 370 DPA. Food reserves consisting mostly of sucrose, were deposited in the cytoplasm and vacuole, Triacylglycerols were a minor food reserve, and the proportion of saturated fatty acids increased with maturation. Water content and water potential of all tissues decreased as embryos matured, and at shedding (400 DPA), embryos contained about 0•45 g H20 g dry mass-1 (-26 MPa) and organelles showed signs of de-differentiation. Embryos harvested 340 DPA survived desiccation to 0.16 g g-1 (-49 MPa), but further drying was lethal. The substantial, but incomplete, desiccation tolerance is considered in terms of allowable cell volume changes during dehydration and provides a framework to explain variation in critical water contents among developing embryos and seed species as well as a new perspective of the continuum of desiccation tolerance that separates recalcitrant and orthodox physiologies.