Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2011
Publication Date: N/A
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: Orthodox and recalcitrant seeds are distinguished by the ability of embryos to survive desiccation. Seeds of many palm species do not conform to the dichotomous classification and storage physiology is considered intermediate or ambiguous. We studied the acquisition of desiccation tolerance in embryos of Pritchardia remota, an endangered fan palm endemic to Hawaii that serves as an exemplar of embryogenesis in Arecaceae. Embryo resistance to the two main strains of desiccation stress, namely deformation and oxidation of cellular constituents, was compared with embryo survival to desiccation challenges. Early development of P. remota embryos was typical of other angiosperms: a metabolically-active embryo experienced massive cell shrinkage and succumbed at relatively high water potentials. About two-thirds of the way through a 400 day gestation period, dry matter accumulation into the embryo ceased and tolerance to desiccation plateaued near -45 MPa (corresponding to water contents of 0.18 g H2O g-1 dry mass). Oxidative activity in mature, dormant-appearing embryos was negligible and antioxidant capacity was comparable to seeds of other species, suggesting that failure to protect against oxygen stress is not an explanatory factor for partial desiccation tolerance in this species. The limits of desiccation tolerance in developing and mature embryos corresponded to cell volume losses of 64 + 4%, a benchmark for lethal compression. Despite the large size of palm seeds, accumulation of food reserves into the embryo is limited, and this may a factor explaining desiccation tolerance that is intermediate between orthodox and recalcitrant seed types.