Combined NMR and MRI Assessment of Water Status and Migration in Quercus texana Seeds During Dehydration

TL;DR

This study uses MRI and NMR to track water loss in Quercus texana seeds during dehydration, identifying critical water levels and regions for maintaining germination.

Contribution

The study combines MRI and NMR to reveal water distribution and migration patterns in recalcitrant Quercus texana seeds during dehydration.

Findings

Seed germination drops to 0% at 13.0% water content, with 71.7% germination at 20.0%.

Water is lost first from the embryo axis and cotyledon center, with the radicle region persisting until 15.0% water content.

Bound water correlates positively with germination, and protecting the radicle and cupule scar can slow water loss.

Abstract

Quercus texana seeds are recalcitrant and thus highly sensitive to desiccation, which makes storage difficult. For practical seed handling, it is important to define their safe water content and to understand how water is distributed during dehydration. The present study utilized magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) technologies to investigate the migration and phases of water, respectively, revealing the underlying reasons for the recalcitrance of Q. texana seeds. The water content of fresh Q. texana seeds was found to be 39.6% and the germination percentage was 93.3%. As the water content decreased, the germination percentage decreased continuously, reaching 0% at a water content of 13.0%. At 20.0% water content, the germination percentage was 71.7%. MRI showed that water was primarily stored in the embryo axis and cotyledon center in fresh Q. texana seeds. Water loss occurs in the following order during seed dehydration: embryo axis, cotyledon center, cotyledon periphery, and cotyledon end. However, water in the radicle region persisted until seed water content decreased to 15.0%, at which point no signal was detected. The NMR T2 relaxation spectrum indicated the presence of bound water (T21 = 0.01–5.44 ms) and free water (T22 = 7.19–1401.93 ms) in the seeds. During the dehydration process, most of the water was lost as free water, and the T22 shifted to longer times. Concurrently, the bound water shifted to shorter T21 times. Overall, for practical purposes, seed water should be maintained at or above 20.0%. MRI further showed that water loss from the radicle plays a decisive role in the decline of seed germination, and that protecting the region of radicle and the cupule scar can effectively retard water loss. Furthermore, the bound-water content is positively correlated with seed germination.

Figures