Challenges for Ex Situ Conservation of Wild Bananas: Seeds Collected in Papua New Guinea Have Variable Levels of Desiccation Tolerance

Abstract

Ex situ seed conservation of banana crop wild relatives (Musa spp. L.), is constrained by critical knowledge gaps in their storage and germination behaviour. Additionally, challenges in collecting seeds from wild populations impact the quality of seed collections. It is, therefore, crucial to evaluate the viability of seeds from such collecting missions in order to improve the value of future seed collections. We evaluate the seed viability of 37 accessions of seven Musa species, collected from wild populations in Papua New Guinea, during two collecting missions. Seeds from one mission had already been stored in conventional storage (dried for four months at 15% relative humidity, 20 °C and stored for two months at 15% relative humdity, -20 °C), so a post-storage test was carried out. Seeds from the second mission were assessed freshly extracted and following desiccation. We used embryo rescue techniques to overcome the barrier of germinating in vivo Musa seeds. Seeds from the first mission had low viability (19 ± 27% mean and standard deviation) after storage for two months at 15% relative humidity and -20 °C. Musa balbisiana Colla seeds had significantly higher post-storage germination than other species (p < 0.01). Desiccation reduced germination of the seeds from the second collecting mission, from 84 ± 22% (at 16.7 ± 2.4% moisture content) to 36 ± 30% (at 2.4 ± 0.8% moisture content). There was considerable variation between and (to a lesser extent) within accessions, a proportion of individual seeds of all but one species (Musa ingens N.W.Simmonds) survived desiccation and sub-zero temperature storage. We identified that seeds from the basal end of the infructescence were less likely to be viable after storage (p < 0.001); and made morphological observations that identify seeds and infructescences with higher viability in relation to their developmental maturity. We highlight the need for research into seed eco-physiology of crop wild relatives in order to improve future collecting missions.

Keywords: Musa; Papua New Guinea; banana; crop wild relatives; desiccation tolerance; ex situ conservation; plant genetic resources; seed conservation; seed storage behaviour.

Figure 1

(A) Germination responses of embryos rescued from 29 accessions of Musa species following drying for 4 months at 15% relative humidity 20 °C and storage for 2 months at 15% relative humidity −20 °C. ‘Hand position’ refers to the position in the infructescence of the hand from which seeds were collected, with ‘1’ being closest to the basal end of the bunch (n = 23 ± 10 seeds). (B) Predicted probability of five embryo rescue outcomes of Musa acuminata subsp. banksii seeds extracted from different hand positions in the infructescence. Probabilities based on the multinomial logistic regression model of the response of seeds from 50 hands (representing 13 accessions; n = 30 seeds for each hand). Shaded areas are 95% standard errors of the estimated regression coefficients.

Figure 2

(A) Embryo rescue outcomes of Musa seeds (batch 2) before desiccation at 16.7 ± 2.4% moisture content (‘Wet’), and after desiccation for seven days in a desiccator to 2.4 ± 0.8% moisture content (‘Dry’). Accession and hand numbers are included above each chart. Seeds were germinated using embryo rescue and results recorded 28 days after transfer to the growth medium (n = 10). (B) Predicted probability of embryo rescue results according to the moisture status of seeds. Plot is on predicted values of the multinomial logistic regression model coefficients in Table S1C, data in Figure 2A. 95% standard errors shown.

Figure 3

Predicted storage of behaviour of Musa accessions (batch 2) using the diagnostic key of Hong and Ellis [65] and Ellis et al. [66]. Area A includes accessions predicted to have intermediate storage behaviour, accessions in area B are predicted to have orthodox storage behaviour. Accessions are coloured according to the germination percentage of seeds after seven days desiccation to 2.4 ± 0.8% moisture content. Moisture content is calculated on the fresh weight basis (‘fwb’). Seeds were germinated using embryo rescues and assessed 28 days after transfer to growth medium.

Figure 4

(A) Moisture content during dry storage, calculated on fresh weight basis (‘fwb’). Seeds from a mature and an immature accession of Musa acuminata subsp. banksii were used. Seeds were dry stored at 15%RH, 20 °C for up to 168 days. (B) The effect of dry storage on embryo rescue outcomes. Outcomes recorded 28 days after the transfer of each embryo to growth medium (n = 48).

Figure 5

Photographs of immature and mature bunches and their seeds of two Musa acuminata subsp. banksii accessions. Seed images taken before and after 1 month drying in a dry room (15% relative humidity, 20 °C). Seed images taken on a Keyence VHX5000 at 150 x magnification.

Figure 6

Collecting locations of seeds used in this study (red circles) and relevant province names. Map is shaded according to elevation (meters above sea level, data from: http://srtm.csi.cgiar.org).