the subsequent gametophyte growth and fertilization [5, 6]. Fern
spores tolerate high levels of desiccation (as low as 5–10% relative
humidity [RH]) and generally tolerate dry storage at sub-zero
temperatures (optimal storage RH between 10% and 25%) [7–
10]. However, some fern spores appear to be adversely affected
by storage at À20
C (the standard temperature in seed banks),
generally showing a reduced longevity during storage at À20
C
when compared to storage at 4
C or LN storage [9, 11]. In
addition, some fern spores (e.g., chlorophyllous spores) present
very reduced life span, and their longevity can be effectively
extended by storage at LN temperatures [5, 10, 12–14]. Fern
spore storage at cryogenic temperatures has been suggested for
their long-term preservation [5, 11].
Pollen (as sperm in animals) is widely used in breeding programs for horticulture, agriculture, and forestry [15, 16], and its
proper preservation is of high importance for these activities
[17, 18]. In addition, although less common in plant species conservation [19], pollen preservation is known as an important way to
preserve some rare and endangered wild species [20, 21]. Indeed,
cryopreserved pollen has been used to retrieve extinct genotypes of
declining plant species [22] and is suggested as a complementary
strategy for the ex situ conservation of recalcitrant seeded species,
such as Quercus sp. [23]. The application of pollen preservation to
wild plants could also increase the conservation possibilities of
many other species, such as dioecious wild species that present a
decrease of mates and/or flowering asynchrony [24, 25]. Pollen
preservation allows to preserve high pollen quantities in very small
space so breeders can use it in the optimal moment to obtain seeds
from plants that are not able to reproduce by themselves (after
hand-pollination), to overcome unfavorable weather conditions in
breeding programs, or to cross plant individuals which grow in
different locations [21].
Pollen can be classified as desiccation tolerant or sensitive
[26]. Desiccation-tolerant pollen usually has water contents
below 30% when dispersed [27], can be dried to a moisture content
of less than 5% [28], and can survive several days in this dry
environment [29]. The optimal moisture contents for storage are
those reached at RH between 11% and 30% [30, 31]. Due to its
desiccation tolerance and the removal of freezable water during
drying [17], this type of pollen can be cryopreserved easily by
drying and storage at low temperatures. Desiccation-sensitive pollen disperses with water content higher than 30% and die rapidly
when dried [32]. For example, in some species like maize (Zea
mays), wheat (Triticum sp.), or wild rice (Zizania texana), pollen
dies in just few hours after dispersal [33–35]. Desiccation-sensitive
pollen requires controlled drying and cooling and may require
some level of cryoprotection in highly recalcitrant species for successful cryopreservation. When desiccation-sensitive pollen can be
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Anna Nebot et al.
spores tolerate high levels of desiccation (as low as 5–10% relative
humidity [RH]) and generally tolerate dry storage at sub-zero
temperatures (optimal storage RH between 10% and 25%) [7–
10]. However, some fern spores appear to be adversely affected
by storage at À20
C (the standard temperature in seed banks),
generally showing a reduced longevity during storage at À20
C
when compared to storage at 4
C or LN storage [9, 11]. In
addition, some fern spores (e.g., chlorophyllous spores) present
very reduced life span, and their longevity can be effectively
extended by storage at LN temperatures [5, 10, 12–14]. Fern
spore storage at cryogenic temperatures has been suggested for
their long-term preservation [5, 11].
Pollen (as sperm in animals) is widely used in breeding programs for horticulture, agriculture, and forestry [15, 16], and its
proper preservation is of high importance for these activities
[17, 18]. In addition, although less common in plant species conservation [19], pollen preservation is known as an important way to
preserve some rare and endangered wild species [20, 21]. Indeed,
cryopreserved pollen has been used to retrieve extinct genotypes of
declining plant species [22] and is suggested as a complementary
strategy for the ex situ conservation of recalcitrant seeded species,
such as Quercus sp. [23]. The application of pollen preservation to
wild plants could also increase the conservation possibilities of
many other species, such as dioecious wild species that present a
decrease of mates and/or flowering asynchrony [24, 25]. Pollen
preservation allows to preserve high pollen quantities in very small
space so breeders can use it in the optimal moment to obtain seeds
from plants that are not able to reproduce by themselves (after
hand-pollination), to overcome unfavorable weather conditions in
breeding programs, or to cross plant individuals which grow in
different locations [21].
Pollen can be classified as desiccation tolerant or sensitive
[26]. Desiccation-tolerant pollen usually has water contents
below 30% when dispersed [27], can be dried to a moisture content
of less than 5% [28], and can survive several days in this dry
environment [29]. The optimal moisture contents for storage are
those reached at RH between 11% and 30% [30, 31]. Due to its
desiccation tolerance and the removal of freezable water during
drying [17], this type of pollen can be cryopreserved easily by
drying and storage at low temperatures. Desiccation-sensitive pollen disperses with water content higher than 30% and die rapidly
when dried [32]. For example, in some species like maize (Zea
mays), wheat (Triticum sp.), or wild rice (Zizania texana), pollen
dies in just few hours after dispersal [33–35]. Desiccation-sensitive
pollen requires controlled drying and cooling and may require
some level of cryoprotection in highly recalcitrant species for successful cryopreservation. When desiccation-sensitive pollen can be
624
Anna Nebot et al.