Gametophyte carbon and light stress relations

Fern gametophytes have intrinsic limitations for taking up and storing water, therefore, water relations may be an important area of gametophyte physiological ecology. As discussed above, and as will probably emerge from future studies, fern gametophytes are true poikilohydric plants. Poikilohydric taxa have evolved complicated morphological, biochemical, and anatomical mechanisms to cope with limited water and are robust in tolerating periods of drought. These plants face the additional complication of stress derived from exposure to excess light when their vegetative tissues experience reduced water contents. Light in excess of that which the photosynthetic machinery can transfer can result in photoinhibition (the light mediated depression of photosynthesis) and eventually significant photodamage (wholesale destruction of photosystems). Such depression can severely limit plant fitness. Low water content combined with excess light can be a deadly mix. Thus, even the most advanced poikilohydric species is faced with special challenges when presented with such combinations.

thba thni bls1 bls2 CABR MIRE PES1 pops Species

Figure 9.5 Maximum photosynthetic rates of terrestrial and epiphytic fern gametophytes from laboratory cultures in high (500 |imol m-2 s-1) and low (100 |imol m-2 s-1) light conditions. Maximum photosynthetic rates were measured using a LICOR 6400 infrared gas analyzer on three groups of 10-20 gametophytes. Error bars represent standard errors; differences among species at the same light levels were calculated by post hoc Tukey tests (p < 0.000 for high light; p = 0.0015 for low light); species sharing the same letter within epiphytes or terrestrial species at the same light level are not significantly different. THBA, Thelypteris balbisii (Spreng.) Ching; THNI, Thelypteris nicaraguensis (E. Fourn.); BLS1, Blechnum species 1; BLS2, Blechnum species 2; CABR, Campyloneurum brevifolium (Lodd. ex Link) Link; MIRE, Microgramma reptans (Cav.) A. R. Sm.; PES1, Pecluma species 1, POPS, Polypodium pseudoaureum Cav. L.

The fern sporophyte contains numerous morphological (Watkins et al., 2006b) and biochemical mechanisms (Tausz et al., 2001) to meet the challenge of excess light. Watkins et al. (2006b) showed that epiphytic species exhibited less photoinhibition at supersaturating light intensities and higher photosynthesis rates at subsaturating light intensities than the terrestrial species (Figure 9.5). These results indicate that there are distinct physiological/biochemical differences between the two groups and provide evidence for gametophytic adaptation to high light.

Because of limited research on gametophyte physiological ecology, discussions here have focused on comparisons of epiphytic and terrestrial taxa. However, the differences demonstrated between these two functional groups substantiate the need to consider gametophyte ecology on a species and habitat basis in order to understand the ecology of individual species. These studies also yield the promise of deeper insight into species ecology when conducted on finer taxon and ecological scales.

Ferns are assumed to have radiated from a terrestrial understory ancestor. As ferns diversified beyond this environment they would have needed several innovations to cope with life in more drought prone and higher light habitats. These innovations would have involved drought and desiccation and light stress tolerance and would have also been necessary in the gametophyte generation. The discussion above demonstrates that variation within such characters exists in modern species and likely plays a significant role in species evolution.

Are there advantages to maintaining an independent gametophyte? Fern gametophytes have historically been viewed as short-lived mesophytes that are limited to wet environments. Whereas this is the case for many species, there is a significant number of species that deviate from such characteristics. Tremendous variation exists in sporophyte ecology. Should we expect less in the game-tophyte stage? Gametophytes can be tough, long-lived individuals (and clones) that produce multiple sporophytes over space and time, grow in areas that are uninhabitable to sporophytes (Farrar, 1967; Peck, 1980; Peck et al., 1990), and exhibit a greater degree of stress tolerance than sporophytes (Sakai, 1980; Sato and Sakai, 1981; Watkins, 2006). An underappreciated role of the gametophyte generation may be its exploratory role in continually testing habitat suitability for sporophytes. Another critical aspect may be the gametophyte's ability to survive harsh times when sporophytes, and thus the species, might otherwise perish from a given area (Farrar, 1985, 1998).

9.4 Ecovalidation

Most of what we know about fern gametophyte development, morphology, physiology, and sexuality is based primarily on laboratory grown populations. Few studies, however, have tested the assumption that laboratory populations mirror natural conditions. Consequently the possibility that laboratory-derived data imperfectly reflect natural systems persists (e.g., Ranker and Houston, 2002). This section reviews relevant literature, then presents new data to address this critical question and to discuss whether differences between laboratory and field systems can be explained. We also explore whether laboratory conditions can be modified to reflect the conditions in nature more accurately.

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