Temperature 841 Physiological temperature optimum and tolerance

8.4.1.1 Temperature optimum Growth experiments on selected bryophytes of temperate and boreal ecosystems suggest that, although temperature optimum varies among species, from low (12-13°C) in mosses such as Dichodontium palustre and Racomitrium lanuginosum to relatively high (23-26° C) in, for example, Bryum capillare and Funaria hygrometrica, the optimum for the majority of species ranges between 15 and 25°C (Furness & Grime 1982b) (Fig. 8.16). Most species exhibit a broad plateau of responses to temperature over the range of

Fig. 8.16. Frequency distribution of the relative growth rate temperature optimum classes in 40 mid-western European bryophyte species (reproduced from Furness & Grime 1982b with permission of Blackwell).

10-25°C. Aquatic bryophytes that develop entirely below the water surface are, by contrast, generally subject to very minor temperature fluctuations and, hence, display a much narrower optimum range than that of terrestrial mosses (Glime & Vitt 1984). In the aquatic moss Fontinalis antipyretica for example, net photosynthesis is optimal between 10 and 15° C, beyond which respiration rapidly exceeds photosynthesis (Glime & Vitt 1984).

These temperature optima are lower than those described for temperate vascular plant species. This may reflect the tendency for bryophytes of temperate regions to grow during seasons (spring and autumn) when moisture is most readily available and when, coincidentally, temperature is relatively low. The moss Brachythecium rutabulum displays, for example, a temperature optimum for relative growth rate of 18°C that is substantially lower than that of the tall herb Urtica dioica (c. 27°C), with which it is commonly associated (Fig. 8.17) (Furness & Grime 1982a). In contrast with the vascular species, B. rutabulum still exhibits considerable growth at 5°C. These differences in temperature optimum may account for the co-existence of B. rutabulum and U. dioica, which exploit different seasonal niches. The

Fig. 8.17. Comparison of the response of mean relative growth rate R to temperature in the tall herb Urtica dioica and the moss Brachythecium rutabulum grown at 25 Wm~2 (reproduced from Furness & Grime 1982a with permission of Blackwell).

productivity of U. dioica peaks in mid-summer, corresponding to maxima in day length, irradiance and temperature, followed by a rapid decline accelerated by autumn frosts. In contrast, B. rutabulum exhibits a complementary phenology, with spring and autumn peaks of productivity. These coincide with the cool, moist conditions at that time of year, when maximum light reaches the moss layer due to the absence of a herbaceous canopy. The comparatively lower temperature optima of bryophytes in comparison with angiosperms also explains certain differences in distribution between the two groups. Along an elevation gradient for example, the maximum of species diversity occurs at a higher altitude in bryophytes than in angiosperms (Section 3.3).

8.4.1.2 Tolerance to extreme temperatures and relation to water status Cold tolerance A feature common among most bryophytes is their ability to grow at low temperatures. More than half of the 40 species investigated by Furness and Grime (1982b) showed a growth reduction of less than 50% at 5°C compared to growth at their optimal temperature. Most species, including tropical ones, seem to be pre-adapted to cold and survive temperatures ranging from -10 to -27°C (Glime 2007a).

As for angiosperms, the tolerance of bryophytes varies seasonally (Rutten & Santarius 1992, 1993). This suggests that mosses develop tolerance in response to changes in environmental conditions. It was experimentally shown that incubation at low, but above freezing temperatures, significantly increases survival rates upon subsequent exposure to negative temperatures. While only 16% of the protonema cells grown at room temperature survived after freezing at — 3°C, a cold treatment at 0°C increased the survival rate to 57% after seven days and over 80% after ten days (Fig. 8.18) (Minami et al. 2006). The mechanism of freezing tolerance is controlled by ABA (Fig. 8.19). ABA causes an increase in soluble sugars, which play a role in protecting cellular membranes and proteins from freezing injury in much the same way as they do for desiccation tolerance; notably, through the process of vitrification (see Section 8.1.3). ABA also seems to induce the synthesis of proteins for freezing tolerance. Indeed, when the ABA treatment is carried out in the presence of nuclear protein-synthesis inhibitors, freezing tolerance dramatically decreases (Fig. 8.20). When cytoplasmic protein-synthesis inhibitors are used instead, freezing tolerance is not affected, suggesting that nuclear-encoded genes are likely to play a critical role in the development of freezing tolerance. These genes encode for the same LEA proteins that are involved in desiccation tolerance, reinforcing the parallel between the mechanisms involved in desiccation and cold tolerance.

Fig. 8.18. Survival rate (%) of protonemal cells of the moss Physcomitrella patens exposed to —2 to —■4°C depending on the number of days of incubation at 0°C (reproduced from Minami et al. 2006 with permission of CAB).
Fig. 8.19. Changes in freezing tolerance of protonemal cells in the moss Physcomitrella patens after different periods of incubation on a medium containing 1 m mol ABA (reproduced from Minami et al. 2006 with permission of CAB).

Although bryophytes share with other cryptogams, including ferns, fungi and algae, but not with angiosperms, other ABA-induced genes possibly involved in freezing tolerance, bryophytes and higher plants thus display similarities in their freezing behaviour as well as in the genes required for the development of freezing tolerance. Such evolutionary conservatism with respect to freezing tolerance suggests that these traits might have been necessary for their common ancestor to adapt successfully to non-aquatic

Fig. 8.20. Survival rate of protonemal cells in the moss Physcomitrellapatens (control), when ABA is added to the growth medium, and when nuclear protein-synthesis inhibitors (CHX) and cytoplasmic protein-synthesis inhibitors (CAP) are added to the growth medium, respectively (reproduced from Minami et al. 2006 with permission of CAB).

Fig. 8.20. Survival rate of protonemal cells in the moss Physcomitrellapatens (control), when ABA is added to the growth medium, and when nuclear protein-synthesis inhibitors (CHX) and cytoplasmic protein-synthesis inhibitors (CAP) are added to the growth medium, respectively (reproduced from Minami et al. 2006 with permission of CAB).

conditions, and hence have been crucial in the evolution of early land plants (Section 1.4.1).

Heat tolerance At the other extreme of the temperature response curve, all of the temperate and boreal species investigated by Furness and Grime (1982b) died at 35°C and most shoots died eventually at > 30°C. Particularly sensitive species were Thuidium tamariscinum, which died after about 20 days at 29° C, and Chiloscyphus polyanthos, which died after 5 days at 27° C. Lethality is attributed to damage to membrane systems, including the photo-synthetic pigment apparatus. Many bryophytes, though, occur in areas wherein they experience much higher temperatures and must hence have evolved heat tolerance.

In bryophytes, acclimatization of water-saturated shoots by short exposure temperatures above 30°C for a few hours results in a small, albeit significant increase in the thermal stability of the photosynthetic apparatus (Meyer & Santarius 1998) (Fig. 8.21). This suggests that short-term thermal hardening of hydrated tissues may take place in a similar fashion in bryophytes and flowering plants through the production of heat-shock proteins. However, the short-term heat hardening capacity of turgid bryophytes is extremely low and, as opposed to that of angiosperms, may not be ecologically relevant (Meyer & Santarius 1998).

Temperature (°C)

Temperature (°C)

Fig. 8.21. Physiological response, measured as intensity of damage to the photosynthetic apparatus (as assessed by the Fv/Fm ratio), to temperatures above 40°C for 10min as a function of the length of the acclimation period in the moss Polytrichastrum formosum. Shoots were acclimated to 38°C for 2h (■) and 4h (□), respectively. Controls (o) were stored at room temperature (reproduced from Meyer & Santarius 1998 with permission of SpringerVerlag).

In bryophytes, heat tolerance strongly increases in the dry state. Under natural conditions, in fact, mosses lose water rapidly when temperature increases, in parallel with a drop in relative humidity, which is accompanied by a prompt rise in thermotolerance. Whereas the lethal thermal limit for metabolically active (i.e. hydrated) gametophytes is 51°C, this limit increases to 110°C when gametophytes are desiccated (Meyer & Santarius 1998).

Differences in response to heat exposure also vary between gametophytes and sporophytes. While all maternal gametophytes of the moss Microbryum starckeanum survive exposures up to 75°C for 1-3 h, no embryonic sporo-phyte remains viable at that temperature (McLetchie & Stark 2006). This suggests that either the inherent thermotolerance is truly lower in the sporo-phyte than in the gametophyte, which is consistent with the observation that sporophyte production is restricted to the coolest, wettest months, or that gametophytic thermal stress response controls sporophyte viability.

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