The Photosynthesis Responses To Water Stress And Light-flecks In Tropical Epiphytic And Terrestrial Ferns

Epiphytic ferns grow above the ground surface, using other plants or objects such as rocks for support and do not obtain nutrients from their hosts. They are an important component of tropical rainforest ecosystems. Epiphytic ferns do not directly absorb water from the soil in the ground and frequently experience drought stress. Therefore, the ability of maintaining necessary water content is a key factor for the survival and growth of epiphytic ferns. The habitats of epiphytic ferns are associated with the dim light environment under the forest canopy or in the forest understorey; therefore, the ability of light-fleck utilization affects their growth and development. In the present study, the differences in adaptation to water stress were assessed by comparing the responses of epiphytic and terrestrial fern species to drought stress. The strategies of light fleck utilization of these two life-form ferns were also studied, with a comparison to those in hemiepiphytic and terrestrial figs. Moreover, the diversity of ferns in tropical montane rain forests in Xishuangbanna of Southwest China was also reported in the study.Using potted plants of two epiphytic fern species (Neottopteris nidus (L.) J. Sm and Microsorium punctatum (L.) Cop.) and two terrestrial fern species (Asplenium finlaysonianum Wall. ex Hook. and Paraleptochilus decurrens (Blume) Copel.) grown under 4% daylight, an experiment of simulated drought stress and subsequent rehydration was done. Leaf relative water content (RWC) in two terrestrial fern species declined more rapidly and to greater extent than that in two epiphytic fern species in the simulated-drought experiment and two epiphytic fern species closed their stomata at relatively high RWC level. Net photosynthetic rate (An), stomatal conductance (gs) and transpiration rate (Tr) decreased with decreasing leaf RWC. The An, gs and Tr of the two terrestrial and two epiphytic fern species studied declined to the much lower values (35 days and 49 days, respectively) and returned to the values of the control quickly after several days rewatering (3 days and 9 days or more, respectively). The values of Fv/Fm at predawn and midday in the two epiphytic fern species did not decline significantly during the whole period of the water-withholding experiment; moreover, the values of Chl NDI values, a spectral index of chlorophyll content, did not significantly decline during the water-withholding experiment in N. nidus. The Chl NDI values declined significantly during the water-withholding experiment in M. punctatum, however, it could return to the level of the control quickly after re-watering. The values of Fv/Fm at predawn and midday began to decline significantly from 28th day in the terrestrial fern species, while the values were still higher than 0.75, which indicated that slight photoinhibition occurred. The slight photoinhibition was relieved after re-watering for 9 d. The Chl NDI values began to decline significantly in A. finlaysonianum and P. decurrens from 14th and 21th day, respectively, and returned to the level of the control after re-watering for 9 d. These results indicated that no irreversible damage occurred in reaction center of photosystem II of all the four fern species studied and the decline of An may be due to stomata closure. The quick recovery in CO2 assimilation rates after rehydration also supported this. In their respective native habitats of the 4 fern species in a rainforest, no significant differences in leaf RWC and chlorophyll content were found between the rainy and dry seasons for all of the four fern species. However, their maximum net photosynthetic rates and maximum stomatal conductance in the rainy season were significantly higher than that in the dry season. The ability of maintaining integrity of photosystems and stable chlorophyll content during the water-stressed periods and rapid recovery of photosynthetic capacities after rewatering in water-stressed epiphytic ferns were probably the reasons for the survival of epiphytic ferns in the drought-prone epiphytic habitat.The photosynthetic induction and induction maintenance were examined on the potted plants of six fern species (three epiphytic and three terrestrial species) and six fig species (three hemi-epiphytic and three terrestrial species) grown under 4% daylight. The time required to reach 90% full induction state (T90) in six terrestrial species except for Pseudodrynaria coronans and Tectaria subtriphylla (15.92 and 16.25 min, respectively) was 5.24 ~ 7.79 min. T90 of three terrestrial fig species was significantly shorter than that in three terrestrial fig species, which was beneficial for light-fleck utilization of the terrestrial figs. The induction time required 50% and 90% maximum photosynthetic rate was nonlinearly and negatively correlated with initial stomatal conductance (gs-initial). Species with higher gs-initial tended to had shorter time required to reach 50% and 90% full induction state, with a threshold value of gs-initial at 50 mmol H2O m-2 s-1.According to the data from 4 sampling plots of 50 x 50 m (divided into 400 small plots of 5 x 5m), floristic composition and ecological characteristics of fern communities in a tropical montane rainforest in Xishuangbanna were studied. Totally, 64 fern species were recorded from the forest. These ferns were dominated by chamaephytes and geophytes with leathery and papery leaves. These ferns showed obvious seasonal change in physiognomy. Six fern species, including Woodwardia japonica, Athyrium dissifolium, Brainea insignis, Microlepia calvescens, Arachniodes spectabilis and Dryopteris pseudosparsa, had 84% of the important value (IV) summed from the total fern species. The IV of Woodwardia japonica was above 1/3 of total. This indicates these six fern species dominate the montane rainforests and are preferential species. The fern species Pteris menglaensis is the exclusive species of the montane rainforests.? ?...