Study On Ecophysiological Characteristics Of Reaumuria Soongorica And Caragana Korshinskii During Dehydration And Rehydration

Reaumuria soongorica and Caragana korshinskii are perennial semi-shrubs. We found that under medium drought stress (when soil water content in the upper layers reached 22.6% and 21.1% respectively), a majority of leaves of those two species turned scorch. Under severe drought stress (when soil water content reached 2.54% and 2.9%), all leaves shedd and the plants have died away. After re-hydration, they can stimulate new leaves and regrowth. Those plants are resurrection plants. This paper presented the studies on the changes of photosynthetic activities, metabolic products enzyme activities, anatomy and chloroplast ultrastructure, in those two species during the period of progressive drought stress and the mergence of new leaves and regrowth after re-hydration. The studies were analyzed and discussed to elucidate further the mechanisms of adaptation to desiccation tolerance during water stress. The main results were summarized as follows:1. Pn declined during drought stress in R. soongorica and C. korshinskii, but intrinsic water use efficiency (WUE) increased remarkably under medium drought stress. The maximal photochemical efficiency of PSII (Fv/Fm) and the quantum efficiency of noncyclic electric transport of PS II (Φ_PSII) decreased significantly under drought stress and exhibited an obvious phenomenon of photoinhibition at noon. The chlorophyll content in R. soongorica had increased under medium drought stress and dropped only under severe drought stress, but had progressive dropped during the drought stress period in C. korshinskii. When soil water content in the layers 0-10cm and 10-25cm reached 2.54%, 3.47% and 2.9%, 12.89% respectively and stem relative water content reached 4.66% and 10.15% in R. soongorica and C. korshinskii after 53 days of drought stress, the leaves died and abscised. But the stems photosynthesis remained and, afterward plants entered dormant state. Upon rewatering, the stems photosynthesis were very feeble and the maximal photochemical efficiency of PSII and the quantum efficiency of noncyclic electric transport of PS II were very low. After 4 to 6 days, the shoots reactivated and the plants developed new leaves, photosynthesis, Fv/Fm andΦ_{PS II} reached normal level. Therefore, R. soongorica and C. korshinskii have the ability to reduce water loss through leaf abscission and maintain the vigor of the stem cells to survive desiccation. 2. PRO and MDA increased in leaves of 2-year-old R. soogorica, in leaves and stems of more than 8-year-old R. soogorica and C. korshinskii during progressive drought stress and decreased in re-hydration period. During progressive drought stress, protein content declined in leaves of 2-year-old, more than 8-year-old R. soogorica, in leaves and stems of C. korshinskii, but increased in stems of more than 8-year-old R. soogorica. During re-hydration period, protein content increased in leaves and stems of more than 8-year-old R. soogorica and C. korshinskii. The progressive drought stress stimulated starch, solube carbohydrate, total nonstructural carbohydrate, sucrose and fructose accumulation in leaves of 2-year-old R. soogorica, in leaves and stems of more than 8-year-old R. soogorica and C. korshinskii, which had a higher accumulation in the stems. After re-hydration, solube carbohydrate, sucrose and fructose content declined in stem of more than 8-year-old R. soogorica, solube carbohydrate and sucrose content declined in stem of C. korshinskii. The content of starch and total nonstructural carbohydrate in stem of more than 8-year-old R. soogorica, the content of fructose, starch and total nonstructural carbohydrate in stem of C. korshinskii increased before the emergence of new leaf, after that, which were declined. The carbohydrate content in leaf of more than 8-year-old R. soogorica and C. korshinskii has high fluctuation in 40 days after re-hydrtion. Changes of carbohydrate content during water stress and re-hydrtion period indicated that R. soogorica and C. korshinskii accumulate high carbohydrate content in stems under extremely drought stress which can stimulate emergence of new leaves and regrowth if the soil moisture reaches a new optimum level.3. During progessive drought stress, activities of catalase increased at first and then declined in leaves of 2-year-old and in stems of more than 8-year-old R. soogorica, but increased in stems of C. korshinskii and kept constant in leaves of more than 8-year-old R. soogorica and C. korshinskii. After re-hydration, activities of catalase declined at first and then kept at constant level in stems and increased stablely in leaves in two species. Activities ofα-amylase andβ-amylase increased in leaves of 2-year-old R. soogorica, in leaves and stems of more than 8-year-old R. soogorica and C. korshinskii during progessive drought stress. After re-hydration, activities ofβ-amylase declined at first and then kept at constant level in stems of more than 8-year-old R. soogorica. Activities ofα-amylase in stems of more than 8-year-old R. soogorica, and activities ofα-amylase andβ-amylase in stems of C. korshinskii increased before the emergence of new leaf, after that, which were declined. Activities ofα-amylase increased at first and then declined in leaves of two species and the activities ofβ-amylase increased stablely. During the energence of new leaf, the disappearance of starch in the stems of R. soogorica and C. korshinskii (starch at 57, 63, 69, 79 and 84 DAT minus starch at 63, 69, 79, 84 and 94 DAT, respectively) was calculated as mobilization parameters. The correlations of starch hydrolytic enzyme activies during a rapid period of starch mobilization in the stems with parameters were determined,α-amylase activity was significantly and positively correlated with the mobilization of starch (r²=0.856, P=0.005 of R. soogorica and r²=0.873, P=0.005 of C. korshinskii, respectively). Slight correlation ofβ-amylase activity was abserved with mobilization parameters (r²=0.449, P=0.117 of R. soogorica and r²=0.3426, P=0.124 of C. korshinskii, respectively).4. The results of anatomical studies in R. soongorica and C. korshinskii indicated that in most case, choroplasts appeared in subcellular organization and choroplasts cling the cell wall. Chloroplasts surrounded by a persistent envelope and with abundant and ordered thylakoid system. With the ongoing of drought stress, Chloroplasts broke away from the cell wall and appeared in the center of cell. The mesophyll ultrastructure and Chloroplasts configuration in leaves were disturbed beyond retrieve in the leaves under severe drought, including the inner and outer membranes destroyed, hylakoid disintergrated, starch grain disappeared and parts of cell tissue dismantled into debris. But in stems, the mesophyll ultrastructure and Chloroplasts configuration remained completely. After leaf abscission, lots of starch grains appeared in choroplasts in stem. 6 days and 4 days after rewatering, the new leaves have already fully outspreaded in R. soongorica and C, korshinskii respectively and have resynthesized large quantities of chlorophyll. Compare with leaf of control, lots of starch grains appeared in choroplasts C. korshinskii and less of starch grains appeared in choroplasts in R. soongorica. The reorganization of the photosynthetic apparatus, whose functional recovery is essential for plant survival.From the above investigations of those two species, we concluded that the major adaptation mechanisms to desiccation depended on protective strategy in photosystem, products metabolism, morphological and anatomical changes and signal regulation. They have the response mechanisms of resurrection plants to drought stress and resurrection mechanisms to re-hydration. It is shown that resurrection plants not only exist in tropic and sub-tropic (Africa), but also exist in temperate zone.