The Adaptive Mechanisms With Drought And Calcium Of Two Species Suitable Growing In Limestone Mountain

Both drought and high calcium are the main characters of soil in the limestone mountain. Triadica rotundifolia is a deciduous tree species of Triadica?Euphorbiaceae?, which is endemic to the secondary forest in limestone mountain. Cinnamomum burmannii was an evergreen tree species of Cinnamomum?Lauraceae?, which mostly grows in calcareous soil and is also common in secondary forest of limestone mountain. Above two kinds of plants as material, the soil moisture was greater than 80% of field capacity for normal level, 50⁷0% for mild drought, 30⁵0% for moderate drought, and less than 30% for severe drought. And with the concentration gradient of 0, 50, 100, 150 mmol·L-1 respectively of Ca2 Cl nutrein solution for studied. The basic adaptation mechanism of Triadica rotundifolia and Cinnamomum burmannii in the limestone mountain region was revealed. In order to provide the scientific proofs for species selection and artificial reforestation in the limestone area in southern China. Main results are listed as follows:?1? The content of chlorophyll a, chlorophyll b, total chlorophyll, soluble protein, soluble sugar and SOD activity of the seedlings of the two species showed a trend of increase first then decrease under continuously drought stress. The change of malondialdehyde?MDA? content was more sensitive in T. rotundifolia and the peak?33.05 mmol·ml-1? appeared at the 28 th day after treatment. Comparatively, the proline content and POD activity were more sensitive in C. burmannii and the proline peak?217.11 ?g·g-1? appeared at the 24 th day. After that, the POD activity showed a significant decrease.?2?Under the continuous drought stress treatment, the active quantum efficiency of PS??Y???? and electron transport rate?ETR? of the seedlings decreased, whereas the non-photochemical quenching?NPQ? showed an inconsistent variation trend, and the minimum initial fluorescence?F0? was continuously increasing. Slight drought treatment in T. rotundifolia caused an increased trend in the maximal fluorescence?Fm?, variable fluorescence?Fv? and light energy capture efficiency?Fv/Fm?, while F0 and Fv increased first then decreased and Fv/Fm continuously declined in C. burmannii.?3? Root characteristics analysis showed that the root activity of T. rotundifolia was significantly higher than C. burmannii. The former increased the adaptive ability mainly via the increasing of total length, volume and diameter of the root, and the latter root characteristics were grew best under the moderate stress. These observation analysis reflected obvious morphological plasticity of these two species.?4? Under the drought stress, the leaves of T. rotundifolia become thicken. Arrangement of spongy tissues became loose and the ratio of palisade to spongy was decreased. Whereas the leave of C. burmannii showed a little different characters: no epidermal hair, inconspicuous differentiation between palisade and spongy tissue, high loose degree of leaves and low packing. Principal component analysis?PCA? showed the drought tolerance of T.rotundifolia was higher than C. burmannii under various drought stresses.?5? Different Ca²⁺ concentrations could influence the seedling growth and the biomass accumulation of T.rotundifolia and C. burmannii. The optimal Ca²⁺ concentrations was 150 mmol·L-1 and 50 mmol·L-1 for these two species, respectively. Root characteristics showed that the root activity of T. rotundifolia was significantly higher than C. burmannii. Compare to control?CK?, the roots traits significantly increased in the treatment group of T. rotundifolia, while they increased in low calcium treatment and was inhibited under the middle and high calcium treatments of C. burmannii. Physiological analyses showed that soluble protein content ranged from 87.87 to 121.51 mg·g-1, and SOD activity varied between 102.77 and 113.10 ?·g-1 in T. rotundifolia, which were significantly higher than C. burmannii. However, the MDA content?3.72⁵.75 ?mol·g-1? was significantly lower of in T. rotundifolia.?6?The results revealed a generally strong ability to absorb calcium by the leaf of T. rotundifolia. There are six main forms of calcium including AIC-Ca, H₂O-Ca, HAC-Ca, Na Cl-Ca, HCl-Ca and Res-Ca in the leaves. The main form of calcium in the leaves was H₂O-Ca, which accounted for 31.3³5.4% of the total calcium, but it was Na Cl-Ca in roots and stems. In C. burmannii, the main calcium forms were AIC-Ca?H₂O-Ca and Na Cl-Ca in roots,but it is HAC-Ca in the stems, and HCl-Ca, Res-Ca in the leaves, which accounted for 32.1⁴3.7% of the total calcium. These findings provided some valuable evidences for further studies on the adaptive mechanisms of edaphic specialization.?7?The anatomical results showed that many traits adapt to rich calcium environment, which included the development of phloem and xylem, pith radius and ratio periderm of root; the thick cuticle of periderm, much xylem vessels and pith radius of stem; concentration of stomatal densely in epidermis, cuticle thickness and development of palisade tissue and spongy tissue. However, the C.burmannii was lack of pith structure, the phloem and xylem were inder-development, big percentage of stem cortex, developed xylem and phloem fiber, thicker leaves and horny, relatively under-developed palisade tissue and spongy tissue. Those results showed the C.burmannii could only adapt to the calcium concentrations about 50 mmol·L-1. In total, the growth, physiological and biochemical results were consistent.