| Changes in precipitation patterns that are driven by the intensification of global warming have important effects on the stability of both individual plants and entire ecosystems.Therefore,it is important to study the effects of water stress on plants and to investigate the responses of plants to water deficits is one of the hot issues in current ecological research.The aim of the present study was to use a 3-yr continuous 100%rainfall exclusion experiment?hereafter referred to as continuous isolated rainfall?to simulate continuous soil water stress and to assess the physiological and ecological effects of water stress on Pinus massoniana,which is a typical adult pioneer plant in areas of severe red soil erosion.The effects of continuous isolated rainfall on the concentrations of stored non-structural carbon?NSC?,related components?soluble sugar and starch?,macro elements?C,N,P,K?,and trace elements?Fe,Cu,Zn,Cd,Ni,Cr?were analyzed and the temporal and spatial dynamic of metabolites in P.massoniana leaves and branches were measured using FTIR.The ecological response mechanisms of P.massoniana leaves and branches to water deficit were also elucidated.The results showed:1.During the study period,the overall trends of soil moisture content at 20 and 80cm in depths were similar in the control group,with extremely significant seasonal fluctuations?p<0.01?.During the same period,the soil moisture content at 20 and 80 cm depths of the drought group decreased continuously.Therefore,the average soil moisture content of the drought group at 20 cm?10.69%,coefficient of variation:4.69%?and 80cm?17.31%,coefficient of variation:10.44%?,was significantly different than the average soil moisture content of the control group at 20 cm?22.56%,coefficient of variation:18.4%?and 80 cm?30.85%,coefficient of variation:8.71%?,respectively,showing an extremely significant difference?p<0.01?.2.The leaf water potentials??d?of control and drought-stressed P.massoniana at6:00 am were analyzed.The?d of the drought group decreased over the course of the experiment,and the mean?d?-3.73MPa,coefficient of variation:7.78%?of the drought group was significantly lower than that of the control group??d:-2.80MPa,coefficient of variation:10.00%??p<0.05?.Continuous rainfall isolation in the past year significantly affected?d.The analysis of leaf morphological indexes showed that the differences of leaf lengh,leaf width and leaf area between control and drought group in the second year were significantly larger than that in the first year,which indicated that water deficit led to the growth restriction.3.During study period,the NSC of the control group exhibited extremely significant seasonal variation?p<0.01?,with peak values in the autumn of 2013 and 2014?194.94mg/g in 2013,209.40 mg/g in 2014?and in the summer of 2015?191.03 mg/g?.However,owing to soil water stress caused by rainfall exclusion,there were no seasonal variation in leaf NSC of the drought group,in which NSC concentration increased 89.73%from August 2013 to October 2014?days 115?542??coefficient of variation:21.19%,p<0.05?,and then decreased by 14.7%from October 2014 to January 2016?days 542?1032??coefficient of variation:3.29%?.This dynamic change process of first rising and then falling was negatively correlated with the 80-cm soil moisture content in the drought group?p<0.01?.It is possible that the mechanism underlying the water stress tolerance of P.massoniana had been initiated after days 542,and the probability of carbon starvation induced mortality was lower under water stress.Furthermore,the adaptative response of P.massoniana to water stress was to synergistically increase soluble sugar and starch concentrations during early rainfall exclusion and then to increase in soluble sugar concentrations by decreasing starch during later drought stress.Therefore,osmotic regulation may be the main drought-tolerance mechanism of P.massoniana.By comparing the changes of NSC concentrations in leaves and branches of P.massoniana,it was found that the response of leaf NSC to water stress was more significant than branch NSC,thereby indicating a time lag effect.4.Leaf N and P concentrations were similar in the control group and drought group during early rainfall exclusion?from August 2013 to October 2014,days 115?542?,but significantly greater in the drought group during late rainfall exclusion?from October2014 to January 2016,days 542–1032??p<0.05?,which indicated that water stress induced increase in the N and P concentrations of P.massoniana leaves.Meanwhile,leaf K concentrations,which were crucial to stomatal conductance and water use efficiency,were consistently and significantly lower in the drought group,except on Day 638?December 23,2014??p<0.05?.Therefore,the negative effection of lower leaf K concentration dut to continuous isolated rainfall maybe one of the main contributors to water shortage induced mortality.The effects of seasonality on both leaf and branch?N,P,K?were great significant in the control group.From 2013 to 2016,leaf P and K concentrations showed peak during summer,whereas leaf N concentrations peaked during autumn in 2013 and 2014,and during summer in 2015.The leaf N:P ratio of the drought group significantly greater than which in the control group during late rainfall exclusion?from December 2014 to January 2016,days638–1032??p<0.05?,which reduced growth rates were the physiological and ecological adaptation of P.massoniana to continuous isolated rainfall in eroded red soil area.5.Because of the decrease of K absorption by Pinus massoniana roots due to isolated rainfall,the K concentrations of leaves and branches reduced correspondingly,which hinder the K-related osmotic regulation mechanism of the leaves and branches of Pinus massoniana,especially the stomatal regulation,and the soluble sugar concentration of leaves and branches would be high.Then when the water deficit was further intensified,the solube sugar concentrations decreased by more than two thirds,the plants died.The results showed that the mechanism of resisting water deficit of Pinus massoniana was multi-channel,and the concentrations of K and NSC were important factors,which was an important physiological and ecological mechanism for Pinus massoniana to become the pioneer vegetation in the red soil erosion area of Changting.6.The leaf Fe,Cu,Zn,Cd,Ni and Cr concentrations of the control group exhibited significant seasonal variation.Leaf nutrient-like elements,such as Cu,Zn,Ni and Cd,decreased continuously throughout the growing season,with the highest levels observed during spring and the lowest levels observed during autumn.Meanwhile,Fe,responded indifferently,with a single peak during summer,and Cr increased continuously throughout the growing season,with lowest levels observed during spring and the highest levels observed during autumn or winter.Compared with the control group,the continuous isolated rainfall failed to significantly affect the trace elements concentrations of either leaves or branches,except for leaf Fe and Cr and branch Cr which were significantly increased?p<0.05?,possibly due to self-selection by P.massoniana under water stress.7.Leaf and branch FTIR spectra of P.massoniana in the drought group indicated,the peak values at 3420 cm-1?reflecting carbohydrates,proteins,nucleic acids?,2920 cm-1?reflecting esters in membrane lipids and cell wall pectin?and 1040 cm-1?reflecting soluble sugar,cellulose and related compounds?in leaves of the drought group first increase slightly and then decrease significantly.Furthermore,the change of water-stressed branches also exhibited a time lag effect,and the sensitivity of response to water stress was weakened.These results indicated that branches may be a sink of P.massoniana and may play an important role in drought tolerance-related osmotic regulation.In contrast,seasonal variation was observed in the peak values at 3420 cm-1,2920cm-1 and 1040 cm-1 in leaves of the control group,with peak values decreasing throughout the season?summer>spring>autumn>winter?and the C and N metabolism of the control branches had less influence on phenology.These results indicated that,to some extent,the metabolites of P.massoniana in seriously eroded granite red soils. |
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