Photosynthetic characteristics and antioxidant protective substances of two coniferous plants in karst fissures and under rainfall distribution
Authors: Zheng Shaojie , Wang Lin , Dong Qiong , Zeng Huiping , Li Xingze , Li Lian , Hua Qian , Wu Yutong , Yang Jiumei , Chen Fuying 
Published: September 2024

Background The widely distributed hard limestone matrix is believed to exacerbate drought and increase the difficulty of restoring fragile karst areas. The cracks in this matrix may alleviate the negative effects of drought on plants, but their underlying mechanisms are still poorly understood.
Studying the physiological growth status of Pinus yunnanensis F. and Pinus elliottii E. seedlings under different karst fissure thicknesses and rainfall distributions is of great significance for the management, vegetation restoration, and tree species selection in karst rocky desertification areas.
In this study, we used a two-factor block experiment and set different rainfall durations, namely reduced rainfall duration (I3d), natural rainfall duration (I6d), and extended rainfall duration (I9d); Different karst small habitats, i.e., stone-free soil (S0), less stone and more soil (S1/4), and half stone and half soil (S1/2), are simulated at these three levels. Analyze the changes in physiological growth and photosynthetic characteristics in two types of coniferous seedlings under different treatments with different karst thicknesses.
Results The results showed that with the increase of karst thickness, the growth volumes of height and diameter of P. yunnanensis F. seedlings, the biomass of various organs, and the accumulation of K+, Ca2+, Na+, and Mg2+ showed a significant change pattern of first increasing and then decreasing (P<0.05); P. elliottii E. seedlings show a gradually decreasing trend (except for Ca2+). The biomass of each organ in two types of coniferous seedlings showed leaf>stem>root, while K+, Ca2+, and Mg2+ in each organ of P. yunnanensis F. seedlings showed leaf>root>stem, and Na+ showed root>leaf>stem. The accumulation of mineral elements in various organs of P. elliottii E. seedlings is as follows: roots>stems>leaves, and the accumulation of mineral elements in both types of coniferous seedlings is as follows: Ca2+>Mg2+>K+>Na+. Root length, root volume, root surface area, root diameter, SOD, POD, SP, photosynthetic pigment content, fluorescence parameters, and gas exchange parameters of P. yunnanensis F. seedlings gradually increase with the increase of karst thickness (except for the 9-day rainfall duration), while those of P. elliottii E. seedlings gradually decrease. Under different treatments, the maximum saturated light intensity and minimum light intensity of P. yunnanensis F. seedlings were 1624.530 and 21.395μmol·m-2·s-1, and 1081.100 and 27.148μmol·m-2·s-1 for P. elliottii E. seedlings, respectively. I6dS1/4 and I3dS0 treatments have the best growth effects on P. yunnanensis F. and P. elliottii E. seedlings.
Conclusions In summary, prolonging rainfall duration has an inhibitory effect on the growth of two types of coniferous seedlings. Reducing rainfall duration promotes the growth and development of P. elliottii E. seedlings, increasing karst thickness inhibits the growth of P. elliottii E. seedlings, and to some extent, promotes the growth and development of P. yunnanensis F. seedlings. Therefore, we give priority to P. yunnanensis F. as the tree species for vegetation restoration or rocky desertification management in karst areas. Our study reveals the role of limestone-filled different karst fissures in mitigating the effects of drought as "containers" for plant growth. These findings help us understand the response of plants to drought stress and provide valuable insights for vegetation restoration in karst environments affected by global climate change.Therefore, further experiments with various karst fissure sizes are necessary to test the universality of the reactions of various plants under different karst fissures. The results of this study can be used to help restore ecosystems damaged by karst rocky desertification processes.