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Tolerance to the effects of drought and subsequent recovery after a rainfall appear to be critical for plants in the karst regions of southwestern China, which are characterized by frequent but temporary drought events. This study investigated the effects of drought intensity and repetition on photosynthesis and photoprotection mechanisms of karst plants during successive cycles of drought and subsequent recovery. Leaf water potential, gas exchange, chlorophyll fluorescence and several associated metabolic processes were studied in six plant species, including Pyracantha fortuneana (PF), Rosa cymosa (RC), Broussonetia papyrifera (BP), Cinnamomum bodinieri (CB), Platycarya longipes (PL) and Pteroceltis tatarinowii (PT) during three cycles of drought treatments at four different intensities. The four treatments were: well-watered, mild drought, moderate drought and severe drought, each followed by rewatering events. We found that limitations to CO(2) diffusion accounted for photosynthetic declines under mild and moderate drought treatments, while metabolic limitations dominated the response to severe drought. Repetition of drought did not intensify the impairment of photosynthetic metabolism regardless of drought intensity in the six species studied. Repetition of severe drought delayed the photosynthetic recoveries in PF, RC and CB after rewatering. Repetition of drought increased thermal dissipation in PF, CB and BP, as well as superoxide dismutase (EC 1.15.1.1) activity in RC and CB. Enhanced photosynthetic performance, measured as increased intrinsic water use efficiency, photosynthetic performance per unit of photosynthetic pigment, maintenance of high thermal dissipation and high ratios of carotenoids to chlorophylls, was observed during the rewatering periods. This enhanced photosynthetic performance allowed for the complete recovery of the six karst species from successive intermittent drought events.

Microbiome plays an important role during the tobacco aging process which was an indispensable link in the production and processing of cigarettes. However, the structure and functions of microbiome have not been clarified during the tobacco aging process. In this study, 16S rDNA and ITS amplicon sequencing techniques were used to analyze the core microbiome of 15 tobacco samples from five different aging stages. The whole bacterial microbiome was classified into 29 microbial phyla and 132 orders. Enterobacteriales (63%), Pseudomonadales (16%), Sphingomonadales (8%), Xanthomonadales (4%), Burkholderiales (4%), Rhizobiales (3%), and Bacillales (2%) comprised the core bacterial microbiome. The whole fungal microbiome was classified into five microbial phyla and 52 orders. Incertae_sedis_Eurotiomycetes (27%), Wallemiales (25%), Sporidiobolales (17%), Capnodiales (5%), Eurotiales (2%), an unclassified Ascomycota (12%), and an unidentified Eurotiomycetes (4%) comprised the core fungal microbiome. FAPROTAX function prediction suggested that the core microbiome has a substantial potential for the carbon cycle, nitrate metabolism, aromatic compound degradation, chitinolysis, cellulolysis, and xylanolysis, but simultaneously, the core microbiome is also a source of human pathogens. The dynamics of the bacterial community were primarily determined by the total nitrogen in tobacco leaves during the aging process, while those of the fungal microbiome were primarily determined by total organic carbon. This study indicated that the core microbiome activities may play an important role in regulating the loss of carbon organic compounds and enhancing the secondary metabolites during tobacco leaves aging process.

Drought stress is one of the most important factors in limiting the survival and growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. The ecophysiological response to drought stress of native plants with different growth forms is useful for re-vegetation programs. Two shrub and four tree species were studied, including Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and other three deciduous trees, Broussonetia papyrifera, Platycarya longipes, and Pteroceltis tatarinowii. The seedlings were randomly assigned to four drought treatments, i.e., well-watered, mild drought stress, moderate drought stress, and severe drought stress. Leaf water relations, gas exchange, chlorophyll fluorescence, and growth of the seedlings were investigated. Under severe drought stress, the two shrubs with low leaf area ratio (LAR) maintained higher water status, higher photosynthetic capacity, and larger percent biomass increase than the most of the trees. The two shrubs also had lower specific leaf area, greater intrinsic water use efficiency, and thermal dissipation than the trees. This suggested that the two shrubs had high tolerance to severe drought and were suitable for re-vegetation in harsh habitats. The evergreen C. bodinieri exhibited higher leaf mass ratio (LMR) and LAR than the deciduous species under mild and moderate stress. However, the low maximum quantum efficiency of PSII photochemistry (F v /F m ) and net assimilation rate, and the sharp decreases of water potential, LMR, LAR, and biomass under severe stress indicated C. bodinieri's weak tolerance to severe drought. In response to drought stress, the three deciduous trees revealed sharp reductions of biomass due to the large drought-induced decreases of gas exchange, LAR, and LMR. Under drought conditions, the deciduous trees minimized water loss by stomatal closure and by reducing transpiration leaf area and light harvesting through shedding leaves. This suggested that the three deciduous trees were more sensitive to water availability than the shrubs and used avoidance strategies against drought stress. However, the better growth performance of the deciduous trees than that of the shrubs under favorable conditions suggested that deciduous trees could be suitable for habitats with mild and temporary drought stress.