| The early regeneration stages（ERS）of plants,including seed imbibition,germination,seedling emergence,and seedling establishment,depend on both intrinsic and extrinsic environmental conditions.Endogenous phytohormones[e.g.abscisic acid（ABA）,indole-3-acetic acid（IAA）,and salicylic acid（SA）],are one of the most important intrinsic factors that impact the developmental and physiological responses in plant growth.They are chemical compounds that are synthesized in plants in minute concentrations.They regulate cell division,differentiation,and plant regeneration and growth,as well as,plant responses to biotic and abiotic environmental stressors.The success of a seed transitioning through the ERS to develop into a healthy seedling is mainly determined by water and temperature,two of the most critical environmental factors that are impacted by climate change.Climate change not only affects the average values in these factors,but also brings about extreme weather events,which can be transitory,but can dramatically alter plant regeneration compared to just increases in averages.Heat waves are extreme weather events that bring temperatures above the normal optimum,which is sensed as temperature stress by all living organisms.High-temperature stress（HTS）during early regeneration in plants can profoundly shape seed germination,seedling growth,and development.Experiencing HTS in ERS could provide stress resilience for plants if they survive such events or,they may succumb and wither or die,and the timing of it could further modulate the downstream effects.How phytohormones affect early plant regeneration processes and respond to environmental stressors are mainly studied in mature plants and specifically in the model plant,Arabidopsis thaliana.Forest species that are adapted to different habitat conditions such as hemiepiphytes（Hs）,which start their life as epiphytic plants in the forest canopy and send their areal roots to the ground to become terrestrial plants as adults may have contrasting responses to climate extremes compared to their counterparts,the non-hemiepiphytes（NHs）,which begin life on the forest floor and become mature adults and continue their terrestrial growth on the ground.The Hs species have an early growth strategy that is more droughts and heat resilient but slower growth with high flexibility in response to growth irradiance.In contrast,the NHs species require higher water levels and grow under shadier conditions but display a faster initial growth rate that help them secure their niche on the forest floor.The species in the genus Ficus,commonly known as figs,in subtropical China and other tropical locations around the globe,fig species are ecologically very important due to their keystone status,mutualistic and parasitic interactions,and role as ecological engineers.They are also important tropical and subtropical agro-bioresources that provide medicine,food and horticultural value to indigenous communities and national economies.Ficus contain both Hs and NHs species,which have contrasting ecological attributes,provide an ideal model system to understand the mechanisms that underlie the temperature stress response and endogenous phytohormones expression during the early regeneration process.This thesis assessed the effect of HTS,implemented as eight hours in 40℃,from 10:00 to 18:00,and growth form affect the early regeneration stages and phytohormone concentration of four Hs and four NHs Ficus species（Hs:F.benjamina,F.concinna,F.microcarpa,F.religiosa;NHs:F.auriculata,F.oligodon,F.racemosa,F.semicordata）that were grown in agar plates placed in climate-controlled growth chambers(growth chamber conditions:25/15°C～12/12 h day/night;humidity:60%relative humidity;light:PPFD 400μmol m-2s-1).Seed germination,seedling emergence and seedling survival probabilities of approximately 136,000 seeds of these eight Ficus species and the concentrations of ABA,IAA and SA were measured after HTS was imposed during imbibition（IMB）where rapid water intake takes place;germination（GER）,defined as 1 mm radical emergence;and emergence（EMER）,defined as seedlings with fully extended cotyledons.Based on preliminary studies these HTS treatments were therefore set to（1）24 h after the experiment began during the time seeds undergo IMB,（2）8 d after the experiment began during GER,and（3）40 d after the experiment began,during EMER.All statistical analyses were used glmm TMB and lme4 packages in R（version 4.0.3）and figures were generated using ggplot 2package.The results showed that in both groups,seeds were more sensitive to HTS in the early regeneration process;stress experienced during imbibition affected emergence and survival,and stress experienced during germination affected subsequent emergence.There was no effect from HTS when received after emergence.Survival was highest in hemiepiphytes regardless of the HTS treatment.The phytohormones showed growth form-and regeneration-stage specific responses to HTS.Due to the HTS treatment,both SA and ABA levels decreased in non-hemiepiphytes during imbibition and germination;during germination,IAA increased in hemiepiphytes but was reduced in non-hemiepiphytes.Due to the HTS treatment experienced during emergence,ABA and IAA concentrations were greater for hemiepiphytes but an opposite effect was seen in the two growth forms for the SA concentration.This study showed that the two growth forms have different strategies for regulating their growth and development in the early regeneration stages in order to respond to HTS.The ability to respond to HTS is an ecologically important functional trait that allows plant species to appropriately time their seed germination and seedling development and will provide an important reference for tree species regeneration and development studies in tropical and subtropical regions.The growth and developmental response regulation of subtropical and tropical hemiepiphytic Ficus species in response to HTS will provide greater resilience and community stability for Ficus communities under climate change.This is the first study to directly test the effects of high-temperature stress on each regeneration stage and endogenous phytohormone concentration in the early regeneration of plants,especially in Ficus tree species.Thus,this investigation provides basic research results on the regeneration of this valuable plant group. |
