Phenotypic Plasticity In Young Leaves Of Sacred Lotus(Nelumbo Nucifera) And Its Adaptive Significance

The phenotypic changes of plants under the influence of different environments are called phenotypic plasticity.Leaf is an important organ performing photosynthesis and transpiration.Leaf phenotype is very important for plant individuals to maximize their access to resources and improve their viability.So far,the research on leaf phenotypic polymorphism and its adaptive significance mainly focuses on late developmental stages or mature leaves,and few studies pay attention to the phenotypic plasticity and adaptability of young leaves during early development.In this study,the leaves of lotus(Nelumbo nucifera Gaertn.)were taken as the research object to observe the phenotypic changes during its development.It was found that the young leaves kept rolling status for about 6 days after protruding from the water surface.During this period,the petiole and leaves grew rapidly,and the inclination angle of the young leaves gradually changed from close to the petiole to almost horizontal.At the same time,the young leaves of lotus showed diurnal phenotypic plasticity,that is,there was foliar nyctinasty during the developmental process.The leaf angle at night showed a more vertical state than that in the daytime.This change in leaf orientation was similar to that of other species.However,the leaf blades of lotus also displayed a more rolled-up morphology at night and more expanded during the day.Such a nocturnal movement characterized by the change of leaf morphology was discovered for the first time.So far,the main hypotheses to explain the later-rolling of leaves are to protect water loss and reduce plant herbivory,but there is little direct evidence.Through field investigation and experiment,I focused on the differences in diurnal water content and degrees of herbivory in young lotus leaves.The results showed that there was no significant difference in water content between day and night across different developmental stages of lotus leaves,indicating that the phenotypic plasticity of lotus leaves is not associated with the protection of water loss.The results of leaf herbivory investigation showed that there was no difference in the incidence of herbivory in lotus leaves across different growth stages,that is,whether they were rolled or not had no significant effect on herbivory.Therefore,the diurnal phenotypic plasticity of lotus young leaves played a limited role in reducing herbivory.It is worth noting that the above hypotheses do not consider the particularity of the initial stage of leaf growth.Starting from the bottom-up growth characteristics of young lotus leaves,this study puts forward a new hypothesis,that is,the adaptive significance of continuous rolling of leaves and diurnal phenotypic plasticity lies in maintaining a more compact shape as much as possible,so that young leaves can stretch petioles through the barrier of upper leaves,so as to obtain more light.In order to test this hypothesis,a comparative experiment of light and shade was set up.Young lotus leaves were subject to altered external environments and their phenotypic indexes were measured.The results showed that the rolling duration and petiole elongation of young leaves in the shade group were significantly greater than those in the light group,indicating that the petiole elongation was improved by maintaining a compact shape.At the same time,the light intensity of leaves in the shade group increased significantly with the lasting days in the treatment,and the dissimilarity of light obtained at different sites of a single leave also decreased gradually,indicating that the continuous elongation of leaves achieved stronger and more homogeneous light acquisition,which supported the hypothesis of leaf growth promotion.At the plant level,lotus individuals that experienced a month of growth displayed a significant increase in the number of leaves and petiole elongation,which expanded the canopy structure and promoted the increase of photosynthetic area.The petiole height of new leaves was also significantly higher than that of old leaves,that is,new leaves grew over old ones by extending petioles.From the perspective of growth dynamics,the interior of lotus plants is constantly changing.The uppermost part has the optimal light condition,which witnesses the replacement of old leaves of weak photosynthesis with new leaves of vigorous growth,thus obtaining more resources as a whole.In this thesis,the phenomenon of later rolling of young leaves was found in lotus,and a novel type of foliar nyctinasty was defined,that is,the diurnal phenotypic plasticity of leaf rolling and orientation.At the same time,this study creatively proposed and verified the growth promotion hypothesis.It is proved that the adaptive significance of the diurnal variation of leaf phenotype is to maximize daytime photosynthesis and maintain the compact shape of leaves at night,so as to pass through the obstruction of upper leaves and achieve effective elongation of petioles.The results reveal the morphological characteristics and important ecological functions of lotus young leaves,broaden the ecological content and evolutionary connotation of leaf vernation,and comprehensively clarify the adaptive significance of leaf behavior from the perspective of growth dynamics and individual fitness.