Atmospheric concentrations of greenhouse gases such as CO2, CH4, and N2O have increased dramatically since the beginning of the industrial revolution due to fossil fuel combustion, deforestation and land development; together, these probably led to a rise in ground-level air temperatures at an unprecedented rate over the past three decades [1, 2]. Moreover, the global mean temperature will continue to rise at a rapid rate, and our climate is likely to warm by 1.1-6.4°C within the next century . Most plant species only grow in a certain temperature range. Thus, some are likely to adapt to warmer temperatures by changing their growth and development or by shifting their ranges, provided that the optimum temperatures are not exceeded. Some species may fail to adapt to this global change and may even become extinct if the air temperature is too high [3–5]. Therefore, projected atmospheric warming is expected to have profound effects on plant physiology and growth, structure and function of plant populations, species distributions, and probabilities of extinction [6, 7]. Moreover, this change in plants may result in complex impacts on vegetation and biodiversity, leading to terrestrial ecosystem consequences [8, 9]. Thus, understanding the changes in plant growth and development in response to simulated climatic warming is important to predict plant responses to global warming in the near future.
Many studies have investigated plant responses to global warming at different scales, with most performed at community level, and only a few at the individual level or a focus on responses of leaves to temperature increase [5, 10]. Because the leaf is the key organ performing photosynthesis and transpiration, its development, which varies with environmental factors, is an important determinant of total plant productivity . In addition, leaves can be indicators of plant community responses to global warming, because their responses are not only the basis of changes at the community level, but they are among those organs that show visible impacts of air temperatures [1, 12]. Furthermore, leaf traits can express phenotypically plastic responses to growth temperature . Consequently, experiments on the effects of global warming on leaf growth and development will provide a better understanding of the mechanism of plant responses to global warming at the community level.
Previous studies mainly investigated the effects of experimental warming on leaf photosynthesis and respiration acclimation, but leaf structure (microstructure and ultrastructure) and biochemical processes were seldom focused on [1, 11, 14]. Because leaf structure is one of the most important traits exhibiting phenotypic plasticity to growth temperature, investigating responses of leaf structure to warming is fundamental to projecting the impact of global change on plant growth. In addition, leaf biochemical and physiological changes are related to leaf structure and function. For example, temperature stress is known to induce plants to produce reactive oxygen species (ROS) and malondialdehyde (MDA), which can damage both the leaf structure and function [15, 16]. To alleviate the damage, plants generally enhance the production of ROS scavenging enzymes, such as superoxide dismutase (SOD) and catalase (CAT), and osmoprotectants like proline and carbohydrates. Although many studies have investigated the effects of high temperature on the production of antioxidant enzymes and osmoprotectants, the periods of high temperature were usually limited to several hours or days; also, few studies examined these biochemical and physiological changes under global warming conditions for one generation [17–19]. Therefore, to obtain an integrative understanding of the responses of leaf growth to global warming, we examined the effects of simulated climatic warming on SOD and CAT activities, contents of MDA, proline, carbohydrates and chlorophyll of Arabidopsis thaliana leaves, and leaf microstructure and ultrastructure, apart from fitness components. Arabidopsis is a model plant widely used in molecular, genetic, and developmental biology. Therefore, studying its responses may represent a valuable assessment of the possible plant changes occurring at the individual level in a future warmer world.