Climate Change, Ultraviolet-B Radiation and Effects on Plants: A Review

Climate change has occurred mainly due to fossil fuel burning and rise in concentration of harmful greenhouse gases (GHGs) in the atmosphere during post-industrialization era. It induced variation in environmental conditions severely affect agricultural crop productivity and yield. Gradual depletion of stratospheric ozone layer in atmosphere has lead to increase in solar ultraviolet-B radiation (UV-B) that reaches earth surface. Relatively little information exists on the effects of UV-B radiation on field crops. The aim of this review is to summarize the results of recent studies on the interaction between climate change, in particular the increase of UV-B radiation, and crops, in terms of yield, stress damage and defence and quality. 1.1 Climate change The climate on Earth has always followed natural cycles linked to the variation of solar insolation resulting from changes in some parameters of the Earth'sorbit. By now it is incontrovertible that the climatic changes that are currently occurring are different and more dramatic than those that have marked the history of our planet: the greater concentration of greenhouse gases in the atmosphere and the rise in temperature and inconsequent sea levels have never been so rapid and had such far-reaching consequences. Extreme climatic events, droughts, storms and catastrophic floods are and will be increasingly intense and frequent. As evidenced by periodic reports from the UN's IPCC (Intergovernmental Panel on Climate Change), the decade 2010-2019 was the warmest on record since reliable and regular records have been kept.

Starting from the 1980s, each subsequent decade has been increasingly hot, with increasingly frequent climatic events. Human activities are primarily responsible for this problem, with a 147% increase in greenhouse gases in the atmosphere, such as CO , compared to pre-industrial levels. Therefore, studies on climate change play an increasingly central role in 2 multidisciplinary scientific research involving researchers from various cultural backgrounds: agronomists, foresters, ecologists, botanists, zoologists, naturalists, geologists, engineers, even physicians and sociologists. Climate change, recognized allover the world by the scientific community, is already having a strong impact on the world population due to changes in the yields, qualitative characteristics and water requirements of agricultural crops [1,29]. There are two elements to consider when it comes to climate change: the greenhouse effect and the thinning of the ozone layer.

While the former can be considered a normal phenomenon for regulating the temperature of our planet, the problem arises when, with the increase of so-called "greenhouse gases" such as CO , CHand N O, in the atmosphere, an increasing amount 2 4 2 of thermal energy from the sun raises seasonal temperatures.

The temperature of the planet has always reported risen in modern times, starting from 1880 until today, rising by about 1.5° Cover the period, a trend that shows no signs of stopping.

The ozone layer is fundamental for the survival of every living species, since it creates a "shield" in the atmosphere against harmful radiation from the sun.

Its gradual decrease in thickness is mainly due to the release into the atmosphere of chlorofluorocarbons (CFCs) [54]. The thinning phenomenon is currently present at the South Pole and is expanding at a rate of 5% every 10 years. This represents a huge risk for survival, since the vanishing of the "shield" effect allows an ever increasing quantity of ultraviolet rays of the types B (UV-B) and C (UV-C) to enter into the atmosphere [35].

Most of our knowledge on the effects of UV-B radiation on plants derives from studies on economically important crops, most of which have emerged as sensitive to UV-B rays. Sensitivity differs between cultivars of the same species [30]. 1.2 UVrays and higher plants Plants are sessile autotrophic organisms that must constantly adapt to changes in the surrounding environment. Plant adaptation to UVrays has been of particular interest to researchers in recent years.

UVrays represent a range of electromagnetic radiation with a well-defined wavelength, divided into 3 specific regions: UV-A, with a wavelength ranging from 315 nm to 400 nm, UV-B, with a wavelength from 280nm to 315 nm, and UV-C, with a wavelength from 200 nm to 280 nm [27].

The global changes in the chemical composition of the atmosphere, with a substantial reduction of the protective ozone layer, have led to an evermore worrying increase in solar radiation on the earth, in particular UV-B rays [2]. Currently, the levels of UV-B reaching the earth during the harvest season are roughly between 2 and 12 kJ m-2 per day at the earth'ssurface, with a 6% to 14% increase in UV-B radiation compared to levels recorded before 1980 [3]. Under normal conditions, only rays included in the UV-A spectrum and a small portion of UV-B rays (0.5%) reach the earth'ssurface. The changing composition of the atmosphere however is allowing an ever greater passage of the latter rays. Although they are a minor component of sunlight, they may potentially cause biological damage that is a cause for concern, due to the high energy emitted (3.94-4.43 eV) [4].

Numerous studies have already shown that the increase in UV-B rays has significant effects on the yield and on the morphological, physiological and biochemical processes of many crops.

Radiative stress has been classified as an abiotic stress.

These notes will briefly examine the various effects found in some herbaceous crops, and the defense mechanisms developed to counteract damage from UV-B rays. 1.3 UVB rays and damage 1.3.1 UV-B and effects on yield The deleterious effects of enhanced UV-B radiation vary markedly within and among species [36,37].

Zeng et al. [38] showed for winter wheat that the supplemental UV-B can cause a decrease in yield of winter wheat of up to 24% with an 11.4% increase in UV-B.

One species that has been studied extensively is soybean (Glycine max). Two soybean cultivars grown with enhanced UVB radiation showed conflicting sensitivities. One of the Essex cultivars showed substantial reductions (20-25%) in yields, while another, the Williams cultivar, was not affected by the increase in UV-B radiation [31]. UV-B rays interfere with the photomorphogenic development of plants and give rise to numerous regulatory effects on the morphology, development, physiology and biochemical composition of plants [5].