Williams, Tom and Paul, Nigel and Dodd, Ian (2020) Leaf temperature and gas exchange responses to ultraviolet radiation. PhD thesis, Lancaster University.
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Abstract
Commercial growers utilising ultraviolet (UV) transparent plastic polytunnel claddings reported enhanced leaf temperature, which they associated with early crop maturity. The general consensus in the literature is that UV radiation reduces stomatal conductance. Thus it was hypothesised that UV radiation induces partial stomatal closure that limits transpiration causing increased leaf temperature. UV-induced partial stomatal closure was evident in a range of experimental environments. Tightly controlled climate cabinet experiments, applying a range of acute (90 minute) UV treatments, identified a non-linear UV irradiance response that decreased stomatal conductance while increasing leaf temperature and instantaneous water use efficiency. In longer term controlled environment experiments, and in polytunnels experiments in the UK and Turkey, the same UV-induced partial stomatal closure resulted in enhanced leaf temperature in UV+ polytunnels compared to UV-, demonstrating the consistency of this response. In the UK, changeable UV radiation conditions due to variable cloud cover led to a reversal of the stomatal response between UV treatments, with greater stomatal conductance observed in UV+ polytunnels. Ultimately leaf temperature decoupled from stomatal conductance, with both variables increasing simultaneously, caused by greater radiation loading in UV+ polytunnels that exceeded transpirational cooling, leading to higher leaf temperatures. This was investigated in polytunnels in Turkey by analysing the net radiation balance between UV+ and UV- polytunnels in terms of upwelling and downwelling solar and far infrared radiation. Downwelling and net solar radiation were far greater in UV+ polytunnels than UV-, but vice versa for downwelling and net far infrared radiation, with an overall balance of greater net total radiation in UV+ polytunnels. This explains the cause of radiative heating in UV+ polytunnels compared to UV- and why leaf temperature decoupled from stomatal conductance when UV radiation levels were reduced by cloud. Thus enhanced leaf temperature in UV-transparent polytunnels is caused by concurrent UV-induced partial stomatal closure and radiative heating resulting from net radiation imbalance, with stomatal closure dominant when total radiation is low but vice versa when total radiation is high. These effects depend on the UV and total radiation transmission properties of the specific plastics used to clad polytunnels, of which there is a vast range available. The conclusive evidence that UV radiation increases leaf temperature in tomato through partial stomatal closure is likely to be relevant to the majority of crops, if not all, produced globally. However, a number of questions still exist in terms of the temperature effect on maturity and yield. There are likely to be benefits and detriments, dependent on geographic location, crop and season, and how those will interact with a changing climate. How will changes in crop temperature affect other organisms? Again, it is likely the effect will be dependent on a number of different factors and these may be beneficial or detrimental to crop production, not least in terms of the interaction between UV radiation and crop temperature on herbivory. Ultimately, there are a number of different complex factors to consider when assessing the implications of enhanced leaf temperature on crop production.