PhD: Growing in Australia – plant strategies to survive intense solar radiation and soils lacking in phosphorus

I am working hard to publish the work from my PhD so please watch this space for more information on published articles. In the meantime please feel free to have a read of the summary of my PhD thesis.

Summary

Sunlight is essential to plants as the main energy source for photosynthesis, yet excess light energy that cannot be processed in photosynthesis can be damaging. Australian plants are regularly exposed to very intense light, enriched in ultraviolet (UV) radiation. This places them at direct risk of photodamage and cellular oxidative stress. Low inorganic phosphate (Pi) availability, characteristic of Australian soils, can reduce the energy threshold of photosynthesis and exacerbate photodamage. Whilst intense light and low Pi are common in Australian ecosystems, relatively little is known about the strategies generalist plant species use to overcome these stressful conditions. This thesis will address this gap in scientific knowledge, increase the general understanding of plant stress responses and identify traits associated with increased stress tolerance that could potentially be exploited in future crops.

This thesis investigates the individual and combined impact of solar UV, light intensity and Pi availability on plant physiology, development and biochemistry in native Australian species and in domesticated sunflower (Helianthus annuus L.). Spectral screens allowed us to assess plant responses to solar UV components; short-term responses to changes in Pi availability were investigated by adding exogenous P, and biochemical strategies to tolerate long-term Pi limitation were ascertained by using paired sub-alpine ecosystems with naturally contrasting availability of Pi, a unique approach in this area of research. In situ gas exchange measurements were complemented with laboratory analyses of leaf biochemistry, leaf optical properties and leaf anatomy.

UV radiation is largely considered detrimental to plant cells, however beneficial effects of solar UV have also been noted. This thesis adds to both bodies of work: I found exposure to solar UV promotes accumulation of epidermal UV screening compounds and up-regulates biosynthesis of foliar antioxidants in native sub-alpine plants and in H. annuus. Solar UV increased rates of photosynthesis and leaf energy content of H. annuus. On the other hand, solar UV also imposed a significant carbon cost to native plant species adapted to intense solar radiation.

Plants exhibit a number of strategies to redistribute internal P resources when supply is limited. My thesis presented examples of native plants reducing allocation of Pi to phospholipids in response to long term P limitation, albeit insufficient to maintain photosynthetic rates comparable to sites without P-limitation. Exposure to solar UV increased substitution of membrane galactolipids for phospholipids, thereby preventing low Pi supply from compromising photosynthesis in H. annuus.

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Rarely do studies consider together the acclimation and adaptation strategies that plants adopt to tolerate stress. In this thesis native sub-alpine plants showed phenotypic plasticity in response to short-term increases in Pi availability (increased capacity for Pi uptake and ability to modify leaf lipid biochemistry) and reductions in light intensity (by reducing foliar antioxidant capacity). There was however no effect on photosynthesis suggesting the latter may well be genetically regulated to cope with long-term trends of Pi availability and light intensity.

Throughout the studies reported in my thesis, it was clear that both native and agricultural plants, have evolved strategies to cope with intense light and soils lacking in phosphorus. Perhaps the biggest distinction is that these adaptive responses were adequate to alleviate potential reductions in photosynthetic rate for domestic agricultural crops, yet in native plants these responses reduce rates of photosynthesis. From an agricultural perspective, these results suggest that genetically inherited traits from native plants could increase tolerance to environmental stress in crop species but at a cost to plant productivity. Future work focussing on elucidating impacts of stress on photosynthesis are needed to fully explore potential benefits of native plant traits for agriculture.