The influence of genome size and limitation of nitrogen and phosphorus on photosynthesis efficiency

Genome size varies 2,400-fold in angiosperms and is an important trait influencing cellular and physiological parameters. One of the major drivers of the astonishing genome size (GS) diversity in angiosperms is polyploidisation and most flowering plant lineages have undergone multiple rounds of polyploidy in their ancestry. Because of the frequency of ancestral polyploidy, one might expect angiosperm genomes to be larger than other eukaryotes, where polyploidy is less frequent. But this is not the case, where GS in angiosperms is skewed towards small genomes, suggesting that, following polyploidy, there is selection over time to reduce GS. It is possible that one selection pressure that acts to reduce the size of the genome is the efficiency of photosynthesis, which may be enhanced in species with small genome sizes. This is because there is a positive correlation between the size of the nucleus and the guard cells across species, which can in turn influence the rate of gas exchange through stomata pores. Photosynthesis may also be influenced by nitrogen (N) and phosphorus (P) availability. These macronutrients are limiting nutrients to plants and play an important role for them, because they are the main constituents of the nucleic acids and they play crucial role in photosynthesis in many processes. Nitrogen is needed to build photosynthetic proteins, but especially for the RuBisCO enzyme and chlorophylls, which are N demanding. Phosphorus is used as ATP and NADPH to give the chemical energy necessary for the fixation of CO2. Both these N and P demands for photosynthesis may compete with the N and P demands of the nucleus, which may be higher in species with large genomes than species with smaller genomes. Thus in considering the selection constraints on genome size in plants it is necessary to consider the effects of GS and nutrient availability on photosynthesis. The overall aim of this PhD project is to determine how GS and nutrient availability impacts photosynthesis. To do that three experimental systems are exploited. These are: (1) The effects of GS on the efficiency of photosynthesis in plant genus Fritillaria, selected because it has particularly large genome sizes, and it has the largest range in genome size, all at the diploid level, for any genus (70 Gb/1C range). These materials enable determination of the impact of GS on cell size, gas exchange and light harvesting properties of photosynthesis. Surprisingly, no effect of GS on cell size 5 was observed, contrary to published expectation, but there was a significant correlation between GS and photosynthesis readings. (2) The effects of GS on the efficiency of photosynthesis in plant genus Nymphaea, selected because it has small genome sizes and polyploidy variants. The polyploid variants enable the effect of step changes in GS associated with polyploidy to be determined. This enables the determination of the impact of polyploidy on photosynthesis and to determine the efficiency of photosynthesis across species in an aquatic plant genus. Exceptionally low non-photochemical quenching (NPQ) was observed in these species, indicative of highly efficient light energy use, perhaps associated with small genome sizes overall and an aquatic habit. There was a relationship between GS and cell size in this genus, despite the range of GS being smaller than for Fritillaria. (3) The effect of nutrient availability and photosynthesis in wheat, selected because of its agricultural importance, its large genome size, and relatives at different ploidy levels with which the data could be compared in future studies. This material enables the effects of nutrient limitation on photosynthesis to be determined, and which components of photosynthesis are most impacted. The results revealed that some components of photosynthesis were significantly impacted by P alone (photochemical quenching (qP, negatively), non-photochemical quenching capacity (NPQ, positively)), others by N alone (maximum rate of carboxylation by RuBisCO (Vcmax, negatively)), whilst both N and P limitation and their interactions reduced biomass. The data show that interactions between photosynthesis, N and P and GS play a role in influencing plant biomass. What we now need to know in future studies is if there are N and P trade-offs between the nucleic acid sink represented by the plant genome and proteins and pigments (chlorophyll) needed for photosynthesis. For example, RuBisCO, essential for the dark reaction of photosynthesis, is likely to compete with the nucleic acid sinks for N, whilst metabolic processes, which require for example ATP, NADPH or protein phosphorylation, are likely to compete with the nucleic acid sinks for P