Mechanisms of photosynthetic and growth inhibition of a southern African geophyte Tritonia crocata (L.) Ker. Gawl. by an invasive European annual grass Lolium multiflorum Lam

European annual grasses are displacing native species along South Africa's West Coast, a region rich in wildflower diversity which forms the basis of a growing nature-based tourist industry. Little is known of the physiological and biochemical mechanisms underlying competitive interactions between these grasses and native taxa and how they are affected by resource supply. Consequently, this study compared Photosystem II function, photosynthetic pigment concentrations, gas and water exchange rates, Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) contents, flowering and biomass accumulation in a native geophyte Tritonia crocata (L.) Ker. Gawl. grown in monoculture and admixed with an alien grass Lolium multiflorum Lam at different levels of nutrient and water supply. Diminished stomatal conductances were the primary cause of reduced net CO2 assimilation rates and consequent decreased biomass accumulation in T. crocata admixed with L. multiflorum at all levels of water and nutrient supply with one exception. These corresponded with decreased soil water contents induced presumably by more efficient competition for water by L. multiflorum whose biomass was inversely correlated with soil water content. At low levels of water and nutrient supply, both stomatal and metabolic impairments to photosynthesis were apparent in T. crocata admixed with L. multiflorum. The metabolic impairments included a decline in the efficiency of primary photochemistry (trapping) (φP0/1 − φP0), associated with a decrease in leaf chlorophyll a and b content, a decreased efficiency of conversion of excitation energy to electron transport (Ψ0/1 − Ψ0) pointing to a reduction in electron transport capacity beyond QA− and a decline in apparent Rubisco activity (Vc,max) and Rubisco content. Under conditions of low nutrient and high water supply, only metabolic impairments to photosynthesis (decreased RC/ABS, chlorophyll a and Rubisco content) were apparent in T. crocata admixed with L. multiflorum. These impairments resulted in a down regulation of photosynthesis and a reallocation of fixed carbohydrate reserves to floral production which increased significantly in T. crocata under these conditions only and corresponded with a decline in the mass of its underground storage organ.