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TitleThe effects of elevated [CO₂] and decreased vapour pressure deficit on the nutrient status of maize and wheat plants under well-watered conditions
AuthorPrayag, Kervin D
SubjectPlant Sciences
TypeMaster Thesis
AbstractIncreases in photosynthetic rates (A), biomass production and grain yield have been measured across a range of C₃ plants under elevated atmospheric [CO₂] ("eCO₂"). However, decreases in the nutritional status of many C₃ plants growing at eCO₂ often occur concurrently with these increases. Several mechanisms have been proposed for these eCO₂-induced decreases, such as dilution effects due to enhanced carbohydrate production, down-regulation of photosynthesis, reduced root development, and decreased transpiration-driven mass flow delivery of nutrients. Reduced mass flow at eCO₂ is generally accepted as one cause for altered nutrient status in C₃ plants. However, eCO₂-induced reductions in mass flow remain understudied in C₄ plants, even though they account for about 18% of the total global net primary productivity and represent a large food source globally (e.g. maize and sorghum). This thesis investigated how mass flow reductions affect the nutrient status of wheat (C₃) and maize (C₄) plants. Reduced mass flow in both maize and wheat plants was induced with eCO₂ and by varying leaf-to-air vapour pressure deficits (VPD). I hypothesised that reduced mass flow at eCO₂ and at low VPD will negatively affect nutrient status in both the C₃ (wheat) and the C₄ (maize) species. In the first experiment, maize and wheat plants were grown at 400 and 800 ppm [CO₂], in three well-watered soils, ranging from sandy to clayey, with and without fertilisation. In the second experiment, plants were grown at three VPD levels, namely 1.613 kPa, 0.773 kPa and 0.350 kPa, in well-watered soil and sand. In the latter experiment, to demonstrate the importance of mass flow, plants grown in sand were supplied nutrients in such a way that they had to rely exclusively on mass flow or diffusive processes (i.e. limited interception) for nutrient delivery to their rhizosphere. eCO₂ stimulated A on average by 22% in maize and by 50% in wheat, while stomatal conductance (ɡₛ) and cumulative water loss (CWL) were respectively decreased by 35% and 31% in maize, and by 26% and 37% in wheat. eCO₂ reduced mass flow delivery of most nutrients on average by 32% in maize, and by 38% in wheat. The hypothesis that eCO₂-induced reductions in mass flow negatively affect nutrient status in maize 33 and wheat was however not supported. This was attributed to the well-watered conditions of the soils, which may have allowed for other processes (e.g. diffusion) to make up for the mass flow reductions. From 0.773 kPa to 0.350 kPa VPD, CWL was decreased on average by 14% and 20% in the maize and wheat plants, respectively. A and ɡₛ were little affected by VPD, but plants of both species always accumulated more biomass at 0.773 kPa. Consequently, there was little evidence to suggest that VPD-induced reductions in mass flow negatively affect nutrient status in maize and wheat. Reduced CWL may have impeded root-to-shoot transport of ions and reduced dry biomass accumulation in the maize and wheat plants at 0.350 kPa (-40% and -22% on average respectively, relative to 0.773 kPa plants). Tissue [NPK] was also decreased (-13%, -41% and -47% respectively) in the 0.350 kPa VPD sand wheat plants, while increases in the proportion of finer roots may have alleviated effects of reduced CWL on tissue [NPK] in the C₄ species. The findings from both experiments imply a decrease in the importance of mass flow for the delivery of nutrients to the rhizosphere under well-watered conditions. However, reductions in mass flow to a similar extent in both species at eCO₂ and low VPD - measured in the present study - suggest that under conditions of low water and nutrient availability, tissue nutrient concentrations could be negatively affected when transpiration is reduced.
PublisherUniversity of Cape Town
PublisherFaculty of Science
PublisherDepartment of Biological Sciences