Responses of wheat photosynthesis and yield to elevated CO2 under heat and water stress

Climate change involves rising CO2 and temperature, varying rainfall patterns as well as increased frequency and duration of heat stress (HS) and water stress (WS). It is important to assess the impact of climate change, including extreme events on crop productivity to manage future food security challenges. Elevated CO2 (eCO2) boosts leaf photosynthesis and plant productivity, however plant responses to eCO2 depend on environmental conditions. The response of wheat to eCO2 has been investigated in enclosures and in field studies; however, studies accounting for eCO2 interactions with HS or WS are limited. My PhD project addresses this knowledge gap. The broad aim of this thesis was to investigate the response of two commercial wheat cultivars with contrasting agronomical traits to future climate with eCO2 and more extreme events, in order to develop a mechanistic approach that can potentially be incorporated in current crop models, which, so far, fail to predict accurate yields under stressful conditions. Consequently, I investigated the interactive effects of eCO2 with either heat HS or WS on photosynthesis, crop growth and grain yield of the two wheat cultivars Scout and Yitpi grown either in the glasshouse or in a dryland field. In the first glasshouse experiment, the two cultivars were grown at current ambient (450 ppm) and future elevated (650 ppm) CO2 concentrations, 22/14oC day/night temperature, supplied with non-limiting water and nutrients and exposed to 3-day moderate HS cycles at the vegetative (38/14oC) and flowering stage (33/14oC). At aCO2, both wheat lines showed similar photosynthetic temperature responses; while larger and greater-tillering Yitpi produced slightly more grain yield than early-maturing Scout. Elevated CO2 stimulated wheat photosynthesis and reduced stomatal conductance despite causing mild photosynthetic acclimation, while moderate HS did not inhibit photosynthesis at 25oC but slightly reduced photosynthesis at 35oC in aCO2-grown plants. Elevated CO2 similarly stimulated final biomass and grain yield of the two wheat cultivars not exposed to moderate HS by variably affecting grain size and number. The main distinct outcomes of this chapter were the insignificant effect of moderate HS on wheat yield and the reduced grain nutrient quality of high tillering Yitpi at eCO2. In the second glasshouse experiment, a single cultivar Scout was grown at current ambient (419 ppm) and future elevated (654 ppm) CO2 concentrations, 22/14oC day/night temperature, supplied with non-limiting water and nutrients and exposed to 5-day severe HS cycle at the flowering stage (39/23oC). Growth at eCO2 led to downregulation of photosynthetic capacity in Scout measured at common CO2 and leaf temperature in control plants not exposed to severe HS. Severe HS reduced light saturated CO2 assimilation rates (Asat) in aCO2 but not in eCO2 grown plants. Growth stimulation by eCO2 protected plants by increasing electron transport capacity under severe HS, ultimately avoiding the damage to maximum efficiency of photosystem II. Elevated CO2 stimulated biomass and grain yield, while severe HS equally reduced grain yield at both aCO2 and eCO2 but had no effect on biomass at final harvest due to stimulated tillering. In conclusion, eCO2 protected wheat photosynthesis and biomass against severe HS damage at the flowering stage via increased maximal rate of RuBP regeneration (Jmax), indicating an important interaction between the two components of climate change, however grain yield was reduced by severe HS in both CO2 treatments due to grain abortion. The field experiment investigated the interactive effects of eCO2 and WS on two wheat cultivars Scout and Yitpi grown under dryland field conditions using free air CO2 enrichment (FACE). Plants were grown at two CO2 concentrations (400 and 550 ppm) under rainfed or irrigated conditions over two growing seasons during 2014 and 2015. Irrigation in dryland field conditions created contrasting soil water conditions under aCO2 and eCO2. Elevated CO2 and WS responses of biomass and grain yield differed in the two growing seasons. Elevated CO2 stimulated photosynthesis, biomass and grain yield, but reduced photosynthetic capacity evident from lower maximal rate of RuBP carboxylation (Vcmax) and flag leaf N only in 2015. Water stress reduced above-ground biomass and grain yield in both cultivars and CO2 treatment more strongly in 2014 relative to 2015. However, marginal growth stimulation by eCO2 did not protect plants from WS. Biomass, grain yield and grain quality were antagonistically affected by eCO2 and WS. When all data were considered together, I observed that Scout and Yitpi responded differently to growth conditions in the glasshouse and responded similarly in the field. Under well-watered conditions, Scout and Yitpi slightly benefited from moderate HS but were adversely impacted by severe HS. At the flowering stage, severe HS caused grain abortion decreasing grain yield in both CO2 treatments. Elevated CO2 alleviated photosynthetic inhibition but did not stop grain yield damage caused by severe HS. Water stress reduced net photosynthesis, biomass and grain yield in both CO2 treatments and no interaction between eCO2 and WS was observed for any of the measured parameters. Grain yield was stimulated by eCO2 more in the glasshouse than in the field. Grain nutrient quality was reduced by eCO2 and unaffected by either HS or WS (in both season average)

Crop Adaptation to Elevated CO2 and Temperature

This book contains a collection of recent peer-reviewed articles on the topic "Crop Adaptation to Elevated CO2 and Temperature" published in Plants. Topics range from meta-analyses of crop responses, to descriptions and results of large-scale screening efforts, to molecular studies of changes in gene expression related to fruit quality