Effects of the parasitic weed Rhamphicarpa fistulosa (Hochst.) Benth. on growth and photosynthesis of its host, Oryza sativa L. Major Research Project Report Stefanie Pflug Utrecht University Crop and Weed Ecology Group Centre for Crop Systems Analysis Wageningen University 24 May 2013 Effects of the parasitic weed Rhamphicarpa fistulosa (Hochst.) Benth. on growth and photosynthesis of its host, Oryza sativa L. Major Research Project Report Stefanie Pflug Student no. 3756815 Utrecht University MSc Environmental Biology Supervisors: Dr. ir. Lammert Bastiaans Dr. ing. Aad van Ast Examiner: Prof. dr. ir. Niels Anten Wageningen University Plant Sciences Group Centre for Crop Systems Analysis Chair group: Crop and Weed Ecology Droevendaalsesteeg 1 6708 PB Wageningen The Netherlands Wageningen, 24 May 2013 Table of Contents Acknowledgements ........................................................................................................................ i Abstract ..........................................................................................................................................ii List of Tables .................................................................................................................................. iv List of Figures ................................................................................................................................. v List of Abbreviations ..................................................................................................................... vi Introduction .................................................................................................................................. 1 Materials and Methods ................................................................................................................. 7 Experiment 1 ............................................................................................................................. 7 Experiment 2 ............................................................................................................................. 9 Data analysis ........................................................................................................................... 10 Results ......................................................................................................................................... 12 Experiment 1 ........................................................................................................................... 12 Experiment 2 ........................................................................................................................... 18 Discussion .................................................................................................................................... 27 Conclusions ................................................................................................................................. 34 Recommendations ...................................................................................................................... 35 References................................................................................................................................... 37 Appendix ..................................................................................................................................... 40 i Acknowledgements First and foremost, I would like to express my gratitude towards my supervisors Dr. Lammert Bastiaans and Dr. Aad van Ast. I greatly appreciate the time both of you have invested in me, your advice during the different parts of my thesis work and your confidence in me. Lammert, thank you for the motivating and inspiring discussions and your critical analyses of my drafts. Aad, your positive and welcoming attitude made the interaction with you always very enjoyable. Thanks for that, and all the jokes and smiles we shared. Peter van der Putten is acknowledged for re-introducing me into the handling of the Li-6400. I would like to thank the staff of Unifarm for the reliable support and the good time I had during my experimental work. I also experienced CSA as a very stimulating and pleasant working environment. I thank Prof. dr. Niels Anten for mentioning the project to me in the first place and I am grateful for the approval of Utrecht University to conduct this external thesis work. My housemates Irma, Laura and Nina are appreciated for being uncomplicated and understanding towards the challenges related to my thesis work. A special thanks goes to Jerry, for simply being my friend. ii Abstract During the past decades, rice (Oryza spp.) has become an increasingly important cereal crop in Africa. Weeds have developed into the most significant biological production constraint. With an increasing utilisation of waterlogged inland valleys for rice production, the facultative hemiparasitic weed Rhamphicarpa fistulosa (Hochst.) Benth. that naturally grows in these areas, has the potential to become a major threat to rice production. Mentions of R. fistulosa in rice fields have already shown an increase in infestation levels during the last years. Until now, little is known about R. fistulosa and the mutual interaction between parasitic weed and host plant. This knowledge is essential for developing effective weed and crop management strategies. This study is based on previous studies on R. fistulosa infected rice plants that showed a reduced growth compared to their uninfected control plants. The dry matter loss of the infected host plant, however, was higher than the additional dry matter gain of the attached parasite. It is not known yet whether this discrepancy is solely due to a cumulative growth reduction caused by disturbances in an early developmental stage or whether R. fistulosa also poses additional detrimental effects on its host, such as a reduction in photosynthetic rate which can lead to a further disturbance of host plant development. The research therefore focussed on two aspects of the host-parasite interaction during the early growth phase of rice: 1) The influence of host plant size, developmental stage and parasite size on host and parasite growth and 2) The effect of the parasite on host photosynthesis. For this purpose, pots with rice plants of two different developmental stages were infested with parasite seeds at one point in time. Monocultures (controls) of both rice and R. fistulosa were also included. The setup allowed detecting how host plant size affects parasite growth and how host plant development is affected when infestation occurs at different developmental stages. The difference in rice age at infestation was 21 days. Growth of both parasitic weed and host plant as well as host photosynthesis was measured for host- parasite associations and monocultures of host and parasite. The study revealed that 1) R. fistulosa which attached faster to relatively taller rice plants did not have a growth advantage over the R. fistulosa plants that attached later to smaller rice plants. Infection with the parasitic weed negatively affected shoot-root ratio of the rice as shoot dry weight was reduced, whereas root dry weight was not affected. Furthermore, plant height and leaf area was reduced in infected rice plants. 2) At the leaf level, leaf greenness and maximum photosynthetic rate of the infected rice plants was negatively affected. It was also observed that reductions in host biomass production occurred prior to reductions in maximum photosynthetic rates. A few weeks after attachment of the parasite, infected rice plants stopped growing. Smaller rice plants were stronger affected by the parasite than the rice plants that were taller at infection. This study provided new insights into the host-parasite interaction of rice and R. fistulosa: R. fistulosa growth was independent of host size, whereas the severity of effects on the host was dependent on the developmental stage at which infection occurred. Furthermore, the study showed that attached R. fistulosa plants did not only induce changes in host biomass partitioning and reductions in host growth. Parasite-infection also led to a reduction in maximum photosynthetic rate which can be seen as an additional impairment of host iii performance and as a contribution to the ‘gap’ between dry matter loss by the host and dry matter gain by the attached parasite. It is suggested that the data obtained in this study is used in a modelling study to evaluate the relative importance of withdrawal of water and other components of the xylem sap and the reduction in maximum photosynthetic rate on rice growth and development. Furthermore, additional experiments with more infestation times and parasite densities can be conducted to obtain a clearer picture of the mutual interaction between parasitic weed and host plant. iv List of Tables Table 1. Relative (%) and absolute (g) reductions of total, shoot, stem and leaf dry weights of rice plants in association with R. fistulosa compared to their respective monoculture. Means and standard deviations are shown………………………………………………………………………………………..18 Table 2. Mean and standard deviation for maximum photosynthetic rate per unit leaf dry weight (Amass), dark respiration and initial light use efficiency of each observation date for rice monocultures (C) and rice in association with R. fistulosa (T1 and T2)……………………………………20 Table 3. Relative (%) and absolute reductions of maximum photosynthetic rate per unit leaf -2 -1 area (Amax, in µmol CO2 m s ) and SPAD meter