Field experiments were conducted in Kentucky and Louisiana in 2008 and 2009 (split-plot in a randomized complete block design with four replications) to investigate it is possible to simulate with manual defoliation the effect of soybean rust (SBR) (Phakopsora pachyrhizi Syd. and P. Syd) injury on a healthy soybean [Glycine max, (L.) Merr.] canopy, understand how defoliation affects the growth dynamics and canopy light interception, and if defoliation affectsleaf senescence and nitrogen remobilization during the seed-filling period. Two manual defoliation treatments based on changes in effective leaf area index (ELAI) (calculated as the reduction in leaf area equivalent to SBR-induced premature leaf abscission, loss in green leaf area, and reduction in photosynthetic capacity of diseased leaves) in infected canopies in Brazil were used to simulate SBR infection at growth stage R2 (full flowering) and R5 (beginning of seed-fill). Both defoliation treatments reduced yield in all experiments and the reduction was larger for the treatments at growth stage R2. The yield losses were equivalent to that observed in infected soybean canopies in Brazil. This suggests that a system of manual defoliation to simulate changes in effective leaf area duration shows promise as a tool to simulate the impact of SBR on soybean yield. The radiation use efficiency and crop growth rate from growth stage R2 to R5 were not influenced by defoliation. Defoliation started at growth stage R2 reduced seed number per unit area, while defoliation started at growth stage R5 reduced seed size due to shortening the seed-fill duration and a lower seed growth rate. There is no evidence that manual defoliation affected leaf senescence or nitrogen redistribution to the seed. This study found that the reduction of light interception by SBR was the main reason for the reductions in soybean growth and yield.

Despite of the documented impacts of the so-called green revolution, food security in the world faces new challenges in terms of population growth, increases in no-agricultural land use (urbanization), and climate change. Trends in food security show that the world community is operating within two limits of food system: (i) the quantity of food that can be produced under a given climate; and (ii) the quantity of food needed by a growing and changing population. Therefore, taking food security successfully into the future requires novel approaches to boost agricultural productivity in order to balance food supply and demand without expanding the agricultural land. To date, progress in wheat yield has been largely the result of the development of dwarf varieties through introgression of reduced height (Rht) genes. The height reductions arising from the presence of these genes increased yield by alteration of partitioning of dry matter and nitrogen in favour of the spike. However, increased partitioning through additional reductions in plant height is not likely; as comparative studies indicate that wheat yield is reduced when plants are shortened beyond a threshold, and most of the modern cultivars have reached the optimal height. Therefore, this dissertation aimed to identify the physiological attributes able to produce yield increases in the Rht genotypes with the optimal heights. Approaches based on physiological understanding of yield are necessary for developing genotypes combining high yielding potential and agronomic traits of superior adaptation, and for understanding yield limiting factors. Yet, direct measurement of physiological variables is often difficult or expensive; as an example, measuring plant water status in the field is problematic, with techniques such as psychrometry generally only being suitable for laboratory studies. Therefore, proxy such as tissue RWC may be a good alternative measure of plant water status. We aimed to address these questions with three components of experimental research :(i) proxy-based screening to increased photosynthetic rate and water use efficiency in wheat; (ii) determinants of increased HI in lines with different Rht genes (b, c) when incorporated into contrasting background wheat genomes (B, D), and the relative effect on N partitioning during grain filling; (iii) analyses of stable isotopes (δ²H, δ¹⁸O, δ¹⁵N and δ¹³C) in an agronomic perspective in alley cropping systems associated with adjacent N₂ fixing trees, in terms of hydraulic redistribution, N availability and crop yields. In this thesis, the proxy-based approach to crop selection was defined as a surrogate-based (proxy and surrogate used interchangeably) screening of cultivars for morphological, anatomical, and physiological traits of performance or crop environmental responses. The research proposed steps for conducting a proxy-based crop selection programme. A comparative screening of 23 Eps cultivars and ranking for traits of photosynthetic and water use efficiency showed the correlative relationships of SLA to An, WUEi, leaf N, Δ¹³C, Kh, leaf RWC, and IVD. Additionally, it was observed that IVD may influence WUE and Amax. It was suggested that these relationships of SLA to traits of photosynthesis possibly resulted from the association of SLA and the leaf biochemical characteristics. Attention was also given to examining the mechanistic foundations that determine the relationship between plant height and yield. The results showed the straw-shortening significantly correlated both with Amax and Kh; and SLA decreased with the level of dwarfing; and the Amax related both Kh and SLA. Therefore, it was proposed that the straw-shortening may affects Amax by exerting a controlling influence over Kh through SLA. Moreover, both the partitioning of N to spike and the flag leaf N were related to plant height and growth stage. Additionally, the increased post-anthesis partitioning of N to grain associated with high N uptake rate and high MRT of N were probably the traits behind increased NUE and NHI. The data also indicated that increased grain number per spike, kernel weight and reduced peduncle length might be the driver of the increased HI in this experiment. The test of the hypothesis that there might be practical application of the analyses of the natural abundance of stable isotopes (δ²H, δ¹⁸O, δ¹³C, and δ¹⁵N) and isotopic mixing model by IsoSource to understand plant interactions in terms of water redistribution and nitrogen transfer and uptake in agroforestry systems, indicated a consistent gradient in depletion of wheat xylem water δ²H, δ¹⁸O, and δ¹⁵N in leaf as moving further away from the tree line. The data also reflected a consistent pattern of isotopic values (δ²H, δ¹⁸O, and δ¹⁵N) in wheat in the proximity of the tree being similar to that of the tree, suggesting they were using the same source of water and N. Similarly, an isotopic mixing model data showed that the crops in the proximity of the trees accessed considerably amounts of the water and nitrogen redistributed by trees. The study also indicated the improvement in water use efficiency, chlorophyll content, grain number per spike, and grain yield for the crops nearest to the trees for a distance up to 5 m. In conclusion, selection for increased HI should shift focus from reduced plant height to include increased grain number and kernel weight, increased partitioning of N to spike, reduced peduncle length, and low SLA. Finally, the hypothesis that efflux of water and N in agroforestry system from tree roots in topsoil and influences a number of physiological functions of neighbouring crops was confirmed by isotopic and physiological data.