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SummaryDespite some notable successes, only a fraction of the genetic variation available in wild relatives has been utilized to produce superior wheat varieties. This is as a direct result of the lack of availability of suitable high‐throughput technologies to detect wheat/wild relative introgressions when they occur. Here, we report on the use of a new SNP array to detect wheat/wild relative introgressions in backcross progenies derived from interspecific hexaploid wheat/Ambylopyrum muticum F1 hybrids. The array enabled the detection and characterization of 218 genomewide wheat/Am. muticum introgressions, that is a significant step change in the generation and detection of introgressions compared to previous work in the field. Furthermore, the frequency of introgressions detected was sufficiently high to enable the construction of seven linkage groups of the Am. muticum genome, thus enabling the syntenic relationship between the wild relative and hexaploid wheat to be determined. The importance of the genetic variation from Am. muticum introduced into wheat for the development of superior varieties is discussed.

Photosynthesis and photoinhibition in field-grown rice (Oryza sativa L.) were examined in relation to leaf age and orientation. Two varieties (IR72 and IR65598-112-2 [BSI206]) were grown in the field in the Philippines during the dry season under highly irrigated, well-fertilized conditions. Flag leaves were examined 60 and 100 d after transplanting. Because of the upright nature of 60-d-old rice leaves, patterns of photosynthesis were determined by solar movements: light falling on the exposed surface in the morning, a low incident angle of irradiance at midday, and light striking the opposite side of the leaf blade in the afternoon. There was an early morning burst of CO 2 assimilation and high levels of saturation of photosystem II electron transfer as incident irradiance reached a maximum level. However, by midday the photochemical efficiency increased again almost to maximum. Leaves that were 100 d old possessed a more horizontal orientation and were found to suffer greater levels of photoinhibition than younger leaves, and this was accompanied by increases in the de-epoxidation state of the xanthophyll cycle. Older leaves had significantly lower chlorophyll content but only slightly diminished photosynthesis capacity.Recent studies show that rice (Oryza sativa L.) yields need to increase by 70% of current levels by the year 2030 to meet the needs of a rapidly increasing human population, and this increase must arise almost exclusively from existing highly irrigated farmland (Khush and Peng, 1996). To achieve this increase in production, the yield potential needs to be increased and the rate of biomass production improved, particularly during the reproductive phase (Cassman, 1994). Of the numerous factors affecting crop yield, the efficiency with which solar radiation is transformed into biomass and the amount of radiation available are the most important (Russell et al., 1989).Light saturation of photosynthesis leads to a decline in radiation-conversion efficiency. For rice this was estimated to be approximately 17% (Murata and Matsushima, 1975) but varies greatly according to variety and growth conditions. In the tropical dry season, high irradiance not only saturates photosynthesis but also subjects the exposed flag leaf to high-light stress. The response of rice leaves to high irradiance has not been characterized in the field. In laboratory and field studies of other plant species, mechanisms operate that down-regulate PSII via an increase in the dissipation of excess excitation energy (Demmig-Adams and Adams, 1996;Horton et al., 1996). Although the major fraction of this is controlled by the thylakoid ⌬pH and the xanthophyll cycle, and therefore responds rapidly to changes in irradiance, some down-regulation is often longlived and in extreme cases damage to thylakoid constituents can occur (Andersson and Barber, 1996). Both damage to and sustained down-regulation of PSII can be considered to be photoinhibitory (i.e. causing a decrease in the quantum efficiency of photosynthesis; Osmond, 1994) and pote...

Rice (Oryza sativa L.) has been used to study the long-term responses of photosynthesis to high irradiance focusing on the composition of the photosynthetic apparatus and leaf morphology. Typical sun/shade differences in chloroplast composition are seen in the fifth leaf following growth in high irradiance compared with low irradiance (1000 and 200 micromol m(-2) s(-1), respectively): higher light-saturated rates of photosynthesis (P(max)), higher amounts of Rubisco protein, and a lower chlorophyll a:b ratio. In addition, leaves were thicker under high light compared with low light. However, responses appear more complex when leaf developmental stage is considered. Using a system of transferring plants from low to high light in the laboratory responses that occur before and after full leaf extension have been studied. Acclimation of photosynthesis is limited by leaf age: the transfer to high light, post-leaf extension, is characterized by alterations in chlorophyll a:b but not in Rubisco protein, which may be limited by leaf morphology. Microarray analysis of gene expression was carried out on plants that were transferred to high light post-leaf extension. A down-regulation of light-harvesting genes was seen. No change in the expression level of Rubisco genes was observed. Up-regulation of genes involved in photoprotection was observed. It was also shown that high-light leaf morphology is established prior to formation of the zone of cellular elongation and division. The endogenous and environmental factors which establish the characteristics of high light acclimation may be important for attaining high rates of assimilation in leaves and crop canopies, and the fifth leaf in rice provides a convenient model system for the determination of the mechanisms involved.