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Increased Phytochrome B Alleviates Density Effects on Tuber Yield of Field Potato Crops1
Hernan E. Boccalandro, Edmundo L. Ploschuk, Marcelo J. Yanovsky, Rodolfo A. Sanchez, Christiane Gatz, ´ ´ and Jorge J. Casal* IFEVA, Faculty of Agronomy, University of Buenos Aires, Avenida San Martın 4453, 1417 Buenos Aires, ´ Argentina (H.E.B., E.L.P., M.J.Y., R.A.S., J.J.C.); and Albrecht-v.-HallerInstitute fur Pflanzenwissenschaften ¨ Allgemeine und Entwicklungsphysiologie der Pflanze, Untere Karspule 2, D–37073 Gottingen, Germany ¨ ¨ (C.G.)

The possibility that reduced photomorphogenic responses could increase field crop yield has been suggested often, but experimental support is still lacking. Here, we report that ectopic expression of the Arabidopsis PHYB (phytochrome B) gene, aphotoreceptor involved in detecting red to far-red light ratio associated with plant density, can increase tuber yield in field-grown transgenic potato (Solanum tuberosum) crops. Surprisingly, this effect was larger at very high densities, despite the intense reduction in the red to far-red light ratios and the concomitant narrowed differences in active phytochrome B levels between wild type and transgenicsat these densities. Increased PHYB expression not only altered the ability of plants to respond to light signals, but they also modified the light environment itself. This combination resulted in larger effects of enhanced PHYB expression on tuber number and crop photosynthesis at high planting densities. The PHYB transgenics showed higher maximum photosynthesis in leaves of all strata of thecanopy, and this effect was largely due to increased leaf stomatal conductance. We propose that enhanced PHYB expression could be used in breeding programs to shift optimum planting densities to higher levels.

The world population is predicted to reach 8 billion by 2025, and this, in combination with current trends in dietary composition, will result in a substantial increase of food demand. Theability to satisfy this demand will rely heavily on the genetic improvement of cultivated plants to increase yield potentials and/or yield stability (i.e. the constancy of performance; Khush, 2001). Although yield per unit area continues to increase for many crops, maximum yield is not showing such an obvious trend, and this has been interpreted as a result of yield potential approaching a ceiling,whereas yield stability continues to grow (Mann, 1999). Although some consider that this ceiling represents the highest physically attainable yield (Sinclair, 1993), others are more confident that incorporation of the current wealth of knowledge in plant biology will overcome the decreasing rate of increase in yield potential. However, genetic modifications that appear beneficial when analyzed atthe cellular or plant level do not necessarily trans-

1 This work was supported by the University of Buenos Aires (grant no. G 067 to J.J.C.), by the National Research Council of Argentina (Consejo Nacional de Investigaciones Cientıficas y Tec´ ´ nicas grant no. PID 888 to J.J.C.), and by the Fundacion Antorchas ´ (grant no. 14116 –16 to J.J.C.). * Corresponding author; e-mailcasal@ifeva.edu.ar; fax 5411– 4514 – 8730. Article, publication date, and citation information can be found at http://www.plantphysiol.org/cgi/doi/10.1104/pp.103.029579.

late into higher yields in the field, either because of negative side effects or simply because they do not operate on the limitations under field conditions. As incident solar radiation penetrates the canopy of commercial crops, it suffers agradual attenuation of the photosynthetic photon flux density (PPFD) and of the red (R) to far-red (FR) ratio (Holmes and Smith, 1977). Changes in the R to FR ratio perceived by phytochromes initiate a number of responses such as increased stem extension growth (Morgan and Smith, 1976), reduced branching (Deregibus et al., 1983), and accelerated leaf senescence (Rousseaux et al., 1999), which...
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