REGULATION OF THE PHOTOSYNTHETIC CAPACITY OF PRIMARY BEAN-LEAVES BY THE RED-FAR-RED RATIO AND PHOTOSYNTHETIC PHOTON FLUX-DENSITY OF INCIDENT LIGHT

The main objective of the present work was to examine the effects of the red:far-red ratio (R:FR) prevailing during leaf development on the photosynthetic capacity of mature leaves. Plants of Phaseolus vulgaris L. cv. Balin de Albenga were grown from time of emergence in a controlled environment room, 25 +/- 3-degrees-C, 12-h photoperiod. with different light treatments: a) high photosynthetic photon flux density (PPFD) 800-mu-mol m-2 s-1 + high R:FR = 1.3; b) low PPFD = 300-mu-mol m-2 s-1 + high R:FR = 1.3; c) high PPFD = 800-mu-mol m-2 s-1 + low R:FR = 0.7; d) low PPFD = 300-mu-mol m-2 s-1 + low R:FR = 0.7. With an R:FR ratio of 1.3, a decrease in irradiance during leaf growth reduced photosynthesis when measured at moderate to high PPFD; but when measured at low PPFD, leaves expanded under low irradiance actually had photosynthesis rates higher than those of leaves grown in high irradiance. A low R:FR ratio during development reduced the photosynthetic capacity of the leaves. In leaves expanded under R:FR = 0.7 and high irradiance photosynthesis was reduced by 42 to 89%, depending on the PPFD at which measurements were made, whereas for leaves developed at R:FR = 0.7 and low irradiance photosynthesis decreased by 21 to 24%, compared to leaves under R:FR = 1.3 and similar irradiance. The reduced photosynthetic capacity under R:FR = 0.7 correlated with lower chlorophyll content on an area basis. As with photosynthesis, the decrease in chlorophyll was more pronounced in leaves under R:FR = 0.7 and high irradiance. In natural environments, leaves may experience low R:FR conditions temporarily during their development, and this may affect their future photosynthetic capacity in full sunlight.