Simultaneous measurements of chlorophyll (Chl) fluorescence and photosynthetic O2 evolution {P^ in leaf disks from Vicia faba (cv. Reina Blanca) leaves were doně during the first weeks of growth in order to evaluate the damage caused by methabenzthiazuron (MBT) under field conditions. The plants were treated at preemergence with two doses of MBT (0.25 .and 0,40 g m'^). Measurements were carried out at constant temperature and saturating CO2 concentration. During the first weeks after plant emergence both doses of herbicide were followed by a drop in and in some fluorescence parameters, such as Fy/Fn, and AF/Fn,' ratios, and in photochemical and non-photochemical quenching, qp and q^. At the same time, increases in minimal fluorescence (Fq and Fq') and in maximal (Fj„') and steady-state fluorescence (Fj) were aiso observed on application of saturating pulses. Fluorescence parameters gave a better indication of the damage caused to the photosynthetic systém than the measurements. The linear relationship found between OO2 and the effective quantum yield, AF/Fn,', suggests that the latter is a good measure of the quantum efficiency of photosynthesis in herbicide-treated plants. Moreover, plant tissues detoxified the herbicide and, one month after plant emergence, photosynthesis was fully recovered in herbicide-treated plants; biomass production recovered and even increased at the end of the growth period.
Light is a limiting factor in plant establishment and growth in the understory of forests. In this paper, we assessed acclimation capacity of Siparuna guianensis, an early secondary successional species. We used seedlings and saplings in three regeneration areas with different irradiance regimes to determine the traits that confer photoplasticity. We examined whether these traits differ at different developmental stages. Anatomical characteristics, photochemical efficiency, photosynthetic capacity, and growth were analyzed. Multivariate component analysis revealed the formation of six clusters: three for seedlings (one for each regeneration area) and three for saplings (following the same pattern of seedlings, considering the area). Increased irradiance favored photosynthetic performance, independently of the developmental stage. The same trend was observed for most data on chlorophyll (Chl) a fluorescence and the ratios of net photosynthetic rate/intercellular CO2 concentration (PN/Ci) and PN/PPFD. No parameter indicated photoinhibition stress. The CO2- and light-response curve data indicated that seedlings were already acclimated to tolerate variation in irradiance. Anatomical adaptations, such as thickness of leaf blade and of adaxial cuticle, were observed in individuals growing in areas with higher irradiation. Thinning of spongy parenchyma and higher investment into a plant height were observed in seedlings, possibly due to the vertical stratification of CO2 and light in the understory; because light is a more limiting resource than CO2 in the lower stratum of the forest. Photoplasticity in S. guianensis is associated with a set of morphological, anatomical, photochemical, and biochemical traits, whereas biochemical performance is best acclimated to variation in irradiance. These traits differed in seedlings and saplings but they were modulated mainly by irradiance in both developmental stages., T. O. Vieira, M. S. O. Degli-Esposti, G. M. Souza, G. R. Rabelo, M. Da Cunha, A. P. Vitória., and Obsahuje bibliografii