This work focused on the effects of aluminum (Al) stress on photosynthetic apparatus using Al-tolerant and sensitive cultivars of sorghum (Sorghum bicolor (L.) Moench). Plants were grown in Clark's nutrient solution aerated continuously and adjusted daily to pH 4.0 and containing two Al concentrations (0 and 185 mmol m À 3 ) supplied as Al 2 (SO 4 ) 3 Á 18H 2 O. Measurements were performed on recently expanded leaves from the eighth to sixteenth day after imposing Al treatments. In general, chloroplastidic pigment concentration, chlorophyll fluorescence parameters, and photosynthetic rates were initially affected by Al stress to a greater extent in the Al-tolerant cultivar than in the susceptible cultivar. However, the Al-tolerant genotype JOURNAL OF PLANT NUTRITION, 25(4), 821-832 (2002) 821 seemed to cope with high Al via progressive, sustained decreases in initial chlorophyll fluorescence and increases in chlorophyll a=b ratio, photosystem II photochemical efficiency, net carbon assimilation rate, and photosynthetic oxygen evolution. In contrast, the Al-sensitive cultivar did not show any apparent acclimation response to high Al application. The results suggest that the harmful effects of Al on photosynthesis might be partially alleviated through time-dependent mechanisms, particularly in the Al-tolerant sorghum cultivar.

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The actual (A) and potential (Apot) photosynthetic rates of C3 and C4 tropical crop species grown under greenhouse conditions was compared. The following species were investigated: Oryza sativa, Phaseolus vulgaris, Glycine max, Helianthus annuus, Gossypium hirsutum, Manihot esculenta, Theobroma cacao, Coffea arabica, Hevea brasiliensis, and Eucalyptus urophylla × E. grandis, all from the C3 group, and Amaranthus sp., Panicum maximum, Pennisetum purpureum, Zea mays and Saccharum officinarum, from the C4 group. A, determined under non-limiting light at ambient temperature and CO2, was measured with an infrared gas analyser, whilst Apot, determined under saturating light and CO2 at an optimal temperature (35 ºC for all species), was gauged with a gas-phase oxygen electrode. On an area basis, A varied from 5.0 up to 26.3 mumol CO2 m-2 s-1, whilst Apot was very similar in 14 of the 15 species, with an average rate of 35.0 ± 2.4 mumol O2 m-2 s-1. The value of Apot in T. cacao was approximately half the mean of the remaining species. On a mass basis, variations in A were much larger, and differences in Apot, although not large, emerged. The overall mean Apot per unit mass in the four tree species was 28.0 ± 2.2 mumol O2 g-1 min-1 against 44.6 ± 5.8 mumol O2 g-1 min-1 in the remaining species. As a whole, the results evidenced a conservative behaviour of the photosynthetic apparatus to fix CO2 amongst the species investigated, despite the large differences in A among them.