The yield of 24 commercial varieties and accessions of common bean (Phaseolus vulgaris) has been determined at different sites in Chile and Bolivia. Statistical analysis was performed in order to characterize whether a particular variety was more or less stable in yield under different environmental conditions. Amongst these, two varieties have been identified for more detailed study: one variety has a higher than average yield under unstressed conditions but is strongly affected by stress, and another has a reduced yield under unstressed conditions but is less affected by stress. The contrasting rate of abscission of the reproductive organs under drought stress was clearly consistent with these differences. The more tolerant genotype shows a great deal of plasticity at the biochemical and cellular level when exposed to drought stress, in terms of stomatal conductance, photosynthetic rate, abscisic acid synthesis, and resistance to photoinhibition. By contrast, the former lacks such plasticity, but shows an enhanced tendency for a morphological response, the movement of leaves, which appears to be its principal response to drought stress.

The aim of the present study was to investigate if the depletion of UV-B radiation affected the most representative carotenoids as well as the ascorbic acid content in tomato fruits, harvested at both breaker and firm red stages. To do this, three tomato genotypes, DRW 5981, HP 1, and Esperanza, were grown inside a greenhouse either covered with polyethylene transparent to UV-B or depleted of UV-B by a special covering film. The antioxidant properties of the fruits were evaluated on the water-insoluble fractions according to the ABTS method. UV-B effect on antioxidant activity was negligible in DRW and HP 1 genotypes, whereas it was detrimental in Esperanza at both ripening stages. This genotype seems to have a negligible capability of accumulating carotenoids and a great susceptibility to detrimental effects of UV-B; conversely, the DRW genotype shows high carotenoid levels under sunlight conditions and a further promotion by UV-B. On the other hand, the HP 1 mutant displays an intermediate behavior and represents the only genotype favored by UV-B with respect to ascorbic acid accumulation.

Experiments designed to study the effect of light quality and quantity and their possible interaction with benzyladenine (BA) in the control of in vitro proliferation of Prunus insititia Schneider GF 655–2 microcuttings are reported. The action of BA as a promoting factor of shoot formation was expressed only in the presence of light. The concentration response curves for BA‐induced proliferation were very similar under the different light sources, irrespective of proliferation rate values. Shoot formation under blue, far‐red and white light was enhanced by the highest photon fluence rates, while the efficiency of red seemed to be independent of this factor. The results suggest the action of a low‐energy response in the red waveband and under the low photon fluence rates of blue and white. The inhibition of shoot elongation induced by BA in the dark as well as under all light treatments indicates that, while the BA‐induced release from the apical dominance is light dependent. BA inhibition of shoot elongation is entirely light independent.

Under natural photoperiodic conditions protandry in hermaphrodite disc flowers of sunflower (Helianthus annuus L.) is determined by the different elongation rates of the style and filaments. The elongation of the filament and style starts simultaneously after the daily dark period, but the style growth rate is slower. When plants close to anthesis are exposed to continuous white light (WL) a loss of protandry occurs: the filaments do not grow far enough to extrude the anthers from the corolla. The histological analyses show that the number of filament epidermal cells remains unaltered after organ elongation and that cells respond to photoperiod only by cell expansion. Emasculation does not substantially inhibit filament cell expansion, whereas isolation of the filament or stamen from the corolla suggests that this organ could be the perception site of the filament growth stimulus. In vitro treatments with auxin (indole-3-acetic acid, IAA or alpha-naphthaleneacetic acid, NAA) reverses the inhibition of cell expansion caused by continuous WL, whereas gibberellic acid (GA(3)) at high concentrations reproduces the effect of continuous WL. Experiments carried out on various Helianthus spp. show that all these plants have evolved the same photo- and hormonal-control of the protandry. In experiments in which the light treatments were continued for 24 h, the auxins drastically reduced the inhibiting effect of red light (R) and dichromatic treatments FR (far red)+R, whereas GA(3) repressed filament extension regardless of light quality. As far as auxins are concerned, the response of sunflower filaments does not appear to be connected with the polar transport of the hormone. Moreover, the promoting effect of darkness is not mediated by an increase of endogenous free IAA in disc flowers. However, sunflower filaments manifested a similar temporal pattern of response to the light/dark cycle and to auxin.

The UV‐mediated induction of anthocyanin and UV‐absorbing compounds was characterized in etiolated hypocotyls of wild‐type and aurea (au) mutant tomato seedlings. Ultraviolet radiation induced significant increases of anthocyanin and UV‐absorbing compounds in hypocotyls of die au mutant and of its isogenic wild‐type, but the differences in the time courses of UV‐induced pigment accumulation indicate mat different photoregulatory mechanisms are involved for each of these two groups of pigments. It appears mat prolonged presence of adequate levels of UVB (290–320nm) energy and consequently the action of a specific UVB photoreceptor are indispensable for the photoinduction of anthocyanin accumulation in UV‐irradiated hypocotyl of the au mutant that is missing the labile phytochrome pool. The large difference found between the wild‐type and the au mutant strongly indicate the involvement of labile phytochrome as the primary functional photoreceptor for the photoinduction of anthocyanin accumulation in wild‐type tomato hypocotyls. The UVB photoreceptor could at least partly replace the action of labile phytochrome (as far as anthocyanin accumulation is concerned) when the functional phytochrome pool is missing as in the au mutant. The general picture of UV‐mediated induction of total UV‐absorbing compounds shows only a macroscopic difference between wild‐type and die au mutant of tomato: the higher initial level (in darkness) of these compounds in die wild‐type in contrast to the au mutant. Although there is UV‐induced accumulation of UV‐absorbing compounds in bom genotypes, the levels in the au mutant never reach mat of the wild‐type under the same UV exposure. A UVB photosensor may play a more important role in the photoinduction of UV‐absorbing compounds. Indeed, in the absence of labile phytochrome, i.e. in the au mutant, a UVB‐absorbing photoreceptor alone is able to establish high responsiveness for the UV‐induced flavonoid accumulation.