When individual seedlings of Datura ferox and Sinapis alba were transferred to populations formed by plants of similar stature, they responded with an increase in the rate of stem elongation. The reaction was detected within 3 days after transplanting and occurred well before shading among neighbors became important. This rapid response, which may be crucial for success in the competition for light, was reduced or abolished when individual internodes were "blinded" to the far-red radiation scattered by the surrounding seedlings. These results show the operation of a localized, photomorphogenetic control of stem elongation that may play a central role in the plastic adjustment of plants during the early stages of canopy development.

We have tested the hypothesis that a plant may detect the ptesence of a neighbour, befote being shaded by it, thtough the petception of the spectt-al composition of reflected sunlight. Within seedlitig canopies the red : far-red t-atio (R : FR) of the light received by a sensor with a geotnett-y appt-oxitnating that of a stetn was significantly teduced by selective reflection. This effect was observed befote any reduction in the atnount of photosytithetic light energy received by an individual seedling could be detected. Stnall gt-een fences of grass, east-west orientated, alteted the specttal distribution of the light on the north (sunlit) side of thetn. Fully illuminated seedlings of Sinapis alba giown on the north side of these gt-een fences produced longer internodes and had a lower leaf: stetn dry weight ratio than those grown in ftont of fences of bleached grasses. A sitnilar tedistribution of gtowth was elicited in seedlings of Chettopodinm album. Datura ferox and S. alba growing iti full sunlight by exposing plants to additional small quantities of fat-red reliected by seleetive tnirtot-s. These t-esults suggest that the change in the R: FR ratio serves as an early warning signal of oncotning cotnpetition.

Colorless phenylpropanoid derivatives are known to protect plants from ultraviolet (UV) radiation, but their photoregulation and physiological roles under field conditions have not been investigated in detail. Here we describe a fast method to estimate the degree of UV penetration into photosynthetic tissue, which is based on chlorophyll fluorescence imaging. In Arabidopsis this technique clearly separated the UV-hypersensitive transparent testa (tt) tt5 and tt6 mutants from the wild type (WT) and tt3, tt4, and tt7 mutants. In field-grown soybean (Glycine max), we found significant differences in UV penetration among cultivars with different levels of leaf phenolics, and between plants grown under contrasting levels of solar UV-B. The reduction in UV penetration induced by ambient UV-B had direct implications for DNA integrity in the underlying leaf tissue; thus, the number of cyclobutane pyrimidine dimers caused by a short exposure to solar UV-B was much larger in leaves with high UV transmittance than in leaves pretreated with solar UV-B to increase the content phenylpropanoids. Most of the phenylpropanoid response to solar UV in field-grown soybeans was induced by the UV-B component ( < 315 nm). Our results indicate that phenolic sunscreens in soybean are highly responsive to the wavelengths that are most affected by variations in ozone levels, and that they play an important role in UV protection in the field.