Monochromatic light was produced by filtering white light
(from air-cooled xenon arc lamps) through band pass filters (AX
about 150 nm) and water-cooled interference filters of a half-band
width of 20 nm (type DAL tandem filters made by Schott, Mainz,
Germany). The monochromatic light was then passed into a box
from which all other light was excluded. (An earlier version of the
equipment is described in reference 14.) Quantum flux density
was determined with silicon cells that were calibrated against
Eppley thermopiles at each of the lamp-filter combinations used.
Plants. X strumarium L. (Chicago strain) and P. frutescens (L.)
Britt. (red-leaved Perilla) were grown in a gravel-soil mixture in
a greenhouse. The natural photoperiod was extended to 20 h with
0.3 w m-2 light from Sylvania Gro-lux fluorescent tubes. Temper-
ature maxima were between 23 and 29 C; the RH was between 70
and 80%1o. For X strumarium, the fifth or sixth leaf from the apex
of 6- to 12-week-old plants was used. For P. frutescens, the
youngest fully expanded leaf pair of one month old plants was
used for the analysis. The other experimental plants were culti-
vated in a growth chamber in which irradiation from General
Electric lamps H 400 DX 33-1 (mercury vapor) and LU 400 (high
temperature sodium vapor) was increased in three steps during the
day to 230 w m-2 (400-700 nm) and then decreased. RH was near
75%. Plants of X strumarium that were required for the experi-
ments with monochromatic light were grown in a 20 h day at 24/
20 C, day/night, in pots supplied with water through 20 cm x 2.5
cm wicks of which 15 cm were in contact with the soil and 5 cm
dipped into a gravel bed containing deionized water. Z. mays L.
cv. Michigan 500, was grown in a soil-Perlite mixture in a 12 h
day at 30/20 C. The fifth leaf (by emergence) of 3- to 4-week-old
plants was subjected to the measurements. The same soil mixture
was used for the cultivation of plants of G. hirsutum L., cv. Acala
SJ- 1, in a 20-h day at 32/22 C; the behavior of fifth leaves (from
the apex down) of 1- to 2-month-old plants was analyzed. Plants
of P. vulgaris L., cv. Montcalm, grew in a gravel-vermiculite
mixture at 23/20 C; the RH was about 80%1o. Irradiance was 180 w
m-2 (between 400 and 700 nm) for 12 h/day, the day length was
extended to 20 h by incandescent lamps. The primary leaves of
plants 10- to 15-days-old were used for analysis.