Photoinduced Changes in the Chlorophyll a to Chlorophyll b Ratio in Young Bean Plants

Abstract

An alternating light-dark system is described under which etiolated bean (Phaseolus vulgaris) leaves form selectively chlorophyll a.This system consists of cycles of 2 minutes of white light alternating with 98-minute dark periods.Etiolated 80 cm above the plants, were used; one light provided to the plants 120 lux (75-w lamp) and the other, 2000 lux (Phillips Ultrilux, 500 w).The chlorophylls were extracted totally by grinding 1 g of leaves in a mortar with 4 ml of pure acetone, followed by several extractions with 80% acetone to a final volume of about 30 ml. The extract was filtered twice, and the absorbance was measured in a Bausch & Lomb Spectronic 505 spectrophotometer. The chlorophyll concentration was calculated accordingto MacKinney (8). In some cases the pigments were transferred to ether, after which absorbancies were measured. The Chl a, Chl b, and protochlorophyllide concentrations were then calculated according to Koski (7). In some samples the pigments were separated by paper chromatography, according to Chiba and Nogushi (5) In some experiments, the leaves, after exposure to the 120-lux light-dark system, were further exposed to 1500 lux for 30 sec. The pigments were then extracted and their concentrations were measured after transfer to ether. It was thus found that exposure of the leaves to 120 lux for 2 min transformed most of the protochlorophyllide to chlorophyllide; only 3 or 4 ,g of protochlorophyllide were left, of which about 1 jg could be phototransformed to chlorophyllide after further exposure of the leaves to 1500 lux for 30 sec.The absolute amounts of Chl a and Chl b synthesized under the light-dark cycles system are shown in Figure 1. It is evident that the amount of Chl a formed increased with the number of light exposures to which the plants were subjected. The rate of Chl a biosynthesis remains constant during the first 40 to 50 cycles. This is expected, since during the 98-min dark period the amount of protochlorophyllide synthesized by the plant tissue has reached its maximal value (4). Eventually a plateau is reached when the net production of protochlorophyllide ceases (4). Figure 1 also indicates a negligible accumulation of Chi b, especially after the first 40 to 50 light-dark cycles. This accumulation was found to start earlier in older plants.In some samples of 4-day-old leaves exposed to less than 30 low intensity light-dark cycles, Chl b was found, spectrophotometrically, to be present in 3-or 4-ug quantities per g fresh weight. However, it was impossible to separate and trace by chromatography any Chl b from these samples. Figure 2 shows the absorption spectrum of an 80% acetone extract of plants subjected to 31 high intensity light-dark cycles. The absorption peaks of pure Chl a are evident, as well as a peak at 473 nm which is due to carotene. Comparison is also made with the spectrum of chromatographically pure chlorophyll a. The concentration of the latter was adjusted in order to obtain equal, maximal absorbancies for the extract and the solution at th...

Development of the Two Heterogeneous Photosystem II Units in Etiolated Bean Leaves

“…The leaves were illuminated by either bright continuous light, or by intermittent light (2-min light and 98-min dark cycles). Other conditions for illumination were described by Argyroudi-Akoyunoglou and Akoyunoglou (3).…”

“…Under conditions of continuous illumination the etioplasts quickly show the characteristics of developed chloroplasts (1,13,22). Intermittent illumination (2 min light and 98 min dark), however, transforms the etioplasts into "protochloroplasts" which show higher photochemical activity per Chl than chloroplasts (1), synthesize selectively Chl a, and are devoid of Chl b and grana (3,6), contain "primary" thylakoids and are deficient in the Chl-protein complex II which is considered to be an essential component of grana stacks (4,6).…”

Chloramphenicol‐induced stabilization of light‐harvesting complexes in thylakoids during development

“…6-day etiolated Phaseolus vulgaris leaves were used, grown and handled as in [1]. The leaves with one cotyledon removed were either exposed to ImL [2 rain white light alternating with 98 rain dark in cycles], or to CL for 18 h and then transferred to the dark.…”

“…Leaves handled in the same manner but immersed in H20 were used as controls. Chl was extracted from leaves as in [1] and determined according to [13]. PS I and PS II activities were monitored as in [ 11 ].…”

“…The rate of Chl accumulation in this case is high and parallels that of the other thylakoid components. In plants exposed to intermittent light, however, Chl is bound selectively on RC proteins and only Abbreviations: PS, photosystem; LHC, light-harvesting complex; Chl, chlorophyll; CPI, P700-Chl a protein complex of PS I; CPIa, PSI Chl-protein complex containing CPI and LHC-I; CAP, chloramphenicol; CL, continuous light; LDC, light-dark cycles; ImL, intermittent light; D, dark; cyt., cytochrome RC core complexes are stabilized in thylakoids [1,2]. Since no LHCs can be detected in the membrane, in spite of the fact that the mRNA for the LHC-II apoprotein is present and can be translated [3], it has been proposed that LHC stabilization is controlled at the post-translational level [4,5]: whenever the rate of Chl accumulation is reduced, while polypeptide synthesis remains unaffected, there is a competition between polypeptides for the small amount of Chl available.…”