J Integr Plant Biol. ›› 1964, Vol. 12 ›› Issue (1): -.

• Research Articles •    

The Presence of Two Light-Dependent Enzymatic Processes Accompanying Fluorescence Decay in Wheat Leaves during Photosynthesis

P.C. Chow, S.C. Lin, Z.C. Chu, K.F. Shien and P.S. Tang   

Abstract: Excised leaves of wheat (var. Lungta 183) previously kept in the dark for 30 minutes were illuminated with light of wave lengths shorter than 525 μm and the time course of their fluorescence decay was followed at 685 μm for up to 6 minutes (or longer) in respect to temperature and certain metabolic inhibitors. The intensity of fluorescence decreased from an initial maximum to a lower constant level within a few minutes with a clearly defined second peak (Fig. 2). The higher initial intensities may be restored by periods of darkness, so that the decay process can be repeated by periods of illumination following periods of darkness (Fig. 3). Within the temperature range studied (2–45℃) the rate of fluorescence decay as well as the time of appearance and amplitude of the second rise were temperature dependent (Fig. 2). An increase in temperature hastened the first two items, while the amplitude of the secondary rise had a negative temperature coefficient (Fig. 2). No peak appeared at 2 ℃ even after prolonging the experiment to 15 minutes, and in leaves which were subjected to 45 ℃ the decay as well as the second rise were abolished (see Chow et al, 1963). The behaviours of the fluorescence decay and that of the second rise toward metabolic inhibitors are different. In general, the former is sensitive to those affecting the TCA (tricarboxylic acid) pathway of carbohydrate oxidation and the latter to those for electron transfer and phosphorylation. Sodium azide (10–4 to 10–2 M) has no effect on fluorescence decay (Fig. 5). DNP (2,4-dinitophenol) and KCN partially inhibit the process at higher concentration (5 × 10-3M) and are without effect at lower (10–4 M) concentrations (Fig. 4). On the other hand, the second peak was significantly inhibited by Na-azide, KCN and DNP, and the inhibition was proportional to concentration (5 × 10–3 M to 10–4 M). Malonate was without effect on the second rise (Fig. 6), but it significantly inhibited (at 5 × 10-2 M) the fluorescence decay (Fig. 6, 1). This inhibition was reversed by addition of succinate (Fig. 6, 2) or fumarate (Fig. 6, 3). It is concluded from these experiments that two light-dependent enzyme catalyzed reactions are related to the phenomenon of fluorescence decay in these wheat leaves. One of these, the second rise, is affected by azide and DNP, and may therefore be concerned with electron transfer and phosphorylation, and the other, which is malonate sensitive, may be concerned with the TCA pathway of carbohydrate metabolism.

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