U. C. Biswal

PHOTOCONTROL OF LEAF SENESCENCE

Senescence is the last phase of leaf development. It is basically a process of degreening and produces changes at all levels of cellular and sub-cellular organizations of leaves. The precise mechanism of senescence induction is not known, although the process, in principle. is known to be under genetic control (Stoddart and Thomas, 1982). Once senescence is triggered, it is subject to environmental modulations and the reports on the effects of various environmental factors on leaf senescence are extensive (Biswal et al., 1983b). Light, one of the important environmental factors, plays a very crucial role in photosynthetic green plants. In addition to its role in photosynthesis, it also decisively controls plant development and morphogenesis (Mohr and Shropshire. 1983). Light absorption by various photoreceptors, transmission of the signal and the final photoresponses in the greening process are well investigated (Kasemir. 1983). However, nothing definite is known about the mechanism of light action in controlling leaf senescence, although light has been known either to stimulate or retard the process (Maunders and Brown. 1983; Biswal et al., 1983a,b,c; Panigrahi and Biswal, 1983; Satler and Thimann, 1983; Iordanov and M. G. Merakchilska-Nikolova, 1983; Rao, 1983; Schmutz et al . , 1983). It has been known that photostimulation of chlorophyll loss during leaf senescence is mostly non-metabolic. The major photoreceptors involved are chlorophylls which are photobleached at high intensity of light. This may cause a significant loss in photosynthetic efficiency of green leaves leading ultimately to their senescence. This type of senescence normally occurs in deciduous plants. Maunders and Brown (1983) have shown light to degrade chlorophyll faster than dark. On the other hand, the differences between the leaves incubated in dark and light with respect to soluble protein, cytoplasmic RNA and free aminonitrogen are much less marked. These results would suggest that light action on chlorophyll degradation is non-metabolic in nature since it does not show its effect on other senscence-associated changes. It is therefore important to differentiate between situations where light has got a direct photochemical role in the Photostimulation of leaf senescence. degradation of chlorophyll and light as a metabolic modulator of pigment degradation during leaf senescence. Whether light should act as a metabolic modulator or a photodecomposer depends on the intensity of light and the plant system. This is clearly demonstrated by Pjon (1981) in leaf senescence of maize and hydrangea. He has shown the differential ability of leaf tissue of maize and hydrangea to dissipate their absorbed light quanta. The loss of chlorophyll in maize leaves has been attributed to the process of photooxidation of pigments, because continuous white light results in loss of maize leaf chlorophyll. but retards the loss of protein. Photobleaching of chlorophyll in this system may be compared with the degradation of the pigments during aging of isolated chloroplasts (Raval et al., 1982; Panigrahi and Biswal, 1983). Recent reports of Lichtenthaler et a l . (1982) and Meier and Lichtenthaler (1983) provide new information on photostimulation of leaf senescence of seedlings grown in high and low light intensities. They have demonstrated that compared to low light, strong light (light quanta fluence rates) leads to the formation of sun type chloroplasts which exhibit high photosynthetic rates and simultaneously an acceleration of leaf development, which also results in an earlier leaf senescence. High light intensities not on ly accelerate chlorophyll breakdown, but also lead to ultrastructural modifications and loss in soluble carbohydrates. These changes, in addition to the breakdown of chlorophyll as shown by the authors, prevent a generalization that light stimulation of senescence is because of simple photooxidation of chlorophylls only. The action of light in controlling leaf senescence is normally attributed to its effect in delaying senescence. It is to the economy of leaves that these should remain in a functional state for as long as possible once the energy has been invested in their formation. Therefore, it is logical to imagine the retarding effect of light in senescence once the leaves are matured. The reports on the precise role of light in retarding the process are controversial with mainly two different views, whether the action is photosynthetic or photomorphogenic (Biswal et al., 1983b). This review is an attempt to report and rationalize the work from various laboratories engaged in this field. Since this is the first review in the field, some of the Photoretardation of leaf senescence.

Effect of u.v.-A on aging of wheat leaves and role of phytochrome

Abstract Leaf aging, characterized by loss of chlorophyll (Chl) and protein, resulting from ultraviolet A (u.v.-A) irradiation, was assessed in primary leaves of wheat seedlings. Irradiation of seedlings with u.v.-A enhanced aging-induced loss of Chl and protein in leaves held either in light or in darkness. Dark incubation of light-grown seedlings caused a faster loss of Chl which was further enhanced with u.v.-A exposure. Enhancement of pigment loss was retarded if the u.v.-A. exposure was followed by a 5 min pulse of red light. The red-far-red reversibility of this response indicated the involvement of phytochrome in retarding u.v.-A-induced leaf damage in darkness. The results of an interaction of u.v.-A and red-light pulses in dark-aging leaves indicated that u.v.-A treatment enhanced sensitivity to phytochrome. The role of phytochrome in retarding u.v.-A-induced damage is attributed to the suppressive action of the photoreceptor on u.v.-induced lipid peroxidation as measured by accumulation of malondialdehyde (MDA).

Ultraviolet-A induced changes in photosystem II of thylakoids: effects of senescence and high growth temperature

Ultraviolet-A (UV-A) radiation induced changes in photosystem II (PS II) of senescing leaves of wheat seedlings were investigated. UV-A radiation did not show any significant effect on the level of photosynthetic pigments. However, the decline in F(v)/F(m) and oxygen evolution rate indicated the damaging effect of the radiation on primary photochemistry of PS II. Modification at the Q(B)-binding site was inferred from the observed downshift of peak temperature of thermoluminescence (TL) B-bands. The UV-A induced changes in PS II of chloroplasts from senescing leaves were found to be synergistically accelerated by high growth temperature.

Senescence-induced loss in photosynthesis enhances cell wall β-glucosidase activity.

A link between senescence-induced decline in photosynthesis and activity of beta-glucosidase is examined in the leaves of Arabidopsis. The enzyme is purified and characterized. The molecular weight of the enzyme is 58 kDa. It shows maximum activity at pH 5.5 and at temperature of 50 degrees C. Photosynthetic measurements and activity of the enzyme are conducted at different developmental stages including senescence of leaves. Senescence causes a significant loss in total chlorophyll, stomatal conductance, rate of evaporation and in the ability of the leaves for carbon dioxide fixation. The process also brings about a decline in oxygen evolution, quantum yield of photosystem II (PS II) and quantum efficiency of PS II photochemistry of thylakoid membrane. The loss in photosynthesis is accompanied by a significant increase in the activity of the cell wall-bound beta-glucosidase that breaks down polysaccharides to soluble sugars. The loss in photosynthesis as a signal for the enhancement in the activity of the enzyme is confirmed from the observation that incubation of excised mature leaves in continuous dark or in light with a photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) that leads to sugar starvation enhances the activity of the enzyme. The work suggests that in the background of photosynthetic decline, the polysaccharides bound to cell wall that remains intact even during late phase of senescence may be the last target of senescing leaves for a possible source of sugar for remobilization and completion of the energy-dependent senescence program.

Response of senescing wheat leaves to ultraviolet a light: Changes in energy transfer efficiency and PS II photochemistry

Response of senescing leaves of wheat seedlings to ultraviolet A (UVA) radiation (365 nm) has been examined. The results indicate that senescence-induced disorganization of thylakoid membrane, decline in carotenoid-to-chlorophyll energy transfer, and enhancement of lipid peroxidation are furthered by radiation. The senescence-induced decline in photochemical activity of photosystem II further declines on irradiation. UVA does not specifically alter any site other than those damaged by senescence.

UV-A Irradiation Guards the Photosynthetic Apparatus Against UV-B-Induced Damage

In clusterbean leaves UV-B radiation caused a reduction in contents of chlorophylls and carotenoids and in the efficiency of photosystem 2 photochemistry. The degree of damage was reduced when UV-A accompanied the UV-B radiation. This indicates the counteracting effect of UV-A radiation against UV-B-induced impairment.

Plastid development in leaves during growth and senescence

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Topology and photoprotective role of carotenoids in photosystem II of chloroplast: a hypothesis.

On the basis of existing evidence, a model is proposed for the topology of carotenoids in photosystem II (PS II) of chloroplasts. suggesting their possible roles in the photoprotection and stability of PS II complex. The presence of one cis and one trans beta-carotene at reaction centre II (RC II), with different photoprotective functions, is suggested. The core antennae (CP43, CP47) are presumed to contain beta-carotene molecules in clusters. The possible molecular mechanism for formation of a quenching complex in the minor LHC II, involving zeaxanthin, chlorophyll a and the glutamic acid side chain of the light harvesting protein, is worked out. This complex is proposed to be an efficient triplet quencher, in addition to its role as a quencher of singlet chlorophyll energy. The migration of triplet energy from RC II to the quenching complex is surmised. It is suggested that the carotenoids in RC II and in different LHCs form an integrated photoprotective unit.

Response of senescing leaves of wheat seedlings to UVA radiation: inhibition of PSII activity in light and darkness

Abstract Ultraviolet A (UVA) induced inhibition in photosystem II (PSII) photochemistry of primary leaves of wheat seedlings senescing in light or darkness has been examined. O 2 evolution and 2,6-dichlorophenol indophenol (DCPIP) photoreduction diminished in the presence of light. Results of thermoluminescence (TL) studies suggest that UVA inhibits the activity of PSII by accelerating the process of senescence-induced modifications of Q B protein conformation and/or S-state transitions.

Alteration in photosystem II photochemistry of thylakoids isolated from senescing leaves of wheat seedlings

Abstract Wheat seedlings, grown for 7 days in the light, were allowed to senesce in the light or dark, and the change in the photosystem II (PS II) photochemistry of chloroplasts isolated from the primary leaves of these seedlings was investigated. The decrease in oxygen evolution and the fast fluorescence results indicated that the impairment of PS II in the leaves of seedlings senescing in the light was different from that in the leaves of seedlings senescing in the dark. Thermoluminescence studies showed a structural modification in the Q B protein of chloroplasts isolated from leaves senescing in the light and an alteration in the S state transition of chloroplasts isolated from leaves senescing in the dark.