Biochemical Changes that Occur during Senescence of Wheat Leaves

Abstract

Changes in activities of photosynthetic enzymes and photochemical processes were followed with aging of vegetative and flag leaves of wheat (Triticum aestivum L. cv Roy). Activities of stromal enzymes began to decline prior to photochemical activities. In general, total soluble protein and the activities of ribulose-1,5-bisphosphate carboxylase and NADPtriose-phosphate dehydrogenase decUned in parallel and at an earlier age than leaf chlorophyll (Chl), leaf photosynthesis, and photosynthetic electron transport activity. Leaves (8) in Phaseolus vulgaris L. observed a continual decline in electron transport per mg Chl, which suggested a gradual degradation of -all chloroplasts. Studies of electron transport as well as other physiological parameters are needed before definitive conclusions can be made as to the loss of whole chloroplasts from wheat leaves.In the present study, the time course of leaf development from full expansion through senescence was investigated in vegetative and flag leaves of wheat to determine the basis for reduction of photosynthesis. Specific objectives of this study were to determine (a) if electron transport activities or stromal enzyme activities declined first and (b) if there was a general decline in the photosynthetic functioning of all chloroplasts or if there was a preferential degradation of whole chloroplasts.Senescence is a coordinated, deteriorative growth process that is initiated at full maturity and ultimately leads to the death of a cell, organ, or organism. To characterize a tissue that is undergoing senescence, it is necessary to identify the sequential order in which various parameters change. Butler and Simon (5) reported from ultrastructural studies that aging is first expressed in the chloroplast. Thus, changes in the physiology of this organelle are important to our understanding of senescence.In leaves, senescence is typically characterized by a decline in MATERIALS AND METHODS Plant Material. Triticum aestivum L. cv Roy was grown in a growth chamber with a photon fluence rate of 3 x 1 -O mol/m2. s (400-700 nm) and a 15-h photoperiod (22°C day/17°C night). For vegetative leaves, the second leaf to emerge was studied from the time of full expansion (about 14 d from planting) through senescence. Flag leaves were sampled from the point of emergence of the head from its sheath (arbitrarily designated day 0) through senescence. Flag leaves were fully expanded at this stage. Reproductive development was induced by vernalization of 12-d-old seedlings for 6 weeks at 2 to 4°C. All plants were fertilized twice weekly with Hoagland solution.Two separate plantings of both vegetative and flag leaf material were used. Two to four replications of each parameter for each age were performed and the results from both plantings were averaged.Leaf Area, Chi, and Protein Assays. Leaf area was measured with a Li-Cor3 area meter (model LI-3000, Li-Cor, Inc.). Chl and Chl a/b ratios were determined by the procedure of Arnon (1).

Photosynthetic Characteristics of Rice Leaves Aged under Different Irradiances from Full Expansion through Senescence

“…2). Similar results were reported by Camp et al (3) in senescent wheat leaves. Also, Makino et al (21) found a strong correlation between Rubisco and ribulose 5-phosphate kinase during leafsenescence.…”

“…Several quantitative analyses indicated that decrease in photosynthesis under ambient air conditions can be predicted from changes in the amounts and kinetics of Rubisco (5,23). Electron transport activities and the reaction-center complexes also decreased during leaf senescence, and their positive correlations with photosynthesis have been frequently reported (3,(12)(13)(14). Thus, it can be expected that a decline in photosyn- 'This work was supported in part by Grants-in-Aid for International Scientific Research (63044018) from the Ministry ofEducation, Science and Culture of Japan.…”

Photosynthesis and Ribulose 1,5-Bisphosphate Carboxylase in Rice Leaves

“…These results are similar to those obtained with barley (7). Recently, Camp et al (6), using wheat leaves, reported that the change in Chl content most closely paralleled the change in photosynthesis and RuBP carboxylase activity was not highly correlated with photosynthesis. One possible reason for this discrepancy is the difference between the photosynthetic measurement methods.…”

“…Friedrich and Huffaker (7) and Peoples et al (24) found that loss of carboxylase activity during senescence is caused by loss of enzyme protein, whereas no correlation between them was reported by Hall et al (9). However, Camp et al (6) have recently reported that RuBP carboxylase activity decreases at a much faster rate than the photosynthetic rate during senescence. Thus, the role of RuBP carboxylase responsible for the change in photosynthesis with leaf age remains uncertain.…”

Regulation of Photosynthesis during Leaf Development in RbcS Antisense DNA Mutants of Tobacco

“…However, our findings raise the question whether the decreases in plastid DNA during senescence occur on a per-plastid basis, as suggested in rice (Sodmergen et al, 1991), or whether they are due to the destruction of entire plastids. Although reduced chloroplast numbers are observed in some species throughout the senescence process (Lamppa et al, 1980;Camp et al, 1982;Wittenbach et al, 1982;Kura-Hotta et al, 1990), in other species plastid function is uniformly impaired during early senescence and whole plastids are lost only late in the process (Mae et al, 1984;Wardley et al, 1984;Ford and Shibles, 1988). This topic has not yet been investigated in tobacco.…”