Characterization of photosynthetic performance during senescence in stay-green and quick-leaf-senescence Zea mays L. inbred lines

Abstract

The net photosynthetic rate, chlorophyll content, chlorophyll fluorescence and 820 nm transmission were investigated to explore the behavior of the photosynthetic apparatus, including light absorption, energy transformation and the photoactivities of photosystem II (PSII) and photosystem I (PSI) during senescence in the stay-green inbred line of maize (Zea mays) Q319 and the quick-leaf-senescence inbred line of maize HZ4. The relationship between the photosynthetic performance and the decrease in chlorophyll content in the two inbred lines was also studied. Both the field and laboratory data indicated that the chlorophyll content, net photosynthetic rate, and the photoactivities of PSII and PSI decreased later and slower in Q319 than in HZ4, indicating that Q319 is a functional stay-green inbred line. In order to avoid the influence of different development stages and environmental factors on senescence, age-matched detached leaf segments from the two inbred lines were treated with ethephon under controlled conditions to induce senescence. The net photosynthetic rate, light absorption, energy transformation, the activities of PSII acceptor side and donor side and the PSI activities decreased much slower in Q319 than in HZ4 during the ethephon-induced senescence. These results suggest that the retention of light absorption, energy transformation and activity of electron transfer contribute to the extended duration of active photosynthesis in Q319. Although the chlorophyll content decreased faster in HZ4, with decrease of chlorophyll content induced by ethephon, photosynthetic performance of Q319 deteriorated much more severely than that of HZ4, indicating that, compared with Q319, HZ4 has an advantage at maintaining higher photosynthetic activity with decrease of chlorophyll although HZ4 is a quick-leaf-senescence inbred line. We conclude that attention should be paid to two favorable characteristics in breeding long duration of active photosynthesis hybrids: 1) maintaining more chlorophyll content during senescence and 2) maintaining higher photosynthetic activity during the loss of chlorophyll.

Figure 1. Chlorophylls content and photosynthetic rate of plants during senescence in field experiments and laboratory experiments.

Total chlorophylls content (A) and net CO₂ assimilation rate (B) in leaves of two inbred lines of maize (Q319 and HZ4) grown in the field after flowering. Total chlorophylls content (C) and net O₂ evolution rate (D) in the detached leaf segments from the two inbred lines of maize (Q319 and HZ4) after treatment with 0.7 mmol L⁻¹ ethephon or water for different days. Means±SE of six replicates are presented. Different letters indicate significant differences between the parameters in different days after flowering and ethephon treatments,, P<0.05. The differences was analyzed by LSD (least significant difference).

Figure 2. Chl a fluorescence transients normalized between Fo to F_P in detached leaf segments during senescence.

Chl a fluorescence transients in detached leaf segments from the two inbred lines of maize (Q319 and HZ4) before the treatment (A), and after 3 days of treatment with 0.7 mmol L⁻¹ ethephon or water (B). Chl a fluorescence transients were normalized between Fo to F_P (Vt = (Ft-Fo)/(Fm–Fo)) (A, B). ΔVt (C) was obtained by subtracting the kinetics of leaf segments before treatment from the kinetics of leaf segments 3 days after treatment. O indicates the O step at about 20 µs; J indicates the J step at about 2 ms; I indicates the I step at about 30 ms; P indicates the P step, the maximum fluorescence. (Each datum is the average of 6 independent measurements.).

Figure 3. Chl a fluorescence transients normalized between Fo to F_J in detached leaf segments during senescence.

Chl a fluorescence transients in leaf segments from the two inbred lines of maize (Q319 and HZ4) before treatment (A), and after 3 days of treatment with 0.7 mmol L⁻¹ ethephon or water (B). Chl a fluorescence transients were normalized between Fo to F_J (Wt = (Ft–Fo)/(F_J–Fo)) (A, B). ΔWt (C) was obtained by subtracting the kinetics of the leaf segments before treatment from the kinetics of leaf segments 3 days after treatment. O indicates the O step at about 20 µs; K indicates the K step at about 300 µs; J indicates the J step at about 2 ms. (Each datum is the average of 6 independent measurements.).

Figure 4. PSII performance obtained by JIP-text in detached leaf segments during senescence.

The maximum quantum yield of PSII (Fv/Fm) (A), the efficiency of electron move beyond Q_A ⁻ (ETo/TRo, B), the absorption flux per CS (ABS/CSo, C), the density of Q_A ⁻ reducing PSII reaction centers (RC/CSo, D), quantum yield for electron transport further than Q_A ⁻ (ETo/ABS, E) and normalized relative variable fluorescence at the K step (W_K, F) in leaf segments from the two inbred lines of maize (Q319 and HZ4) treated with 0.7 mmol L⁻¹ ethephon or water for different days. The means±SE of six replicates are presented. Different letters indicate significant differences between the parameters in different days after ethephon treatments, P<0.05. The differences was analyzed by LSD (least significant difference).

Figure 5. Chl a fluorescence transients normalized between Fo to F_290µs in detached leaf segments during senescence.

Chl a fluorescence transients in leaf segments from the two inbred lines of maize (Q319 and HZ4) before treatment (A), and after 3 days of treatment with 0.7 mmol L⁻¹ ethephon or water (B). Chl a fluorescence transients were normalized between Fo to F_290µs (Wt = (Ft–Fo)/(F_290µs–Fo)) (A, B). ΔWt (C) was obtained by subtracting the kinetics of the leaf segments before treatment from the kinetics of leaf segments 3 days after treatment. L indicates the L-band at about 130 µs. (Each datum is the average of 6 independent measurements.).

Figure 6. ΔI/Io in detached leaf segments during senescence.

The change in the amplitude of 820 nm transmission (ΔI/Io) in leaf segments from the two inbred lines of maize (Q319 and HZ4) treated with 0.7 mmol L⁻¹ ethephon or water for different days. The initial values of ΔI/Io before treatment were taken as 100%, whereas those after treatment were taken as the percentage of the initial values. The means±SE of six replicates are presented. Different letters indicate significant differences between the parameters in different days after flowering and ethephon treatments, P<0.05. The differences was analyzed by LSD (least significant difference).

Figure 7. The relationship between chlorophyll content and photosynthetic performance in leaf segments during senescence.

Relationships between the quantum yield for electron transport further than Q_A ⁻ (ETo/ABS) (A), W_K (B), net O₂ evolution rate (C), maximum quantum yield of PSII (Fv/Fm = TRo/ABS) (D), efficiency of electron move beyond Q_A ⁻ (ETo/TRo) (E),absorption flux per cross section of leaf (ABS/CSo) (F), density of Q_A ⁻reducing PSII reaction centers(RC/CSo) (G), maximum PSI redox activity (ΔI/Io) (H) and leaf chlorophyll content in leaf segments treated with 0.7 mmol L⁻¹ ethephon or water. “•” and “○” represent the stay-green inbred line of maize Q319 and the quick-leaf-senescence inbred lines of maize HZ4, respectively. The initial values of all the parameters in leaf segments before treatment were taken as 1, whereas those after treatment were taken as the proportion of the initial values. (Each datum is the average of 6 independent measurements.).