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.
Highlights
To meet the demand for food for the growing world population, a significant increase in the world grain production is required, in crops grown in developing countries
Extents of the decreases of both chlorophyll content and net CO2 assimilation rate in Q319 were significantly less than those in HZ4 (P,0.05), which indicates that Q319 is a functional stay-green genotype
According to the later and slower decrease of chlorophyll content at the later developmental stage in Q319, we could not confirm that Q319 has a longer duration of photosynthetic activity
Summary
To meet the demand for food for the growing world population, a significant increase in the world grain production is required, in crops grown in developing countries. The photosynthesis in leaves, especially in leaves of quick-leaf-senescence genotypes, begins to decrease during the later developmental stages, which severely limits grain yield [4,5]. Spano et al [3] reported that extending the duration of active photosynthesis will elevate the yield of crops and that delaying leaf senescence is one of the ways to accomplish this. The elevation of grain production in hybrids with a long duration of active photosynthesis might be much more obvious under stress conditions than under normal conditions [5,9]. The photosynthetic rate in some of the staygreen genotypes decreases at the normal rate, the chlorophyll content decreases much slower or later than traditional hybrids. The relationship between the retention of the photosynthetic rate and the retention of the chlorophyll content has not clearly been known yet
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