Leaf senescence in maize hybrids:: plant population, row spacing and kernel set effects

Maize crop management involves decision making on several cultural practices aimed to maximize grain yield, like plant population and row spacing. These practices affect the light environment perceived by plants and the post-flowering source-sink ratio, but there is scarce information on the way they influence plant leaf senescence. The objectives of our research were to: (i) characterize the development of leaf area senescence for contrasting canopy architectures (i.e. plant population x row spacing), and (ii) analyze the response of leaf senescence to changes in the light environment and the post-flowering source-sink ratio. Field experiments were conducted in Argentina between 1997/1998 and 2000/2001. Four hybrids were grown at a wide range of plant populations (3, 9, 10 and 12 plants m(-2)), row spacings (0.35, 0.7 and I m) and pollination treatments (natural and restricted pollination). Senescence development was well described (r(2) = 0.61-0.99, P < 0.05) as a bilinear process, starting always at around 500 degreesC day (base temperature of 8 degreesC) before silking. Senescence progressed at a lower rate during the first phase of the process than during the second one (similar to1.4 vs. similar to5.5 cm 2 per plant per degreesC per day). The second phase always started between silking and 400 degreesC day after silking. Increased plant population increased senescence rate during the whole plant cycle, but never affected the ontogenic stage when senescence was initiated or accelerated in all hybrids. Increased plant population promoted: (i) an enhanced light attenuation within the canopy (k coefficient = 0.43, 0.55 0.53 and 0.65 for 3, 9, 10 and 12 plants m-2, respectively), (ii) an augmented post-flowering source-sink ratio (11.6 cm(2) of green plant leaf area per kernel at 9 and 12 plants m(-2) compared to 8.3 cm(2) per kernel at 3 plants m(-2)), and (iii) a decreased grain protein concentration. Senescence was reduced by kernel set restrictions that enhanced post-flowering assimilate availability, indicating the process was accelerated by assimilate starvation at high plant populations independently of the green leaf area established per growing kernel. Row spacing altered light quality (red:far-red ratio) perceived at the lowermost leaf stratum at the highest plant populations, but had no effect on senescence development. Senescence during grain filling was related to the local light quantity perceived by leaves and to N availability for actively growing kernels. Although senesced leaf area was influenced by crop growing conditions, senescence initiation and the onset of increased senescence rate were not. (C) 2003 Elsevier Science B.V. All rights reserved.