Physiological observations of maize (Zea mays L.) hybrids differing in photosynthetic capability

Three studies were conducted in an effort to elucidate differences in growth physiology and leaf stomatal characteristics of maize hybrids which differed in photosynthetic capability;Experiment 1 was designed to determine if maize hybrids differing in photosynthetic capability differed in leaf area expansion or dry matter accumulation and distribution. It was conducted in the field using two dates of planting, one month apart, during the 1978 and 1979 growing seasons. Carbondioxide exchange rate (CER), leaf area, and dry weights of main culms and tillers of maize hybrids were measured weekly through six weeks of development. The results suggested that high-CER maize cultivars did not differ from low-CER cultivars in main culm leaf area but in environments conducive to tillering, the high-CER lines tillered more profusely. Dry weight accumulation paralleled leaf area development in both groups of hybrids. The high-CER lines exhibited a tendency to distribute dry weight out of main culms to tillers or unmeasured below-ground organs. It was concluded that a tendency for greater production of nonproductive, competitive sinks for assimilates may deny high-CER maize cultivars any advantage in grain yield;Experiment 2 sought to determine if higher CER maize hybrids were able to expand leaf area or accumulate dry weight to a greater extent than low-CER hybrids at three levels of nitrogen (N) fertility. Photosynthesis is linked physiologically to N uptake and assimilation in plants. It was hypothesized that cultivars with greater photosynthetic efficiency would have an advantage in N use efficiency (NUE). The experiment was conducted using two high- and two low-CER maize hybrids grown in three N fertility solutions (210, 17, 3 ppm) in sand culture. Maize plants were grown in pots in the open during the summer and repeated in the greenhouse during the fall of 1979. Leaf CER, total leaf area, and dry weight of roots and shoots were measured every two weeks for six weeks. Results revealed that in some environments (high temperature and high solar radiation), high-CER lines may be able to utilize greater amounts of N for increased growth rates. Environmental conditions had an effect on the photosynthesis/NUE relationship. Conclusions were made cautiously due to apparent environmental limitations (e.g., water, temperature, and/or N);Experiment 3 compared stomatal resistance (r(,s)) and stomatal frequency (SF) of maize hybrids with different photosynthetic capabilities. Although leaf porosity should affect photosynthesis, previous studies have shown inconsistent relationships among photosynthesis, r(,s), and SF. The relative influence of r(,s) is believed greater in C(,4) and C(,3) carboxylating species. It was hypothesized that r(,s) and SF may have a role in varietal CER differences in maize, a C(,4) species. Experiment 3 was conducted by returning to the experimental plots of experiment 1 shortly after silking and measuring adaxial r(,s) and SF of the uppermost ear-leaf of the different photosynthetic lines. A diffusion autoporometer was used for r(,s) measures. High-CER maize hybrids exhibited lower r(,s) and greater SF both years. Although r(,s) and SF were not correlated in these studies, it was concluded that variation in r(,s) and SF is associated with CER in maize. Lower r(,s) of high-CER maize hybirds would increase transpiration which may limit use of these lines to environments with nonlimiting moisture