Our previous laboratory study revealed that insufficient Ni supply can limit nitrogen fixation in Trichodesmium, a primary diazotrophic phytoplankton in the tropical and subtropical oceans. Here we show that light intensity and Ni availability interrelate to influence Trichodesmium growth. Trichodesmium growth is severely inhibited under high light (670 μE m–2 s–1) and insufficient Ni condition. On the contrary, the sufficient supply of Ni in seawater can sustain the growth of Trichodesmium under either high or low light conditions. We also observed elevated intracellular Ni uptake in Trichodesmium grown under relatively high light condition, supporting that the Ni requirement is used for removing superoxide generated through photosynthetic electron transport. This study shows that light saturation condition for Trichodesmium growth is Ni concentration dependent. This finding may exhibit implications for interpreting temporal and spatial distributions and activities of Trichodesmium in both modern and ancient oceans when light intensity and Ni concentrations have significantly varied.

Short stature and early maturing mutants were induced in two japonica rice (Oryza sativa L.) cultivars by 60Co gamma irradiation. Frequency of useful mutants in the M2 generation grown from bulked seed of M1 plants was 1.89% in ‘Calrose’ and only 0.62% in ‘Colusa.’ The most promising mutant was the short stature Calrose selection D7.D7 was about 25 cm shorter than its parent cultivar Calrose. In 11 large‐plot tests conducted over 2 years, D7 equaled the yield of the tall check cultivar ‘CS‐M3’ (a cultivar similar to Calrose), was 35 cm shorter and had more lodging resistance. The short stature of D7 was controlled by a single recessive gene.Two early maturing Calrose mutants, D18 and D31, may be useful as germplasm sources of early maturity. One short stature, early Colusa mutant, D38, may also be useful as germplasm.Several different short stature phenotypes were induced in Calrose. When Calrose M1 plants from the 25 kR treatment were handled in a pedigree fashion, nearly 10% of the M1 plants produced mutants possessing some degree of short stature.

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Semidwarf indica varieties of rice (Oryza sativa L.) were developed by induced mutation. They generally have a yield capacity equal to that of the semidwarf varieties (T(N)1, IR 8 and others) selected from hybridization . Genetic studies showed that semidwarf genes induced in the mutants and spontaneous ones in existing varieties are at the same locus . These semidwarf mutants, selected in Taiwan and from an American long-grain variety, Centura Patna 231, were compared with their parents, the semidwarf variety Dee-goe-woo-gen, T(N)l and IR 8 . The semidwarf mutants average about 35 % shorter than their tall parents . The plant height is reduced by shortening five internodes from the top, while panicle length is not affected . The mutants, Dee-geo-woo-gen, and improved short-statured varieties had similar patterns of internode elongation . The mutants derived from varieties sensitive to photoperiod have lost the sensitivity . One more crop can be grown within a year . All induced and hybrid-derived semidwarf varieties were highly susceptible to bacterial leaf blight when Xanthomonas oryzae was inoculated although some of their parents were moderately resistant . The mutation rate from irradiation of this semidwarf gene was estimated to be 0 .1 % in I-kung-bau, a native variety of Taiwan. This particular type of mutation has not yet been reported in japonica varieties . However, several promising strains with reduced plant height, earlier maturity and more grain yielding were recently obtained from the California japonica variety, Calrose, by X-irradiation . The merits of mutation breeding in rice are discussed .

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