ANALYSIS OF STARCH ACCUMULATION AND GERMINATION IN ONOCLEA SPORES

Previous studies have shown that low fluences of light accelerate starch accumulation and enhance germination of Onoclea spores. Fluence response curves for induction of starch accumulation were compared with fluence response curves for enhancement of germination in order to determine if the two photoresponses in Onoclea spores have a common photoreceptor. Fluence response curves indicate that both responses were proportional to the log of the fluence and that the relative fluence efficiencies of the four wavelength regions tested were similar for induction of both germination and starch accumulation. Red (600-720 nm) irradiation was the most efficient, while green (500-600 nm), blue (400-520 nm), and far-red (720-900 nm) irradiations showed a decreasing order of efficiency for induction of the responses. A correlation coefficient between the amount of starch accumulated as a result of red irradiation and the final percent germination was calculated to be 0.964. These results support the hypothesis that a common photoreceptor mediates both photoinduced germination and starch accumulation. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) inhibits photosynthesis by blocking the flow of electrons from Photosystem II to Photosystem I. At 0.1 mM DCMU failed to inhibit both photoinduced starch acumulation and germination. This result and the greater efficiency of red than blue light, the low fluence functional for induction, and the fluence dependency argue against the participation of photosynthesis in photoinduced starch accumulation. A similar conclusion has been previously drawn for photoenhancement of Onoclea spore germination. Additionally, the effects 0.01-1.0 mM cycloheximide and 100 ,ul/l ethylene on photoinduced starch accumulation were investigated. Neither agent inhibited starch accumulation, whereas both substances inhibited germination 70-90% when applied at a time coincidental with the period of rapid starch accumulation. These results indicate that the photoinduction of starch accumulation does not have an ethylene sensitive stage nor does it require protein synthesis as does photoenhancement of germination of Onoclea spores. LIGHT accelerates the rate of conversion of sucrose to starch as well as stimulating germination of Onoclea sensibilis L., the sensitive fern. In the dark a small percentage of the spores germinate, and there is a slow rate of conversion of sucrose to starch. The acceleration of sucrose degradation and starch accumulation occurs within 4 hr after low fluences2 of red or far-red irradiation (Towill and Ikuma, 1975c). Photoinduced germination has been shown to have an oxygen requirement (Towill and Ikuma, 1975a), to be dependent upon at least on constitutive protein (Towill and Ikuma, 1975b), and to be inhibited by ethylene (Fisher and Miller, 1978) at a time which 1 Received for publication 9 March 1979; revision accepted 26 July 1979. I thank Mr. E. Shelton, Mr. C. Helms and Ms. C. Nelson for their assistance. I also thank Ms. C.-Y. Chen and Dr. H. Ikuma for the supply of Onoclea sporophylls. This research was supported in part by a Faculty Grant-In-Aid from Arizona State University and in part by a National Science Foundation Grant (PCM76-81532). 2 The term fluence implies energy fluence which is defined as the amount of energy crossing a unit area normal to the direction of propagation (Rupert and Latarjet, 1978; Jagger, 1977). is coincidental with the period of rapid sucrose degradation and starch accumulation. Onoclea spores undergo a single cell division approximately 20-24 hr after irradiation (Fisher and Miller, 1978). Elongating rhizoidal and protonematal cells protrude from the spore coat 120140 hr after irradiation (Towill and Ikuma, 1975b). During the period of cell division and elongation there is a decline in the rates of sucrose degradation and of starch accumulation and an initiation of hydrolysis of storage proteins and lipids (Towill and Ikuma, 1975c). The relationship between accelerated carbohydrate metabolism and photoinduced germination is unknown. The two processes may be initiated by light absorbed by different photoreceptors or by the same photoreceptor. Even if the photoreceptor is the same, accelerated carbohydrate metabolism may not be causally related to germination. A photoreceptor may undergo a primary transducing event which then initiates a multitude of independent secondary processes (Smith, 1975). In this communication data are presented which suggest that the photoreceptor responsible for acceleration of the rate of starch accumulation in Onoclea spores is the same as