514 NATURE VOL. 224 NOVEMBER I 1969 cultures may have played in producing the illness were left as dark controls. Irrigation was discontinued 1esponses. In previ~us inoculations, however, passage in after March, but rainfall in April and May was sufficient man and human t1ssue cultur·e did not increase the to keep the soil fully moist. frequency of infections or enhance the virulence for man Such shoots as were available were harvested from the beyond that seen in the initial inoculum which had been unpruned plots throughout the winter season and subse­ passaged .in egg and HK only. This suggests that passage quently during April and May. The results showed that m man d1d not contribute to virulence. In spite of these irrigation alone increased tho winter crop by about 40 considerations the essential point is that an influenza per cent and illumination increased it by another 45 virus of equine origin produced typical human influenza! per c~nt: Illumination did not increase the winter crop illness which lends credence to the possibility that in on umrr1gated plots, although it must have induced early natural conditions equines and equine influenza. viruses bud break as the crop harvested from this plot during might contribute to the emergence of new variants of April and May clearly shows. With the advent of long human influenza. days dormancy was also broken in the unilluminated plots, This work was supported in part by the Clinical R esearch but because of the early start tho illuminated plots Centers Branch, US National Institutes of Health, and the produced more crop in April- May. Vaccine Development Branch, National Institute of In mid January the pruned and illuminated bushes had Allergy and Infectious Disoases. twice as many new leaves as the corresponding unillumin­ R. B. Cou cH ated bushes. R. G. DouGLAS, JUN. In north-east India. the tea plant flowers from November J. A. KAsEL to January. Illumination during this period reduced the S. RIGGS number of flowers from 376 to 65/m2 of unpruned bush V. KNIGHT surface. Baylor College of Medicine, These results demonstrate that increasing the day to Departments of Microbiology and Medicine, 13 h during winter, using weak supplementary artificial Houston, Texas. illumination, enhanced shoot growth, hastened bud break ltec~ived August 4, 1969. and inhibited flowering. If winter dormancy of tea induced by short days is tho 1 Coleman, M. T., Dowdle, W. R., Pereira, H. G., Schild, G. C., and Chang, W. K., Lancet, ii, 1384 (1968). result of a. decrease in the relative concentration of 'Alford, R. H., Kasel, J·. A., Lehrich, J. R., and Knight, V., Amer. J. gibberellic acid (GA) in the pla.nt, it should be possible Epidemiol. , 86, 185 (1967). to break dormancy and induce growth in short day condi­ ' Knight, V., in Progress in Medical Virology, 6 (edit. by Barger, E., and Melnick, J. L.), l (Karger, Jlasel and New York, 1964). tions by increasing the amount of GA in the dormant ' Chanock, R. ~i. Parrot.t, lt. H., Cook, K., Andrews, D. E., llell, J. A., plants. This possibility was tested by injecting 10 p.p.m. Reichelderfer, 'f., Kapikian, A. Z., Mastrota, F. M., and Huebner, R. J., and 40 p.p.m. respectively of GA into young, single New Eri{Jl. J. llled., 258, 207 (1958). ' Stuart-Harris, C. H., Influenza and Other Virus Infections of !he Respiratory stemmed tea plants on December 30 (1027 h), when all Tract, 9 (Williams and Wilkins, Baltimore, 1965). tea plants at Tocklai were completely dormant. As well as GA, kinetin was injected into dormant tea plants to investigate whether it directly or in combination with GA affected bud break and subsequent growth. (Kinetin 2 Seasonal Dormancy in Tea has been reported to inhibit IAA activity .) The com­ pounds were injected into three plants each of two clones (Camellia sinensis L.) by the leaf-stalk injection method3 • TEA is cultivated between latitudes 45° north and 35° By the middle of February, the plants supplied with GA south, and at or near the equator it produces almost the completed a flush of growth consisting of two to four same yield every month. But farther from the equator leaves when other plants were still dormant. These the winter harvest gradually declines and at latitudes treated plants started the second flush growth in early beyond about 16° there is almost complete winter dorm­ Mat·ch, coinciding with bud break and the first flush ancy which lasts longer with increasing latitudes. growth of the untreated plants and those treated with The cause of winter dormancy cannot be ascribed solely kinetin. The plants treated with GA completed second to low temperature because tea. flushes throughout the flush growth by early May. year at high elevations near the equator, although tem­ Clearly, plants treated with GA grew in the short day peratures there are even lower than winter temperatures conditions of winter while untreated plants remained in the plains of north-east India1 (25°-27° N), where bushes completely dormant. Thus gibberellic acid replaced long a.re dorma.nt for about 3 months. Soil water stress and days as it did in the case of Camellia japonica4 and Pinus nutrition can also be ruled out as major causes of dormancy elliottii•. Kinetin in the concentration.'! tried had a. because even with an ample supply of water and nutrients significantly antagonistic effect on GA activity. throughout the dry winter months tea bushes fail to flush Winter dormancy of tea therefore seems to be a matter a.t Tocklai (26° 47' N). of short day length (or long night length), acting through Monthly crop distributions and average day lengths internal plant growth regulators, and it cannot be broken for several tea growing regions from the equator to 30° by the conventional methods of commercial ma.na.gement, north or south indicate that tea. bushes pass through but the results reported here open up the possibility of a period of complete dormancy when the winter day is inducing growth in short day conditions either by artificial shorter than a critical length of about 11 h 15 min for at illumination or by treatment with gibberellic acid. least 6 weeks. The longer the period of short days, the I thank S. C. Barua for his help and D. H. Laycock for longer the dormancy. valuable suggestions. To test this suggestion experimentally, pruned and D. N. BAHUA unpruned mature tea bushes were exposed to 13 h days from November 15 to March 21 by providing weak supple­ Tea Research Association, mentary artificial illumination during the morning and Tocklai Experimental Station, evening. The intensity of illumination from incandescent J orhat-8, Assam, India. lamps in the visible part of the spectrum was less than Received July 14; revised September 1,1969. 1 per cent of the visible solar radiation on a bright summer day and was considered too weak to affect net photo­ ' Hadfield, W., Nature, 219, 282 (1968). 'Ropp, R. S. De, Plan! Physiol., 31,253 (1956). synthesis. Half the experimental area was irrigated with 'Jlarua, D. N., Curr. Sci.,25, 249 (1956). 50 mm of water every month from November to March 'Lockhart, A. J., and Bonner, J., Plan! Physiol., 32,492 (1957), and the other half was not irrigated. Four similar plots • Boardean, P. F., Nature,l82, 118(1958). © 1969 Nature Publishing Group.