The aotivity of (±)-absoisin II' [(±)AbII] and its interaction with a number of plant growth regulators in the oontrol of lettuce germination, lettuoe hypoootyl and radiole elongation, ououmber seedling growth, radish lea( senesoenoe, and barley vernalization were examined. The interac.tions between (± )AbII and th«;l promoting h~rmones were of two general types. In gibberellio acid (GAs)-promoted lettuoe germination and kinetin-oontrolled leaf senesoenoe, the effeots of low ooncentrations of ( ± )-!\bII were oompletely overoome by high oonoentratioileof the other substanoe. In .the second type of interaction, (± )AbII was inhibitory only in the presenoe of high oonoentrations of the promoter (kinetin in lettuce germinatipn and allogibberio acid in lettuoe radicle elongation). Cuoumber radiole elongation, on the other hand, was promoted by (± )AbII in the presenoe of a mixture of GA4 and GA7.These various responses suggest that linking the.meohanism of AbII action with that of one growth hormone alone may be' too restriotive.

SummaryThe effects of variations in light intensity, photoperiod, and light quality on shoot dry weight, tillering, and leaf growth of barley (cv. Prior) have been examined in controlled environments. The rates of tillering and of dry matter production were primarily dependent upon the total radiant energy incident upon the plants. Tilleriug was unaffected by changes in the photoperiod (independent of light energy) or in the spectral composition of the light which profoundly affected apical development. At low light intensities, tiller buds on the main axis only elongated, whereas at higher intensities secondary and higher·order tillers were produced. The largest number of tillers was associated with the coieoptile node, and tiller production declined regularly with each successive node up the main axis.The rate of leaf emergence on the main axis was relatively insensitive to changes in light intensity. At higher intensities there was an accumulation of expand· ing but unemerged leaves as the formation of leaf primordia progressively outstripped the rate of leaf emergence. Mature leaf size and shape were determined by the inter· action of two control systems: a heteroblastic change from node to node linked with apical development and hence indirectly affected by the spectral composition of the light source; and a direct effect of light intensity in the short photoperiod, probably mediated through carbohydrate supply.

SummaryThe growth of the root systems of barley seedlings was measured at daily intervals for the first 2 weeks after germination. The lengths and numbers of primary and higher-order branches were recorded and relative rates of extension and multiplication together with mean extension rates were derived from the data.An earlier suggestion that root growth rates are high early in plant development but then fall to a lower level was supported by the data which also demonstrated changes in growth rate associated with the initiation of secondary and tertiary roots. A high nutrient concentration reduced root growth -both by a transient effect on germination rate and a continuing reduction in mean extension rate. Nutrient effects on branch root production were traceable to these effects on elongation of the parent root. I. INTRODUOTIONIn the previous paper in this series (May, Chapman, and Aspinall 1965) it was demonstrated that the average relative rates of both extension and multiplication of roots of plants growing in a tenfold range of nutrient concentration were the same and, moreover, essentially constant over a period of 6 weeks beginning 2 weeks after seedling emergence. There were, however, substantial differences in both numbers and lengths of roots from different nutrient concentration at the time of the first sampling. Furthermore, extrapolation of the root growth curves, both of extension and of multiplication, back towards seedling emergence indicated that the relative rates of root development must have been considerably greater in the first 2 weeks of growth than in the period under observation. These data suggested the importance of this early period of developntent in establishing the pattern of root growth.This early period of seedling root growth has been directly explored in the present investigation. The changing rate of root growth in this period and the origin of the effects of nutrient concentration have both been examined. II. EXPERIMENTAL METHODSIn general, the techniques used in the work reported in this paper were identical with those previously described (May, Chapman, and Aspinall 1965). Barley plants (cv. Piroline) were grown singly in perlite-filled cylinders (5·5 cm diameter by * Part I, Aust.

The influences of light intensity, rooting medium, and the excision of various organs on the early phases of root growth in barley seedlings (cv. Piroline) were explored. The emergence of root axes was affected by light intensity and excision of roots, endosperm or shoot, but was not altered by the rooring medium. Elongation of the emerged axes was similarly responsive. The elongation rate of both the set of axes and individual axes fell wi4h time. The elongation rate of laterals was less than that of the axes and was sensitive to the excision of seedling organs. The data are discussed in relation to the hypothesis that the growth of the root system is determined by the availability of substrate.

SummaryFour varieties of barley (Prior, Noyep, CI 3576, and CI 5611) were grown in field plots and subjected to varying intensities of incandescent light as a 2-hr night interruption.The light treatment accelerated spikelet development, primordium production, and stem elongation and reduced the number of leaves on the main stem and of grains on the ear. These effects were most pronounced in the variety CI 5611 where illumination commenced prior to floral initiation but were also manifest in the other varieties which were not illuminated before initiation, demonstrating the photoperiodic sensitivity of barley in the later stages of development.The data are discussed in relation to the control of apical morphogenisis in cereals, the use of night interruption in plant breeding, and the investigation of the effects of the environment on cereal growth.