Plants of yam, taro, tannia, cassava and sweet potato were raised under shade or in full sunlight and the effect of shade on leaf chlorophyll and carotenoids was examined to determine and compare the relative shade tolerance and adaptability of the var ious species. All five species of root crops adapted to shade. The chlorophyll concentration was higher, while the chlorophyll a:b ratio, carotenoids per unit chlorophyll and the weight per unit area of leaf were lower in the shade than in the su n in yam, tannia, taro, cassava and sweet potato. All species had larger leaves and more chlorophyll per leaf in the shade. The extent of the changes, however, differed between species. The aroids (taro and tannia) appeared to be shade-tolerant species as their chlorophyll a:b ratios changed less than those of the other species in the shade, suggesting that their light-harvesting systems may be normally adapted to shade conditions. Taro and tannia also adapted to shade by a greater proportional i ncrease in leaf size, a smaller reduction in leaf weight per unit area and a greater proportional increase in chlorophyll and carotenoids per leaf than the other species. Yam compensated for shade by having a large proportional increase in leaf size and a ppeared to be moderately tolerant of shade. Sweet potato and cassava appeared to be the least tolerant of shade of the major tropical root crops.

The origin and distribution of cassava in Asia is discussed. The other topics covered include: agronomy and utilization; area, production and yield; cassava trade in Asia; and cassava industry in the Asia-Pacific Region (i.e. Thailand, Indonesia, India, China, Philippines, Vietnam, Malaysia, Sri Lanka, Oceania, Papua New Guinea, Fiji, Tonga and Australia).

SUMMARYTwo methods were developed for the rapid estimation of heat tolerance in plants using excised tissue pieces. The first method was a modification of the conductivity-bridge method and could yield results in less than 3 h. The second method combined plasmolysis with vital staining for the estimation of tissue injury following a regulated heat stress. This method was first developed and perfected using onion bulb epidermis tissue. It was later adapted for estimation of heat tolerance in intact tomato plants. Results from this method could be obtained within 1 h. The advantages and limitations of the two methods are compared and discussed.

SUMMARYThe progression of sprouting was observed in water-yam (Dioscorea alata) and whiteyam (D. rotundata) tuber pieces planted in moist sawdust. Soon after planting, white callus-like protuberances (sprouting loci) were seen on the tuber surface, followed by the appearance of one or more differentiated shoot buds on the sprouting locus. Sprouting loci appeared more readily on the upper and lower parts of each tuber piece than on the sides. Large tuber pieces sprouted more rapidly, had larger and more numerous sprouting loci, and gave rise to more shoot buds than small tuber pieces.Anatomical examination showed that the shoot bud resulted from the activity of a layer of meristematic cells lying close to the tuber surface. Extensive cell divisions commenced in this layer after planting, and some of the resulting cells soon differentiated into the shoot bud.

S U M M A R YSince shading often occurs under normal growing conditions in various cropping systems (for example, intercropping, alley cropping), field experiments were carried out to determine the effects of shading on stomatal density, leaf size, leaf dry matter, and leaf lamina thickness in the major tropical root and tuber crops, tannia (Xanthosorna sa&ifooliurn), sweet potato (Zpornoea batatas), yam (Dioscorea esculenta), cassava (Manihot esculenta), and taro (Colocasia esculenta). Shading decreased stomatal density in the lower epidermis of tannia, sweet potato, yam and cassava, and in the upper epidermis also in tannia and sweet potato; the upper epidermis of yam and cassava were devoid of stomata. In contrast to the other species, taro under shade had an increased stomatal density in both the upper and lower epidermis, a finding which was confirmed in subsequent pot experiments. This response of taro was postulated as a possible manifestation of greater shade adaptation by the species.For all the species, shading generally resulted in the production of larger (in terms ofsurface area) but thinner leaves, with a decreased dry matter concentration. For the two species (yam and cassava) that had stomata only on the lower epidermis, normal sun-grown plants had about twice as many stomata per unit area of lower epidermis than did corresponding plants of the other three species. The agronomic and physiological significance of the findings are discussed.t Corresponding author