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.

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

Methods for the determination of chlorophyll were compared in Amaranthus tricolor, Kochia childsii and Chloris gayana. From sequential extr-action data, 96% ethanol appeared to be more efficient than 80% acetone in extracting chlorophyll from these plants.The chlorophyll alb ratio was significantly lower in sodium-deficient compared to normal C4 plants. Of the group I elements, only sodium, irrespective of the salt supplied to deficient cultures, restored the chlorophyll alb ratios to the value observed in normal plants. The concentration of sodium required to increase the chlorophyll alb ratio in leaves of sodium-deficient plants was similar to that required to bring about the growth responses. The increase of the chlorophyll alb ratio occurred at an early stage during recovery from sodium deficiency preceding the increase in chlorophyll concentration and the growth response. It is therefore likely that the low chlorophyll alb ratio may be intrinsically associated with the condition of sodium deficiency.

Changes in C4 pathway intermediates in response to sodium nutrition, consistent with the hypothesis that there is a limitation in the conversion of pyruvate to phosphoenolpyruvate (PEP) in the mesophyll cells in sodium-deficiency, were observed in the C4 species, Kochia childsii, Chloris gayana, Amaranthus edulis, Amaranthus tricolor and Atriplex spongiosa. In the C3 species Lycopersicon esculentum (tomato), no differences were observed when grown with or without added sodium. Of the group 1 elements, only sodium, irrespective of the salt supplied to deficient cultures, effected these changes in the C4 species. In the light, concentrations of aspartate, PEP and 3-phosphoglycerate (3-PGA) were lower and those of pyruvate and alanine were greater in sodium-deficient than normal plants. In the dark, concentrations of aspartate, pyruvate, alanine, PEP and 3-PGA were similar in sodium-deficient and normal plants. In the C4 species Atriplex spongiosa, the concentration of sodium required to bring about these changes corresponded to that required for growth responses. Rapid increases in the concentrations of malate, PEP and 3-PGA and decreases in pyruvate and alanine were observed following the direct application of sodium to leaves.

Significantly lower chlorophyll a b ratios were observed in mesophyll thylakoids extracted from sodiumdeficient than normal C , plants of Amaranthus tricolor, Chloris gayana and Kochia childsii. The chlorophyll a l b ratio of bundle sheath thylakoids was not generally as significantly affected by sodium nutrition as the mesophyll thylakoids.The lower chlorophyll a l b ratio in mesophyll thylakoids of sodium-deficient plants was associated with a lower photosystem I1 activity. The effect of sodium nutrition on photosystem 1 activity in the mesophyll thylakoids was dependent upon the species. Although no effect of sodium nutrition on photosystem I was observed in mesophyll thylakoids of K. childsii, photosystem I activity was greater in sodium-deficient than normal plants in A. tricolor.The photosystem I1 and I activities of the bundle sheath thylakoids were not affected by sodium nutrition in either K. childsii or A . tricolor.