Excised leaves of silver maple {Acer saccharinum L.) exposed to 0, 0.045, 0.090, or 0.180 mM Cd^+ exhibited reduced net photosynthesis and transpiration, and increased dark respiration. Rates of net photosynthesis and transpiration diminished with time and were strongly correlated with solution concentration and tissue content of Cd^+. Net photosynthesis and transpiration were reduced to 18 and 21%, respectively, of the untreated controls after 64 h. Dark respiration increased as much as 193% of the untreated controls but was poorly correlated with solution concentration or tissue content of Cd^+, Diffusive resistances of leaves to carbon dioxide and water vapor transfer increased with both increasing Cd^^ concentration and time. These findings are discussed in relation to stomatal function.

The plastochron index provides a morphological time scale which has proved more reliable than chronological age in studies relating morphological and physiological development of a whole plant or plant organ. Since its inception in 1957, the index has been utilized in a variety of investigations from leaf ontogeny in cottonwood trees to rhizoid cluster initiation in algae. The plastochron index has been extensively used in studies involving source and sink relationships, leaf anatomy, cell differentiation, and primary vascularization. It has been used in investigations of hormonal regulation of plant growth and in studies of the effects of various environmental factors on developmental processes in crops. This paper reviews some of the literature from 1957 to present concerning the development and use of the plastochron index. 1

Effect of cadmium chloride on dry weight accumulation, height growth, and relative conductivity of stems of silver maple (Acer saccharinum L.) seedlings exposed to 0, 5, 10, or 20 ppm CdCl2 · 2½ H2O (on weight basis of rooting media) was determined. Leaf, stem, and root dry weight were significantly reduced by all CdCl2 treatments. Seedling height at 8 weeks was severely reduced and strongly correlated with applied CdCl2 levels. Relative conductivity of excised stem sections was significantly reduced by Cd treatment. The reduction was caused by (i) a progressive decrease in the proportion of xylem tissue available for water conduction, (ii) reduced size of vessels and tracheids, and (iii) partial blockage of xylem elements by cellular debris or gums. These findings are discussed in relation to water stress.

Two commercial corn (Zea mays L.) hybrids, ICI 8532 IT and Pioneer 3180 IR, designed for tolerance to imidazolinones, were evaluated for tolerance to various acetohydroxyacid synthase‐inhibiting herbicides at the whole plant and enzyme levels. The purpose was to establish and compare the underlying enzymatic basis for tolerance in these plants, which were produced by contrasting methods and contain different genetic complements of the altered target enzyme. ICI IT plants exhibited significant tolerance (40‐fold higher rates for 50% inhibition of growth than the control hybrid) to imazethapyr, and somewhat less to imazaquin and pyrimidyloxybenzoate. ICI IT plants were no more tolerant to chlorsulfuron or flumetsulam than the control hybrid. Pioneer IR plants were highly tolerant to all of the AHAS inhibitors tested, requiring rates 200‐ to 2000‐fold higher than those of the control hybrid for 50% inhibition of growth. The basis for tolerance was evaluated on acetohydroxyacid synthase (AHAS, EC 4.1.3.18) in vitro. Relative to unmodified AHAS isolated from the control hybrid, AHAS isolated from ICI IT corn was seven‐fold less sensitive to imazethapyr, five‐fold less sensitive to pyrimidyloxybenzoate and two‐fold less sensitive to imazaquin, but was as sensitive to chlorsulfuron and flumetsulam. AHAS from Pioneer IR corn was highly insensitive to all of the AHAS inhibitors tested. Sensitivity to the feedback inhibitor leucine was not detectably altered in the modified enzymes. Substrate saturation kinetics of AHAS isolated from ICI IT corn were identical with those of unmodified AHAS, while AHAS from Pioneer IR required a two‐fold higher concentration of pyruvate for half‐maximal saturation. The results support the theory that because the catalytic and herbicide binding sites of AHAS are distinct from each other, crops can be designed for tolerance to AHAS‐inhibiting herbicides with little effect on performance.

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