Abstract. Biogenesis of thylakoid membranes in the conditional chlorophyll b-deficient CD3 mutant of wheat is dramatically altered by relatively small differences in the light intensity under which seedlings are grown. When the CD3 mutant is grown at 400 lxE/m 2 s (high light, about one-fifth full sunlight) plants are deficient in chlorophyll b (chlorophyll a/b ratio > 6.0) and lack or contain greatly reduced amounts of the chlorophyll a/b-binding complexes CPII*/CPII (mobile or peripheral LHCII), CP29, CP24 and LHCI, as shown by mildly denaturing 'green gel' electrophoresis, by fully denaturing SDS-PAGE, and by Western blot analysis. High light CD3 chloroplasts display an unusual morphology characterized by large, sheet-like stromal thylakoids formed into parallel unstacked arrays and a limited number of small grana stacks displaced toward the edges of the arrays. Changes in the supramolecular organization of CD3 thylakoids, seen with freeze-fracture electron microscopy, include a reduction in the size of EFs particles, which correspond to photosystem II centers with variable amounts of attached LHCII, and a redistribution of EF particles from the stacked to the unstacked regions. When CD3 seedlings are grown at 150 ~tE/m 2 s (low light) there is a substantial reversal of all of these effects. Thus, chlorophyll b and the chlorophyll a/b-binding proteins accumulate to near wild-type levels (chlorophyll a/b ratio = 3.5-4.5) and thylakoid morphology is more nearly wild type in appearance. We show that this partial rescue of the CD3 high light phenotype is accompanied by large changes in thylakoid structure. The CD3 mutant, which defines a new class of chlorophyll b-deficient phenotype, is discussed in the more general context of chlorophyll b deficiency.

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A non-lethal chlorophyll deficient mutation was induced by use of the chemical mutagen ethyl methanesulfonate. Chloroplasts from the control and mutant plants were found to be very similar ultrastructurally. Thylakoid membrane volume was only slightly greater in plastids from the control as compared with plastids from the mutant. The chlorophyll content of the mutant was reduced by over 60%. This decrease in chlorophyll was not accompanied by a similar decrease in electron transport. Uncoupled electron transport rate based on a unit chlorophyll basis was nearly twice as great for mutant chloroplasts as for control plastids. However, electron transport rate based on a unit membrane volume was similar in mutant and control plants. At high irradiance the relative quantum requirement of the control and mutant was similar when expressed on membrane volume.

Kochia [Kochia scoparia (L.) Schrad.; syn. Bassia scoparia (L.) A.J. Scott] is a weed that infests cereal crops in the Great Plains of the USA, often severely reducing yields. Herbicides have controlled kochia, but recently kochia has developed resistance to many herbicides. Nonherbicide alternatives are therefore needed for the integrated management of kochia. Greenhouse and growth chamber competition studies were conducted between kochia, a C4 weed, and barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) to determine the environmental conditions that would render kochia most vulnerable to competition by a small‐grain crop. Replacement‐series experiments between kochia and wheat or barley were conducted under various temperature, soil moisture, and light conditions. Unlike wheat, kochia growth and photosynthesis were suppressed under cool temperatures. Barley suppressed kochia more than wheat did because of its larger canopy, despite its lower photosynthetic rates. Under high radiation conditions and warm temperatures, growth and photosynthesis were greater for kochia than wheat. Warm temperatures also increased dark respiration and reduced water use efficiency under low radiation conditions, however, thus limiting kochia's competitiveness under a closed canopy. Water stress did not affect competition, although net photosynthetic rates of kochia were greater at photosynthetically active radiation (PAR) values > 400 μmol m−2 s−1. Growth and CO2 exchange rates varied among four different kochia accessions, but growth of all accessions was reduced by shade. Results suggest that a leafy, cold‐tolerant crop or cultivar, grown early in the season to produce necessary ground cover, should provide opportunity to suppress kochia.

Total water potential of barley (Hordeum vulgare L.) leaves from plants grown under greenhouse and growth chamber conditions was divided into pressure and osmotic potential components, and their relationship to leaf relative water content was determined. Pressure potential approached zero at a water potential of about −32 bars, and a relative water content of about 65%. A change in the elasticity of leaves occurred at about 2 bars pressure potential and about −12 bars water potential. First visible wilting was observed between 75 and 80% relative water content. Transpiration decreased as leaf relative water content decreased but transpiration was independent of soil water content until about 16% (0.6 bar soil suction). First visible wilting of barley leaves was observed at soil water content between 9 and 13% (1–5 bars soil suction). Water potential and leaf relative water content decreased as the soil matric potential decreased. There was a shift to lower relative water content and water potential values as plants became older when the soil matric potential decreased.