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As leaf irradiance is decreased in increments, a single transient CO2 burst is exhibited by C3 plant leaves. This post-lower illumination CO2 burst (PLIB) is sensitive to changes in irradiance, to changes in the concentrations of 02 and CC2, and to temperature. Increasing 02 concentrations above ambient produces a progressively larger PLIB while increasing CO2 concentrations above ambient produces a progressively smaller PLIB. The PLIB, which exhibits many responses to environment common with other methods for measuring photorespiration and photosynthesis, is proposed as a measure of photorespiration in illuminated leaves of C3 plants. Although the PLIB cannot be used as a quantitative measurement of photorespiration, we propose that the PLIB is a rapid, easy, relatively inexpensive, nondestructive method for evaluating photorespiration in intact illuminated C3 leaves in air.Leaves of some plants release a burst of CO2 immediately following illumination. This phenomenon was first referred to as a dark CO2 outburst by Decker (6,7) and is now commonly referred to as PIB.' A photo-stimulated dark 02 uptake also appears to be an expression of the same phenomena (13). The PIB was initially interpreted as being a remnant of light respiration (6, 7), and the same interpretation is widely accepted today (3,15,21). In relating the PIB to photorespiration, the amplitude of the PIB was shown to be a function of previous light intensity (7,17,18), temperature (8), atmospheric 02 concentration (1 1, 14, 17, 19), atmospheric CO2 concentration (7,9,13,16,21), leaf age, and photosynthetic CO2 assimilation pathway, i.e. C3 and CAM plants exhibit a PIB but most C4 plants do not (10-12, 15, 17-19). In some CAM plants such as pineapple, the PIB is exhibited as two peaks-the first associated with photorespiration, being sensitive to C02, 02, and light intensity but the second CO2 outburst attributed to decarboxylation of organic ' and Department ofBiochemistry (Z-P. T., S-S. C., C. C.acids during CAM (5). Multiple PIB peaks of CO2 release were observed in the earliest C3 plants studied (6,18) Biochemically, it is commonly accepted that photorespiration occurs during photosynthesis in the presence of 02 when the enzyme RuBP carboxylase functions as RuBP oxygenase. In its oxygenase capacity, it adds 02 to RuBP to produce P-glycolate and ultimately amino acids, glycine and serine, as transitory intermediates en route to completing the photosynthetic carbon oxidation cycle. The balance between carboxylation and oxygenation in leaves shows similar responses as does the PIB to environmental parameters such as 02, C02, and light intensity.We recently reported (20) a transitory release of CO2 from illuminated leaves when the illumination intensity was suddenly reduced. This post-lower illumination CO2 burst appeared to be a relatively simple means of measuring photorespiration at specific illumination intensities (20). These studies were conducted to document this hypothesis. In the initial work, a release of CO2 was detected follow...

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Leaf water status, carbohydrate levels, net photosynthesis, stomatal conductance, ABA, dihydrozeatin riboside (DHZR), and trans-zeatin riboside (ZR) levels were determined in a greenhouse during rooting of stem cuttings of Acer rubrum L. `Red Sunset' taken on 3 Sept. 1987 and 28 May 1988. Leaf water status deteriorated before rooting and improved after root emergence. Leaf carbohydrate concentrations (glucose, sucrose, total soluble sugars, and total carbohydrates) increased until rooting and decreased after rooting, while changes in starch concentrations were trendless. ABA levels increased after insertion of cuttings into the rooting medium, but decreased before rooting. No correlation between timing of rooting and concentrations of the cytokinins ZR or DHZR was observed. Photosynthetic rates during rooting were higher for the Sept. 1987 cuttings and did not decrease to the compensation point as did those for May 1988 cuttings. Low photosynthetic rates and stomatal conductance of the cuttings during rooting were associated with water stress. The relationship between photosynthetic rates of such cuttings and cytokinin (CK) or ABA content was unclear. Chemical names used: [S-(Z,E]-5-(1-hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3-methyl-2, 4-pentadienoic acid (abscisic acid, ABA); 2-methyl-4-(1H-purin-6-ylamino)-2-buten-1-ol (zeatin, Z).

The influence of temperature, irradiance, photoperiod and growth retardants on growth and flowering of Angelonia angustifolia Angel Mist series was evaluated. When temperature was increased from 15 to 30 °C, time to visible bud and time to flower decreased in a quadratic manner but total plant height and flower stem dry weight increased linearly. As irradiance increased, time to flower, time to visible bud, and height decreased quadratically. Changes in photoperiod had no effect on growth or flowering, suggesting that A. angustifolia is a day-neutral species with regards to height and flowering time. Daminozide, ancymidol, and paclobutrazol resulted in significant reduction of plant height compared with control plants but did not influence flowering time. Chemical names used: K-cyclopropyl-K-(4-methoxyphenyl)-5-pyrimidinemethanol (ancymidol); butanedioic acid mono (2,2-dimethylhydrazide) (daminozide); K-[(4-chlorophenyl)methyl]-K-(1,1-dimethyethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).