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...

A transient CO2 burst is exhibited by irradiated leaves of the C3 plant geranium (Pelargonium X hortorum, Bailey) after the irradiance is quickly lowered. The light CO2 burst appears to be related to photorespiration because of its irradiance dependency and its sensitivity to other environmental components such as CO2 and 02 concentration. The term postlower-irradiance CO2 burst or PLIB is used to describe the phenomenon. The PLIB appears to be a quantitative measurement of photorespiration with intact geranium leaves. The PLIB has been observed with intact leaves of other C3 plants but not with C4 leaves. Therefore, it is proposed that, after maximizing intact leaf photosynthetic rates and leaf chamber gas measuring conditions, photorespiration can be measured with intact C3 leaves such as geranium as a transient post-lower-irradiance CO2 burst.An estimated 30%7o to 50% of the photosynthetically assimilated carbon is lost through the process of photorespiration, particularly in leaves of C3 plants (6, 7). Technically, photorespiration is difficult to measure accurately because in light CO2 is assimilated from the atmosphere while at the same time CO2 is evolved via photorespiration. Therefore, an accurate and simple method of measuring photorespiration is needed.This a report of a serendipitous observation made while measuring leaf photosynthetic rates at progressively reduced irradiance levels in the C3 plant geranium. At each reduced irradiance intensity, a transient CO2 burst occurred prior to obtaining a steady-state rate of photosynthesis. This CO2 burst from an illuminated leaf appeared kinetically to be similar to the PIB4 described initially nearly three decades ago (3,4 was postulated to be a brief remnant of photorespiration inasmuch as it was observed in the dark within 1 to 3 min (3, 4). These early ideas are generally accepted today (2, 5-7). We reasoned that this reduced irradiance intensity CO2 burst also might be associated with photorespiration. The PLIB is proposed to describe this phenomenon. In this manuscript, we will describe our discovery and measurement of the PLIB in intact geranium leaves and present initial observations regarding the PLIB with other leaves.We will propose that the PLIB may be a direct, easy, and useful measurement of photorespiration in intact leaves. MATERIALS AND METHODSOne hundred geranium plants (Pelargonium X hortorum, Bailey cv. Razmatazz) were grown in a standard peat:vermiculite (1:1 v/ v) mix in 10-cm pots. Plants were watered to saturation when needed and fertilized weekly with 800 mg/l of 15-0-12.5 N-P-K at each irrigation for 10 weeks and were vigorously growing in a standard greenhouse at 30 ± 5°C day and 21 ± 3°C night.The fourth leaf from the base was used for all photosynthesis measurements while still attached to an intact plant. All CO2 measurements were obtained by the standard method differential analysis between CO2 concentration entering and exiting a photosynthesis chamber using a Beckman model 215B IR CO2 analyzer. The circular chamber ...

Foliar application of 0.3% CCC applied to geranium (Pelargonium x hortorum Bailey) resulted in increased net photosynthesis (Pn), transpiration (Tr), chlorophyll concentration, and reduced photorespiration as measured by post lower illumination CO2 burst (PLIB) compared with plants treated with 0.5% SADH. Pn and Tr rates were enhanced beginning 2 to 3 days after the CCC treatment compared with control plants and remained elevated for a least 4 to 5 more days.

Cineraria plants (Senecio cruentus DC) were transplanted into medium either with or without a hydrogel (polyethylene oxide). Half the plants in each medium were treated with a film-forming antitranspirant while half were not. Plants then were placed either in a clear glasshouse or a shaded glasshouse (40% shade), and no additional water was applied. Water loss was lowest for plants where both the foliage and medium were treated, whereas control plants (no treatment) lost water most rapidly regardless of light intensity. Plants which received only the hydrogel were similar in water loss to control plants at both light intensities. As water stress developed, net photosynthesis (Pn) decreased, reaching a zero rate at wilting; however, Pn measurements of treated leaves showed few significant differences due to treatment during the water stress period.