Environmental Responses of the Post-lower Illumination CO(2) Burst as Related to Leaf Photorespiration.

As leaf irradiance is decreased in increments, a single transient CO(2) burst is exhibited by C(3) plant leaves. This post-lower illumination CO(2) burst (PLIB) is sensitive to changes in irradiance, to changes in the concentrations of O(2) and CO(2), and to temperature. Increasing O(2) concentrations above ambient produces a progressively larger PLIB while increasing CO(2) 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 C(3) 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 C(3) leaves in air.

A Transient Burst of CO(2) from Geranium Leaves during Illumination at Various Light Intensities as a Measure of Photorespiration.

A transient CO(2) burst is exhibited by irradiated leaves of the C(3) plant geranium (Pelargonium X hortorum, Bailey) after the irradiance is quickly lowered. The light CO(2) burst appears to be related to photorespiration because of its irradiance dependency and its sensitivity to other environmental components such as CO(2) and O(2) concentration. The term post-lower-irradiance CO(2) 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 C(3) plants but not with C(4) leaves. Therefore, it is proposed that, after maximizing intact leaf photosynthetic rates and leaf chamber gas measuring conditions, photorespiration can be measured with intact C(3) leaves such as geranium as a transient post-lower-irradiance CO(2) burst.

Determination of the buoyant density of canine distemper virus by radioassay.

Canine distemper virus was labelled with tritiated uridine and, following precipitation with saturated ammonium sulphate solution, was concentrated 66-fold by centrifugation through a discontinuous sucrose gradient. When this preparation was centrifuged to equilibrium in density gradients of potassium tartrate or sucrose, radioactivity was distributed over the density range 1.218 to 1.180 with a pronounced peak at around 1.195. This corresponded closely to the distribution of infectivity and also to that of virus particles revealed by electron microscopy. In density gradients of caesium chloride, a plateau of radioactivity was present over the density range 1.26 to 1.24 with a peak at around 1.240 but most of the infectivity was limited to the range of 1.24 to 1.22. Since the amount of infectious virus recovered from potassium tartrate was greater than that recovered from the other two materials and the radioactive peak occurred over a narrower density range, it was concluded that potassium tartrate was the material of choice for the isopycnic centrifugation of canine distemper virus.