Prediction of Growth Rate at Different Light Levels from Measured Photosynthesis and Respiration Rates

Abstract

S'umnarY'. Light integrators with a linlear responise are not suitable for measurinig the light climates of plants because plants are not linear integrators. It should be possible to make a quantitative allowanice for this nonlinearity by using the C,O, uptake curve of the plant. To test this, we have subjected white clover plants to different levels of constanit light, comqparing the rate of increase of total dry matter with the net rate of uptake of CO. per day. Tenmperature, humidity, daylength and nutrient supply were kept coinstanlt. The growtth rate calculated from CO, uptake agreed well with the observed rate over the light levels tested (3.7-88 wm-2, 0.4-0.7 micron : 1 w m-2 = 103 erg-sec-'mC-2). All plants put on weight over the few days of the experiment, even those)laced at light levels below their compensation point. The plants adapted their respirationi rates to he a conistant proportion of their growth rates. Most of the adapt-ationi occurred within 24 hours of the light change. The a(laptation of respiration has implications for miiodels of light/growth relations in plant communities, almost all of whichi assume that respiration is proportional to leaf area and(independenit of growth rate or light level. The onily miiodel which does not is that of (le WVit, and(this gave good agreemiienit with ouir results. In order to he able to predict the influence of light on the rate of growth of a plant, one must measure the light in a way which is meaningful to the plant. Since l)lants are nonlinear devices, the rea(liilgs of a liniear light integrator are not mean-inigfutl in this sense (10). In principle. one could use the CO.,-uptake curve of the planit to predict growth rate froml light. T'he present experiment was designe(l to test this princil)le, iuni(ler the simplest possible coni(litionis. Materials and Methods Pla(11t .lIatcrioil (id Grolztl C(on ditionis. The material ulse(l wN,as Trifoliitm rcpcns L., Grassland(s D)ivision Ntucleus N.Z. White Clover C 1852 1962/3. The planits were grow%n in controlled-environment cabinets, at conistanit temperature, humidlity, daylength anid niutrien-t supply (20 + 1°, 85 + 5 % R.H., 12-hour (lay, Hoagland-type nutrienit in Perlite). The cabinets were lit from above with a bank of mercury tungsten reflector lamps, a water filter being used to remove heat radiation, The plants were raised from seedl at a high light level. The experiment was begun 50 days from germination , when the plants had reached a leaf area of 0.6 dM2 (leaf area index of 0.8). At this point there was no appreciable self-shading, so that the light received by the flat horizontal leaves could be accurately measured with a flat photocell. The plants were separated into 4 batches, which were grown for about a week at 4 different light levels, all other conditionis remaining constant. The rate of increase of dry weight was compared with wlhat would be expected from the rates of photosynthesis antd respiration , measured at the same light level. Light Measirenments. The illumination was measured in the plane of the majority of the leaves. 30 cm above the Perlite, with a color-corrected seleniuimi photocell calibrated in ft-c. This was converted to the irradiance within the wavelengths 0.4 anid 0.7 micron, in units of w*m-'. For reasoins given elsewhere (11), the irradiance is the quantity which is more meaningful to the plant. A conversion factor of 35 X 10-3, appropriate for this type of lanip, was applied to convert illumination in ft-c to irradiance 559