Design Calibration and Field Use of a Stomatal Diffusion Porometer

We grew velvetleaf (Abutilon theophrasd Medic.) and cotton (Gossypium hirsutm L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 peinsteins m-2sec-'. In air containing 21% 02, velvetleaf and cotton grown at 750 peinsteins m-2sec-I had maximm photosynthetic rates of 18.4 and 21.9 mg of CO2 dm-2hr-', respectively. Maximum rates for leaves grown at 320 and 90 peinsteins m-2sec' were 15.3 and 10.3 mg of C02 dm-2hr-' in velvetleaf and 12 and 6.7 mg of CO2 dm-2hr-' in cotton, respetively. In 1 O2, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air conting 21%°2, and plants grown at medium and high irradiance dW not differ in rate. In both species, stomatal conductance was not signflcantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with diferences in mesophyl con- Velvetleaf is a malvaceous annual weed of increasing importance to cotton growers (6). Chandler (6) reported cotton yield reductions of over 90%o with full season competition from dense infestations of velvetleaf. Both Chandler (6) and Bjorkman (4) have shown the need for comparative physiological studies to help explain competitive and environmental responses ofclosely related plants. Comparisons of such closely related weed and crop plants as velvetleaf and cotton should lead to a better understanding of weed-crop interactions.As part of a series of investigations of the comparative ecophysiology of weeds and crop plants, we studied the effects of irradiance during growth on the photosynthetic characteristics of velvetleaf and cotton. We determined the degree of adaptability of the photosynthetic apparatus to irradiance and the changes in some leaf characteristics which help to account for the adaptability. We also studied the effects of irradiance on certain growth parameters determined from mathematical growth analysis and related the differences in the growth parameters to differences in photosynthetic characteristics. were grown in a 1:1 (v/v) mixture of Bosket2 sandy loam and sand in a controlled environment chamber at a 31/25 C day/night temperature, 80%o relative humidity, and a 15-hr photoperiod. Light was supplied by a mixture of fluorescent and incandescent lamps. The temperature regime was chosen according to the method of Went (20) to represent average July day/night temperatures for Stoneville, Mississippi. The plants were grown in 15-cm pots and watered daily to field capacity with Hoagland solution. Screening in the chamber provided three irradiance levels during growth: 90, 320, and 750 ,tE m-2sec-1 PAR,3 400 to 700 nm. MATERIALSMeasurement of CO2 Exchange. The rates of CO2 uptake of single leaves of cotton and velvetleaf were determined at ambient C02 and 02 concentrations with an IR gas analyzer and pincer cuvette described previously (16). Photosynthetic rates were determined in air containing 21% 02 at 30 C at six irradiances ranging from 90 to 2,000 ,uE m-2sec' ...

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

Patterson

Duke²,

Hoagland³

1978

“…For the period of vegetative growth (May through early August), the average daily maximum and minimum air temperatures were 29 C and 18 C, respectively. From these values, average day and night temperatures were calculated by the method of Went (22) (15) for both leaf surfaces on well exposed canopy leaves in the two environments. Resistances to H20 flux were converted to resistances to CO2 flux by multiplying by 1 .605 (14).…”

Section: Methodsmentioning

confidence: 99%

Photosynthesis in Relation to Leaf Characteristics of Cotton from Controlled and Field Environments

Patterson¹,

Bunce²,

Alberte³

et al. 1977

In situ and light-saturated net photosynthetic rates per unit leaf area were greater in cotton (Gossypium hirsultum L.) plants grown in pots in the field than in similar plants from a phytotron growth chamber. Lightsaturated stomatal resistances did not differ in leaves of similar age and exposure on field and chamber plants; lower photosynthetic rates in chamber leaves were associated with greater mesophyli resistance. Differences in net photosynthetic rates were related to differences in leaf thickness. When In the present study, we found differences in in situ photosynthetic rates under ambient conditions in the field and chamber environments. Measurements of the response of photosynthetic rate to changes in light intensity were then made under standard conditions in the laboratory to determine whether the differences observed in situ could be related to the plant material itself rather than to differences in the ambient light environments. Stomatal diffusive resistance, leaf anatomy, and chloroplast lamellar characteristics were also studied as possible explanations for the observed differences in photosynthetic rates.MATERIALS AND METHODS Growing Conditions. Cotton (Gossypium hirsutum L. var. McNair 612) was grown in a growth chamber and out-of-doors in 25-cm plastic pots containing a 1:1 gravel-vermiculite mixture. All of the plants were watered to excess, morning and afternoon, with modified half-strength Hoagland solution. Plants were grown in a chamber programed for a 12-hr thermoperiod with a 26 C/20 C day/night temperature. The chamber photoperiod was 14 hr, comprised of 12 hr fluorescent and incandescent lighting (650 ,ueinsteins m-2 sec-' photosynthetically active radiation, 400-700 nm) with 1-hr extensions by the incandescent lamps (50 /ieinsteins m-2 sec-' PAR5) preceding and following the 12-hr full light. Daytime relative humidity was maintained at 70% and an automatic CO2 supplementation system (18) maintained the chamber CO2 concentration at or above 300 ppm.

“…Cleaf -Ccup ra + ri rO + ri (1) During the past few years the water vapor diffusion porometer (1,4,8,9) has become a widely used tool for field studies of stomatal resistance, replacing, to a considerable degree, other types of porometers which measure viscous air flow or some other parameter.…”

mentioning

confidence: 99%

Leaf Temperature Effects on Measurements of Diffusive Resistance to Water Vapor Transfer

Morrow

Slatyer

1971

ABSTRACTis low (ri = 1-2 sec cml). When Tleaf-Tup = 5 C, the ratio drops below 0.5. Under high leaf resistance conditions (rz = 10-50 sec cm-') the comparable ratios are approximately 0.9 and 0.7, respectively. sensor which is sensitive over a narrow range of relative humidity at the low end of the relative humidity range. In operation, the cup is dried by air from a silica gel dehydrator. It is then placed over the leaf. Transpiration proceeds, the humidity in the cup increases, and the time required for the sensor to respond over a narrow, prescribed (usually 1-2%) range is determined.Since the relative humidity at the sites of evaporation within the leaf is close to 100% (it is generally assumed to be the saturation vapor pressure at the leaf temperature [7]), and since the humidity sensor is observed over a narrow range at the low end of the relative humidity scale (say, 18-20%), the water vapor concentration difference from leaf to air is assumed to be constant during the period of measurement. Consequently, the transpiration rate (E) can be assumed to be steady during the period of measurement and inversely proportional to the time-lapse (At) which occurs as the sensor responds over the measurement range.Calibration is based on the frequently used transpiration equation:During the past few years the water vapor diffusion porometer (1,4,8,9) has become a widely used tool for field studies of stomatal resistance, replacing, to a considerable degree, other types of porometers which measure viscous air flow or some other parameter.The attraction of the diffusion porometer is that it measures the actual diffusive resistance of the leaf to water vapor movement, and the transfer processes and pathways that are involved are essentially the same as in natural transpiration. These where eleaf (mm Hg) is the saturation vapor pressure at Tleaf and ecup (mm Hg) is the actual vapor pressure at the mid-point of the measurement range, and at Tcup; Cleaf and ccup (g cm-') are the corresponding water vapor concentrations. The coefficient a converts vapor pressure in mm Hg to water vapor concentration in g cm73. (It has the value 2.89 X 104 e/T when T is in degrees Kelvin). The term ra (sec cm7l) is the diffusive resistance external to the leaf (i.e., of the porometer cup itself) and r1 (sec cm-l) is the diffusive resistance of the leaf. Since E = k/At, where k is a coefficient of proportionality, this expression can be rewritten asAt any one level of Tleaf and T,u,X the right hand side of equation (2) is a constant, so (ra + rl) can be uniquely determined, for any one porometer system, by measurements of At. Calibration is generally carried out by determining At when the porometer is located over a range of perforated Plexiglas plates, the diffusive resistance of which is calculated from diffusion theory (4). At the base of each plate is a layer of wet blotting paper to simulate the sites of evaporation within the leaf.Since water vapor must diffuse from the sites of evaporation through the perforations in each plate, and...