Abstract
Diurnal leaf and canopy gas exchanges of well-watered field grown cotton were measured. Our objective was to scale leaf-level values of transpiration and net assimilation to the whole canopy level using estimates of canopy leaf area. Single leaf gas exchange measurements were made with two portable photosynthesis systems and canopy measurements with four open Canopy Evapo-Transpiration and Assimilation (CETA) chamber systems. Canopy leaf area was measured at the end of the experiment and estimated during gas exchange by fitting values to a growth curve. Leaf level measurements were arithmetically scaled to estimate canopy level gas exchange based on canopy leaf area and then compared to the measured values. Scaled values of single leaf transpiration were very similar to canopy transpiration measurements, although both whole canopy transpiration and assimilation were overestimated around mid-day. We conclude that canopy cotton transpiration of well-watered field grown plants could be estimated within 5% throughout the day by scaling leaf level measurements to the whole canopy using measured canopy leaf area. Estimating canopy assimilation from leaf level measurements remains problematic.
Highlights
Crop production is restricted to a range of suitable environmental conditions in which the plants are able to function
Scaled values of single leaf transpiration were very similar to canopy transpiration measurements, both whole canopy transpiration and assimilation were overestimated around mid-day
Attempts to estimate whole canopy net assimilation (Aest) by scaling leaf level responses by either whole canopy leaf area or by intercepted radiation were less successful and either over- or under-estimated assimilation (Figure 4(A) and Figure 4(B)), depending on whether scaling was based on absorbed photon flux or on a simple leaf area
Summary
Crop production is restricted to a range of suitable environmental conditions in which the plants are able to function. It follows that there exists a set of environmental conditions within which a crop functions optimally, resulting in maximal potential harvestable yield. Since crop yield is the product of plant function, agronomic “plant stress” can be defined as a reduction in realized yield resulting from sub-optimal whole plant physiological, metabolic, or developmental processes. Plant based irrigation scheduling seeks to determine the strain within the production system resulting from organismal water deficits, rather than the environmental conditions surrounding the crop; “Plant Stress” will continue to be used because it is a commonly used term. It should be borne in mind that “Biological Strain” more accurately describes organismal responses and physiological dysfunction in response to sub-optimal environmental conditions and to specific stressors [5]
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