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Influence of Plant Size on the Carbon and Water Relations of Cucurbita foetidissima HBK

A. K. Knapp and J. T. Fahnestock
Functional Ecology
Vol. 4, No. 6 (1990), pp. 789-797
DOI: 10.2307/2389445
Stable URL: http://www.jstor.org/stable/2389445
Page Count: 9
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Influence of Plant Size on the Carbon and Water Relations of Cucurbita foetidissima HBK
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Abstract

Plant size varies by two orders of magnitude within populations of Cucurbita foetidissima HBK. a perennial, prostrate vine of deserts and grasslands in the western USA. Differences in plant size in this species growing in a Kansas (USA) brome grassland were most evident in leaf size (10 times greater in larger vs small plants) and taproot mass (c 7000 g in large vs 13 g in small plants). This study examined the influence of plant size on net photosynthetic CO2 uptake (A), stomatal conductance to water vapour (g), transpiration rate (E), xylem pressure potential (ψ) and leaf temperature (Tleaf). During a period of drought in which co-occurring grasses and forbs experienced significant water stress (ψ < -2.5 MPa), ψ in large- and intermediate-sized plants of C. foetidissima was always >-0.6 MPa. In small plants, minimum ψ was c -1.3 MPa. No substantial diurnal reductions in A or g were measured in the two groups of larger plants, but CO2 uptake was reduced to zero after midday in small plants, presumably due to water stress. Increased mass of succulent taproots and greater rooting depth may have contributed to greater ψ in larger vs small individuals of C. foetidissima. Water use efficiency (WUE) was greater in leaves from large- and intermediate sized plants relative to small plants under all environmental conditions. During brief periods of low irradiance due to clouds or artificial shading, 5-10C reductions in Tleaf in plants with large leaves resulted in low E, enhancing WUE. Much smaller reductions in Tleaf and E occurred during shade in the small leaves of small individuals. These results indicate that greater size in C. foetidissima, while not increasing interception of irradiance in this grassland, resulted in greater independence from resource limitations as well as water conservation during periods of variable sunlight.

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