Radiative balance and temperature of differently pigmented cotton canopies.

Pigments are known to modify the spectral properties of foliage, which in turn affect the amount of radiant energy stored by the plant canopy. Studies have shown that red pigments (anthocyanin) increase leaf absorptivity of solar radiation, but little is known about their effect on canopy net radiation and temperature. We hypothesized that increased absorptivity of solar radiation caused by red pigments would result in higher canopy temperature when compared to that of a green canopy. To better understand the role of red pigments on canopy net radiation and temperature, we conducted a study where we measured leaf spectral properties, canopy spectral reflectivity, stomatal conductance, net radiation, and leaf and canopy temperature of red and green cotton (Gossypium hirsutum L.) canopies. On average, albedo of the red canopy was 0.02 lower than that of the green canopy, and most of the differences in reflected solar irradiance were in near-infrared wavelengths. Red canopy had greater net radiation than the green canopy throughout the measurement period, and that was due to its lower albedo. Red canopy was about 1 °C warmer than the green canopy on average; however, computer simulation indicates that albedo was of secondary importance in controlling canopy temperature. Contrary to our hypothesis, results suggest that lower stomatal conductance in the red leaves was the main driver of canopy temperature differences between red and green canopies.

Diurnal changes in water conduction in loblolly pine (Pinus taeda) and Virginia pine (P. virginiana) during soil dehydration.

We studied diurnal changes in water conduction during soil dehydration in 37-month-old seedlings of one Virginia pine (Pinus virginiana Mill.) and two loblolly pine (P. taeda L.) sources, one from North Carolina (NC) and the other from the "Lost Pines" areas of Texas (TX), in an environmentally controlled growth chamber. For seedlings of similar biomass, the TX source had higher values of transpiration, needle conductance, and plant hydraulic conductivity under well-watered conditions than the NC source. Under dry soil conditions, the TX source had lower values of water conduction than the NC source. The Virginia pine source responded similarly to the TX source under both well-watered and dry soil conditions. For all three pine sources, gradients between soil and needle water potentials were greatest when the seedlings were moderately stressed. The TX and Virginia pine sources had higher gradients and lower daytime needle water potentials under moderate stress conditions than the NC source. Predawn needle water potentials did not differ among the pine sources. We conclude that the TX and Virginia pine sources have decreased daytime needle water potentials and increased water potential gradients during the daytime under moderate stress conditions, but with no disruption of recovery at predawn. The greater rates of transpiration and water conduction by the TX source compared with the NC source under well-watered conditions suggest a means by which growth can be maximized prior to the onset of drought, thereby enhancing survival of loblolly pines in drought-prone environments.