Like many species, the model plant Arabidopsis thaliana exhibits multiple different life histories in natural environments. We grew mutants impaired in different signaling pathways in field experiments across the species' native European range in order to dissect the mechanisms underlying this variation. Unexpectedly, mutational loss at loci implicated in the cold requirement for flowering had little effect on life history except in late-summer cohorts. A genetically informed photothermal model of progression toward flowering explained most of the observed variation and predicted an abrupt transition from autumn flowering to spring flowering in late-summer germinants. Environmental signals control the timing of this transition, creating a critical window of acute sensitivity to genetic and climatic change that may be common for seasonally regulated life history traits.
stricting their application in retrospective or validation studies (Hutchinson, 1991). Crop growth models require solar irradiance as input data, yetThe need for solar irradiance data for crop models there are few places where such data are routinely measured. For has led researchers to develop a number of methods for locations where measured values are not available, solar irradiance simulating such data. For example, some crop modelers can be estimated using empirical models such as the Bristow-(e.g., Rosenthal et al., 1989) have incorporated stochas-Campbell (B-C) model. This study was conducted to assess the spatial and seasonal accuracy of the B-C model for midcontinental locations tic weather generators into their simulations. These in Kansas. A 30-year data set from Manhattan, KS, was used to weather generators simulate irradiance and other metecalibrate and evaluate unmodified and modified forms of the B-C orological and climatological inputs based on probabilismodel. The effect of seasonality was investigated by subdividing the tic criteria. This approach eliminates the need for meayearly data into two subsets, a high noontime solar elevation angle sured solar irradiance; however, it seems reasonable period, ranging from DOY 121 to 273, and a low noontime elevation that estimated, rather than randomly generated, solar angle period comprising the remainder of the year. The B-C model irradiance values would also result in improved yield eswas also evaluated without seasonal division of the year. The calitimates. brated models were then tested against measured solar irradiance A number of techniques are available for estimating values for 10 sites distributed across the state of Kansas. Results solar irradiance. These vary in sophistication from simindicate that, for the calibration site at Manhattan, irradiance was ple empirical formulations based on common weather more accurately estimated using a modified form of the B-C model. For the yearly data, root mean square error (RMSE) was 3.9 MJ m Ϫ2 or climate data to complex radiative transfer schemes d Ϫ1 (25% error), compared with 5.2 MJ m Ϫ2 d Ϫ1 (24% error) for the that explicitly model the absorption and scattering of high solar elevation angle period and 3.6 MJ m Ϫ2 d Ϫ1 (32% error) the solar beam as it passes through the atmosphere. for the low solar elevation angle period. The RMSE for the 10 test Hall, Kansas State University, Manhattan, KS 66506-0801; R.L. Vanwhere A, B, and C are empirical coefficients. Although derlip,
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