Despite widespread application in studying climate change impacts, most crop models ignore complex interactions among air temperature, crop and soil water status, CO 2 concentration and atmospheric conditions that influence crop canopy temperature. The current study extended previous studies by evaluating T c simulations from nine crop models at six locations across environmental and production conditions. Each crop model implemented one of an empirical (EMP), an energy balance assuming neutral stability (EBN) or an energy balance correcting for atmospheric stability conditions (EBSC) approach to simulate T c . Model performance in predicting T c was evaluated for two experiments in continental North America with various water, nitrogen and CO2 treatments. An empirical model fit to one dataset had the best performance, followed by the EBSC models. Stability conditions explained much of the differences between modeling approaches. More accurate simulation of heat stress will likely require use of energy balance approaches that consider atmospheric stability conditions.

Marketing of phosphite-containing preparations for foliar application, together with recent reports of positive yield responses, has revived the question as to whether phosphite (HPO À 3 ) is a suitable P source for plants. Two experiments using zucchini (Cucurbita pepo L. convar. giromontina) have been conducted to evaluate the P-nutritional effect of phosphite either provided via the substrate or as a foliar spray. Plants grown in a P-deficient substrate were severely damaged when phosphite was applied as foliar fertiliser and more drastically when provided via the substrate. Growth of P-deficient plants receiving phosphite as a foliar spray was impaired in a dosedependent manner after foliar P application (concentrations 0.0, 0.9, 2.7, and 4.5 g P L -1 ), while foliar provision of phosphate improved plant growth and yield. In the youngest leaves of phosphite-treated plants, which had developed after foliar spray, phosphite accumulated to considerable extent, reaching a similar concentration as phosphate at tissue level. These results confirm that P-deficient plants are very sensitive to phosphite, which represents a nutritionally ineffective form of P. It should thus not be considered as a form of P suitable for fertiliser manufacture.

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We investigated the leaf : stem partitioning of winter wheat (Triticum aestivum L. varieties 'Dekan' and 'Batis') with and without drought influence. Irrigated and drought-stressed winter wheat, grown in a rainout shelter in 2009/10 and 2013/14, were sampled during shoot elongation phase at the experimental Farm Hohenschulen located in Northern Germany. The data set contains leaf (DM L ) and stem dry masses (DM S ), as well as measured water contents for several soil layers. A reduced relative dry matter allocation to leaves was observed under drought stress. Our results clearly show that, if simulated leaf : stem partitioning is not sensitive to drought, this will cause a positive bias in simulated leaf and a negative bias in simulated stem dry matter under water-limited conditions. This is in accordance with previous studies which revealed that crop simulators often overestimate the impact of drought on light-use efficiency, whereas the consequences on leaf area development are underestimated. However, the drought stress-induced shift in leaf : stem partitioning is yet not considered by most common wheat crop simulators. Our aim was to fill the gap in simulation of drought stress implications on leaf area development. A simple allometric model for leaf : stem partitioning (InðDM S Þ ¼ g Á InðDM L Þ þ h) was parameterized. Starting from the allometric leaf : stem relationship observed under optimum water supply, a correction term was introduced, which allows to adapt the partitioning to drought stress conditions. The lg-transformed root-weighted soil water potential in the rooting zone (lgw root , lg(hPa)), calculated as a function of measured water contents and simulated root distribution, was used as a drought stress indicator. The linear correction term assumes an increase of the stem fraction, proportional to the difference between lgw root and a drought stress threshold (pF crit , lg(hPa)). The analysis revealed that the shift in allometric partitioning towards stem fraction starts with lgw root greater than 1.92 [lg (hPa)]. The slope of the relative increase of dry matter allocated to the stem fraction was determined with 0.26 [lg(hPa) À1 ]. Both parameters of the correction term were found to be highly significant. Implications for crop modelling are discussed.