No abstract

Frost risk assessment is of critical importance in tropical highlands like the Andes where human activities thrives at altitudes up to 4200 m, and night frost may occur all the year round. In these semiarid and cold regions with sparse meteorological networks, remote sensing and topographic modeling are of potential interest for understanding how physiography influences the local climate regime. After integrating night land surface temperature from the MODIS satellite, and physiographic descriptors derived from a digital elevation model, we explored how regional and landscape-scale features influence frost occurrence in the southern altiplano of Bolivia. Based on the high correlation between night land surface temperature and minimum air temperature, frost occurrence in early-, middle-and late-summer periods were calculated from satellite observations and mapped at a 1-km resolution over a 45000 km² area. Physiographic modeling of frost occurrence was then conducted comparing multiple regression (MR) and boosted regression trees (BRT). Physiographic predictors were latitude, elevation, distance from salt lakes, slope steepness, potential insolation, and topographic convergence. Insolation influence on night frost was tested assuming that ground surface warming in the daytime reduces frost occurrence in the next night. Depending on the time period and the calibration domain, BRT models explained 74% to 90% of frost occurrence variation, outperforming the MR method. Inverted BRT models allowed the downscaling of frost occurrence maps at 100-m resolution, illustrating local processes like cold air drainage. Minimum temperature lapse rates showed seasonal variation and mean values higher than those reported for temperate mountains. When applied at regional and subregional scales successively, BRT models revealed prominent effects of elevation, latitude and distance to salt lakes at large scales, whereas slope, topographic convergence and insolation gained influence at local scales. Our results highlight the role of daytime insolation on night frost occurrence at local scale, particularly in the early-and midsummer periods when solar astronomic forcing is maximum. Seasonal variations and interactions in physiographic effects are also shown. Nested effects of physiographic factors across scales are discussed, as well as potential applications of physiographic modeling to downscale ecological processes in complex terrains.

No abstract

Several studies conducted under high input conditions have indicated little susceptibility of pearl millet to water deficit untill early grain filling, because the losses in main shoot production were fully compensated by increased tiller fertility. The present study assessed the impact of water deficits at three development stages: prior to flowering (S30), at the beginning of flowering (S45), and at the end of flowering (S60) in pearl millet grown in experimental conditions similar to Sahelian farming conditions. It included a control irrigation treatment simulating the natural distribution of rainfall throughout the cropping season. Both biomass production and grain yield were severely reduced by S30 and S45, while S60 had no effect. In S30 and S45, the flowering of tillers was delayed or totally inhibited. In both of these treatments, the low number of productive tillers did not compensate for damage to panicle initiation and flowering of the main shoot. All treatments maintained green leaves on the main shoot during the grain filling period, and in S30 leaf growth recovered from mid-season drought. These results illustrate how pearl millet mostly escapes drought by matching its phenology to the mean rainfall distribution in the Sahel. In the case of mid-season drought, some late productive tillers and the maintenance of green leaf biomass of the main shoots limited, but did not overcome, the yield losses. This study stresses the importance of agro-ecological conditions in control treatments, particularly the water regime and crop density, when assessing crop drought resistance.

A comparative study of soil-plant water relations was conducted on three grapevine cultivars (Vitis vinifera L. cvv. carignane, merlot, shiraz) to investigate their adjustment to short-term and long-term water stress under field conditions. Adjustment was a function of the relative stability of the internal plant water status on diurnal and seasonal scales. On a diurnal scale, stomatal closure in response to water vapour pressure directly contributed to this stability. Indirect evidence suggested an influence of the soil water status on the diurnal stomatal activity. On a seasonal scale, sufficient leaf hydration required high whole-plant hydraulic conductance. This was achieved by either daily stomatal regulation or limitation of leaf area. Physiological adjustment to water stress through stomatal control was well developed in cv. carignane, which originated in a Mediterranean environment. However, cv. shiraz, which was of mesic origin, apparently adjusted to water stress by reducing leaf area. Our study demonstrates the utility of integrating data on stomatal conductance, leaf water potential and whole-plant hydraulic conductance to interpret whole plant adaptation to water stress, and elucidates two mechanisms by which genotypes differentially acclimate to water stress.

Reviewing the situation of quinoa production in southern Bolivia, Jacobsen (2011, J. Agron. Crop Sci. 197: 390) argues that the booming export market has a negative effect on the environment and on the home consumption of quinoa, thereby leading to an environmental disaster in the region. In view of the scarcity of scientific knowledge on the rapid social and environmental dynamics in the region, we consider that Jacobsen’s review misrepresents the situation of quinoa production in southern Bolivia. Specifically, we argue that (i) the data presented by Jacobsen (2011, J. Agron. Crop Sci. 197: 390) do not support any drop in quinoa crop yield supposed to reflect soil degradation and (ii) his demonstration regarding home consumption of quinoa is ill‐founded from both a nutritional and a cultural point of view. We suggest that the diffusion of the arguments exposed by Jacobsen (2011, J. Agron. Crop Sci. 197: 390), because of their flaws, might have strong negative impacts on those concerned with sustainable food production and fair‐trade with developing countries. We conclude that, rather than reinforced agro‐technical controls on local farmers, the rising competition in the international quinoa market requires a shift towards an ethical economy and ethical research cooperation with quinoa producers.

No abstract

A semi-empirical model of stomatal conductance was used to analyse porometer measurements on different grapevine cultivars under field conditions. Stomatal conductance was expressed as a function of quantum flux density, water vapour deficit, air temperature and leaf water potential.Where possible, the parameters describing the partial functions were estimated from field data and provide physiological insights into the transpiration process. They indicated differential stomatal sensitivity to air humidity in the various cultivars, a characteristic which could be related to their geographical origins.