L'hybride double est le produit du croisement de deux hybrides simples. Du fait que l'on croise des génotypes plus ou moins hétérozygotes, il y a des disjonctions au niveau des produits du croisement. C'est donc une variété hétérogène. En amélioration des plantes, ce type de variétés est utilisé lorsque la vigueur des lignées parentales ne permet pas de produire des semences hybrides simples de bonne qualité et avec un rendement suffisant. On perd ainsi en niveau de vigueur, mais on gagne en stabilité de production due à l'hétérogénéité génétique de la variété. De plus, le prix de revient de la semence est inférieur à celui d'un hybride simple. Les hybrides doubles sont souvent utilisés en début de sélection, puis sont remplacés par des hybrides simples lorsque les lignées parentales sont de vigueur suffisantes.
La production de la semence hybride double s'obtient par semis alterné, en champ isolé, de l'hybride simple choisi comme femelle (organes mâles castrés) et de l'hybride simple choisi comme mâle.
Hybride trois voies.
L'hybride trois voies est le produit du croisement entre un hybride simple (femelle) et une lignée pure (mâle). C'est encore une variété hybride hétérogène, mais moins que l'hybride double. Elle est utilisée pour les mêmes raisons que l'hybride double, lorsque la vigueur des lignées prises comme mâles est suffisante pour assurer une bonne pollinisation au moment de la production des semences.
Hybride producteur direct.
En viticulture, l'hybride producteur direct est une variété de vigne obtenue tout d'abord par croisement entre une vigne sauvage d'origine américaine et une variété européenne de Vitis vinifera, puis par recroisements consécutifs entre eux des hybrides obtenus précédemment. Cette variété hybride présente une résistance plus ou moins élevée aux champignons parasites qui provoquent le mildiou, l'oïdium, le black-rot et l'anthracnose. En revanche, la qualité des fruits et des vins fournie par l'hybride producteur direct est inférieure à celle des variétés de Vitis vinifera. On ne peut donc pas l'utiliser dans les vignobles où la production est renommée.
Hybride naturel.
L'hybride naturel est le produit d'un croisement fortuit entre deux plantes. Au cours de la production de semences des variétés, l'hybridation fortuite avec des plantes ne faisant pas partie de la génération multipliée doit être évitée. Pour cela, la production de semences doit se faire à toutes les générations en conditions parfaitement isolées, sans plantes de la même espèce au voisinage du champ de multiplication.
Gallais
hydro-injecteur
Système de remplissage rapide des cuves de pulvérisateurs. SYN. : hydro-emplisseur.
Dans l'hydro-injecteur, un jet d'eau sous pression, issu de la pompe principale, arrive au niveau d'un étranglement à la canalisation de remplissage, ce qui crée une dépression aspirant l'eau avec un fort débit.
Aubineau
hydrolyse
1. Chimie. Décomposition chimique d'un corps par fixation d'eau.
2. Pédologie. Mécanisme d'altération des aluminosilicates individualisant l'aluminium sous forme de composé insoluble, la silice restant en solution à l'état d'acide non dissocié.
On distingue l'hydrolyse totale, appelée « allitisation », de l'hydrolyse partielle, appelée « siallitisation ». La seconde se différencie en monosiallitisation lorsque le processus conduit à une argile 1/1 dont la charge est nulle électriquement (cristallisation de kaolinite en régions tropicales), ou en bisiallitisation lorsque le processus conduit à une argile 2/1 de type smectite et qui présente une certaine charge, compensée par la rétention de cations basiques en position interfoliaire (régions tempérées).
MCGirard
hydromorphe
Qualifie un sol soumis à un excès d'eau permanent ou temporaire.
MCGirard
hydromorphie
Phénomène dû à un excès d'eau dans la couverture pédologique ou un horizon.
L'excès d'eau peut résulter des précipitations atmosphériques, mais aussi des apports d'eau superficiels (inondations, ruissellement, etc.) ou profonds (remontées de nappe, etc.). Il peut être plus ou moins durable, se traduisant par une saturation de toute la porosité accessible par l'eau. Il y a donc un déficit d'oxygène (anaérobie) entraînant une modification de l'activité biologique, ce qui a pour principale conséquence des processus d'oxydoréduction conduisant à une ségrégation du fer. L'hydromorphie prolongée ou proche de la surface est défavorable au développement de la plupart des plantes cultivées (à l'exception notable du riz).
Dans les horizons organiques, l'anaérobiose provoque une minéralisation de la matière organique fraîche et une diminution de la décomposition. Si l'hydromorphie est temporaire se développent des hydromulls, hydromoders et hydromors dont la couleur est gris foncé, la structure fine devenant massive quand la durée de saturation augmente ; le taux de carbone organique est souvent inférieur à 8 %. Si la saturation est longue se développe l'horizon anmoor de couleur noire, à structure massive avec un toucher onctueux quand le matériau est saturé d'eau, et un taux de carbone organique supérieur à 8 %.
MCGirard
hydroxy-coumarine
Produit issu de la coumarine et dont de nombreux dérivés (coumafène, coumatétralyl, difénacoum, flocoumafen, bromadiolone) sont utilisés pour la lutte contre les rongeurs nuisibles aux cultures et aux produits récoltés.
Les dérivés de l'hydroxy-coumarine provoquent des hémorragies internes chez les animaux qui les ingèrent. Ils sont généralement utilisés en appâts ou en toxiques de piste. Certains sont très toxiques pour l'homme, les animaux domestiques et le gibier.
Raynal
hygiène
Discipline de la médecine humaine et de la médecine vétérinaire étudiant les moyens propres à conserver ou favoriser la santé.
Bougler/Gallouin
hygrométrie
Mesure de l'humidité de l'air.
Cette mesure se fait par détermination du point de rosée (hygromètre d'Alluard), par détermination de la masse volumique de l'eau dans l'air au moyen d'un analyseur de concentration (absorption de rayonnement, par exemple), par détermination de la température humide (psychromètre) ou encore par celle de l'humidité relative (hygromètre à cheveux).
Perrier
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