Plant Propagation from Seed
Hardening or Acclimating Seedlings
During the seed propagation cycle, the objective is to promote ideal seedling growth and development. This is largely accomplished by providing optimal levels of all potentially limiting factors and minimizing environmental stresses in the greenhouse. Following seed propagation, however, seedlings must be hardened prior to transplanting. To induce plant hardiness and properly condition plants to resist stresses, growing conditions are gradually adjusted such as moisture, light, and nutrients. These adjustments must not be too severe because since overly stressed plants will actually be less hardy. Proper hardening takes time, and it is a common mis¬take to rush the process. With¬out proper hardening, seedlings are likely to suffer from transplant shock, grow poorly, or die on the outplanting site. In general, transplants should have well-balanced shoot and root development.
Irrigation Deficit and Water Stress
Reducing irrigation duration or frequency creates a mild moisture stress, slows shoot growth, leaf area expansion, promotes carbohydrate level in the leaves, and helps condition plant stock to withstand drier conditions on transplanting. This reduced water availability decreases the possibility of producing top-heavy plants and encourages the formation of smaller leaves with thicker cuticles that transpire less (lose less water) after outplanting.
Light Adjustment
The use of sunlight and shading as a conditioning treatment depends on the conditions on the outplanting site and the light needs of the species. Plants that will be planted into full sun conditions should receive minimal or no shading during greenhouse cultivation, especially during the hardening phase.
Nutrition Deficit
Reducing or stopping fertilization along with reduc¬ing irrigation slows shoot growth and hardens plants. Among the mineral nutrients, nitrogen, particularly in the ammonium form, is the primary driver of shoot growth. During hardening, it can be helpful to reduce or stop nitrogen fertilization to induce a mild nutrient stress.
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