Principles and Practices of Seed Production
Flowering is a prerequisite for seed production. It starts after floral induction which provokes a meristem to start flower bud formation (also referred to as flower initiation) after certain internal or external signals. Plant age or size and more specifically endogenous level of certain hormones are considered as internal cues; while, length of day/night and low temperature are external signals. These external stimuli allow synchronized flowering in a population at optimal time during a year to ensure successful pollination and seed setting before inclement weather conditions. In some conditions, two different developmental signals are required in succession, such as, two different photoperiods or low temperature treatment (vernalization), followed by certain photoperiod. Vegetables and flowering annuals vary in their vernalization and/or photoperiodic requirements to pass from juvenile (vegetative) phase to reproductive phase, which is a transitional process. For some species, vernalization (exposure to low temperature) is obligatory for flower induction and differentiation. These species are biennial and cannot start flowering without completion of their vernalization for examples, crucifers (cabbage, Chinese cabbage, cauliflower, turnip, kale, and kohlrabi), carrot (European types), onion, red beet, parsnip, celery and lilium (Lilium logiflorum). In such species, vernalization and day length synergistically promote floral induction. Some obligate species such as celery, globe artichoke, and carrot, require short days during vernalization for floral induction and long days after vernalization (during flower differentiation). While, some other species have facultative vernalization requirement and cold exposure is required just for flower induction and flower differentiation, and bolting is regulated by long days. In facultative vernalization requiring species, long days also compensate for unstable vernalization due to very short exposure to cold temperature. Examples of facultative species are leek, broccoli, radish, spinach, lettuce, and peas.
Usually, a temperature of 0–5°C is required to fulfill vernalization requirements of several crops. Higher vernalization temperature can results in delayed, incomplete and/or poor flowering, even de-vernalization in some species. However, summer cauliflower and broccoli can flower at 20-30°C without vernalization. Onion requires 2-13°C, temperate types vernalize at low temperature, while tropical types can vernalize at 9-13°C. Several vegetables and herbaceous (annual flowers) species respond to vernalizing temperature at certain developmental stage. Most of the crucifers can be vernalized when stem diameter is 10-15 mm. Carrot, onion, cauliflower, cabbage, coreopsis (Coreopsis grandiflora), gaillardia (Gaillardia × grandiflora), rudbeckia (Rudbeckia fulgida) and tobacco must have 8-12, 4-7, 4-12, 4-15, 8, 16, 10, 37 leaves, respectively, to respond to vernalization temperature.
Seeds of some crops, if exposed to low temperature during imbibition, can be vernalized, for example lettuce, turnip, spinach, Chinese cabbage, red beet, and white mustard (Sinapis alba L.). Exposure of ripening beetroot seeds (on the mother plant) to low temperature can also reduce the vernalization requirements. Moreover, vernalization requirement (duration of exposure to low temperature) of various cultivars of a crop are different. Therefore, sowing time should be adjusted according to the vernalization requirements i.e. cultivars requiring prolonged exposure should be planted earlier than those requiring short exposure time.
Vernalization response is common in winter annuals and biennials. Summer vegetable crops and summer annuals (flowers) usually require long days. Cucurbits are long day plants but, long days and high temperature promote production of staminate flowers and mild temperature and relatively short days promote gynoecy (femaleness). Flowering in short day plants, the native of low latitude on both sides of the equator, starts when day length is less than a particular critical time. Amaranth (African spinach), chrysanthemum, and poinsettia are short day plants.
Some vegetables like eggplant, tomato, cucumber and watermelon do not have specific day length requirement for flower initiation. Although cucumber is day insensitive but, long days promotes maleness and short days favour gynoecy (femaleness). Similarly, Asiatic carrot cultivars under long day conditions behave as annual and do not require vernalization temperature. In some crops, such as radish, cultivars without vernalization and specific day length requirement flower earlier when grown under long day conditions. So, for successful seed production, one must be familiar with photoperiod and low temperature requirements of crop(s). Other components of climate, such as irradiance and precipitation, also have significant role in flowering. Length of juvenile period can be reduced in pelargonium (Pelargonium × hortorum) by increasing the irradiance, through supplemental lighting or by providing growth promoting conditions (Armitage and Tsujita 1979).
Among other climatic requirements of flowering and seed setting is the prevalence of suitable temperature and absence of rainfall during flowering. Continuous rainfall during flowering can wash out stigmatic fluid and suppress anther dehiscence. High as well as low temperature can result in slow growth of pollen tube and/or embryo abortion. Increase in average daily temperature reduces number of flowers per inflorescence, e.g., in Pelargonium spp. Temperature during seed maturation can also affect germination. Some seeds have higher germinability when matured under higher temperature, while others showed more germination when matured at lower temperature. This effect is due to pre-conditioning effect of high or low temperature on seed development.