Crop responses to environmental factors
- Temperature:
a) Chilling injury: Plants adapted for warm regions if exposed to prolonged chilling temperature above freezing point are injured and the growth is also retarded. Transport of water from roots is reduced and plant cannot cope with transpiration loss. Rice and cotton are killed near freezing temperature if it prevails continuously for 2 -3 days.
b) Freezing injury: Generally it occurs in temperate regions where the water is frozen in ice crystals which results in dehydration and killing the cells.
c) Heat injury: At very high temperature there may be the problem of heat injury which triggers high respiration, defoliation and premature dropping of fruits. Seedlings are more susceptible to heat injury.
- Light:
- Photo-periodic effects relate directly to the timing of both the light and dark periods.
a) Long-day plants: A long-day plant flowers when the day length exceeds their critical photoperiod of 14 hrs. e.g. Carnation (Dianthus), Henbane (Hyoscyamus), Oat (Avena),
Ryegrass (Lolium), Clover (Trifolium), Bellflower (Campanula carpatica). Some long-day facultative plants are:Pea (Pisum sativum), Barley (Hordeum vulgare), Lettuce (Lactuca sativa), Wheat (Triticum aestivum, spring wheat cultivars), Turnip (Brassica rapa), Arabidopsis thaliana (model organism).
b) Short-day plants: Short-day plants flower when the day lengths are less than their critical photoperiod. They cannot flower under long days or if a pulse of artificial light is shone on the plant for several minutes during the middle of the night; they require a consolidated period of darkness before floral development can begin. Some short-day obligate plants are:Chrysanthemum, Coffee, Poinsettia, Strawberry, Tobacco, var. Maryland Mammoth,
Common duckweed, (Lemna minor), Cocklebur (Xanthium), Maize – tropical cultivars only. Some short-day facultative plants are:Hemp (Cannabis), Cotton (Gossypium), Rice, Sugar cane.
c) Day-neutral plants: Day-neutral plants, such as cucumbers, roses and tomatoes, do not initiate flowering based on photoperiodism at all; they flower regardless of the night length. They may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalisation (a period of low temperature), rather than in response to photoperiod.
- Water:
a) Aquatic plants, hydrophytes, or hydrophytic plants:
- They are plants that are naturally adapted to growing in water or waterlogged soil.
- They are able to thrive with watery places as their natural habitat due to special morphological and anatomical modifications like the presence of modified roots called “knee pneumatophore”.
- Some crops belonging to this plant classification are gabi or taro (Colocasia esculenta), lowland rice (Oryza sativa), members of water hyacinth family (e.g. Monochoria vaginalis), water lily (Nymphaea spp.), papyrus and umbrella plant (Cyperus spp.).
b) Lithophytes or lithophytic plants:
- They are plants with special adaptation to growing on rocks or in rocky terrain having scant humus.
- They absorb nutrients from the air, rain, and decaying organic matter which accumulate on the rocks. Examples are orchids belonging to the genera Vanda, Ascocenda, Ascocentrum, and Trudelia.
c) Mesophytes or mesophytic plants:
- also called moist-loving plants, belong to the terrestrial plants.
- Their natural habitats consists of moderate conditions for plant growth. These conditions are described as not excessively dry but not too wet. Many cash crops which are grown in tropical climates having even distribution of rainfall are mesophytes. Examples are corn and many fruit trees and vegetables.
d) Terrestrial plants:
- also called land plants, are those that grow on land with their roots in the soil.
- Their body parts are divided into two main groupings: the aerial parts and the underground parts.
- Most farm crops belong to this plant classification and are further grouped into special classifications such as heliophytes, mesophytes, sciophytes, etc.
e) Phreatophytes:
- are plants adapted to arid conditions by growing long roots which obtain water from underground reserves.
- The mere presence of these plants indicate a stable supply of underground water and such knowledge has been applied by digging wells close to them.
- Examples are the mesquite (Prosopis), cottonwood tree (Populus) and California fan palm (Washingtonia filifera).
f) Lithophytes or lithophytic plants
- are plants with special adaptation to growing on rocks or inrocky terrain having scant humus.
- They absorb nutrients from the air, rain, and decaying organic matter which accumulate on the rocks. Examples are orchids belonging to the genera Vanda, Ascocenda, Ascocentrum, and Trudelia.
g) Halophytes or halophytic plants,
- also called salt loving plants, are plants that can tolerate growing under saline conditions or in natural habitats which are excessively rich in salts.
- Included under the halophytic plant classification are the nipa (Nypa fruticans), talisay (Terminalia catappa), bakawan (Rhizophora mucronata) and many other mangrove species
h) Xerophytes, xerophytic or xeric plants,
- also called dry loving plants, are those having plant adaptation to regions with scant or no water and hot conditions.
- Their natural habitats are the arid and semi-arid regions and those places with long summer drought.
- A specialized type of landscaping which emphasizes the use of xerophytes is termed xeriscaping.
- Examples of xerophytes are the members of the Lamiaceae and Compositae, the olive (Olea), and the succulents such as the cacti and those belonging to the genera Asparagus, Euphorbia, Agave, Aloe, Crassula and Sansevieria (botany.wisc.edu, 2009). The pineapple and other bromeliads are also included under this plant classification.
I) Psammophytes:
- the plants which grow on sand or small pebbles usually in dry habitats like Acacia Senegal.
j) Psychrophytes:
- are the plants which grow in the soil which become very cold and the availability of water is limited on the account of freezing injury.