Agriculture for Engineers 4 (3+1)

Lesson 20. Effect of Different Weather Parameters on Crop Growth and Development -I

20.2 DIFFERENCE BETWEEN WEATHER AND CLIMATE

Weather is a state or condition of atmosphere at a given place and at a given time. It is daily variations or conditions of lower layers of the atmosphere. Weather pertains to smaller area like village, city, or even district and smaller duration of time i.e.  part of a day, or complete day. Some examples are hot day, rainy day, cloudy weather, dry weather, etc.

Climate is a generalized weather or summation of weather conditions over a given region during a comparatively longer period. Climate is related to larger areas like zone, state, country, part of continent and longer duration of time like month, season, or year and best described by the normal and averages e. g.  cold season, tropical climate, temperate climate, etc.

When it is desired to give the state of weather at any particular place, the following weather elements are considered. (1) solar radiation (2) air temperature (3) rainfall (4) wind movement (5) relative humidity (6) atmospheric pressure and (7) clouds.

20.3 SOLAR RADIATION

Solar energy provides light required for seed germination, leaf expansion, growth of stem and shoot, flowering fruiting and thermal condition necessary for the physiological function of the plant. The effect of solar radiation on plant communities can be divided into four categories (a) Thermal effect of radiation (b) Photosynthetic effect of radiation (c) Photoperiodic effect and (d) Other effects.

20.3.1 Thermal effect of radiation

More than 70 per cent of the solar radiation absorbed by the plant is converted into heat. This heat energy is utilized for transpiration and for convective heat exchange with the surroundings. This exchange determines the temperature of leaves and of other plant parts.

20.3.2 Photosynthetic effect of radiation

A portion of solar radiation, up to 28 per cent in terms of energy is used in photosynthesis. Solar radiation influences the production of enzymes useful in photosynthesis, development of photosynthetic apparatus, growth, yield formation and finally yield.

20.3.2.1 Enzymes

The reduction of carbon dioxide to carbohydrates is catalyzed by enzymes, namely phosphophenol pyruvate carboxylase and ribulose biphosphate carboxylase. Light intensity increases activity and amount of these enzymes.

20.3.2.2 Development of photosynthetic apparatus

The different pigments necessary for photosynthesis are produced in the presence of light. Chlorophyll formation is promoted by light. Light influences the orientation of leaves also. With higher light intensity, leaves become horizontal.

 20.3.2.3 Growth

Interception and utilization of solar radiation can be increased by proper management practices, such as adjustment of row spacing, plant population and selection of most advantageous time for planting.

20.3.2.4 Yield formation and yield

Light intensity affects yield attributes and finally yield. In groundnut, low light intensity during peak flowering reduce number of flowers per plant. Flower open during cloudy period do not produced pegs. Low light intensity at pegging and pod filling reduces peg and pod number. In cereal, number of tillers increase with increase in light intensity. Reduction in grain yield of rice in rainy season compared to summer season is attributed to solar radiation.

20.3.3 Photoperiodic effect

Crop developmental processes like rate of leaf production, flowering, etc. are influenced by duration of sunlight. Most plants are influenced by relative length of day and night especially for floral initiation. The duration of the night or complete darkness is more important than the day light. This effect of light on plant is known as photo-periodism. Depending on the length of photoperiod required for floral initiation, plants are classified as long day, short day and day neutral plants.

Long day plants require comparatively long days (usually more than 14 hours) for floral initiation. They put forth more vegetative growth when days are short. Most of the temperate crops like wheat, barley and oats fall under this category.

In short day plants, floral initiation takes place when days are short (less than 10 hours) or when the dark period is long. Most of the tropical crops like rice, sorghum, maize, etc. are short day plants.

Day neutral plants do not require either long or short dark periods. Photoperiod does not have much influence for phasic change for these plants. The crops belong to this group are cotton, sunflower, buck wheat etc.

20.3.4 Other effects

20.3.4.1 Assimilation of nutrients: In maize accumulation of phosphorus is high under white, yellow, orange and light blue light and high under darkness. Higher solar radiation alone gives higher protein content due to greater assimilation of nitrogen.

20.3.4.2 Translocation of photosynthates: sucrose and fructose accumulated in culms up to two to three weeks after anthesis in cereals. Fructose appears to be the most important storage carbohydrate in leaves and culms prior to grain filling, plants shaded during grain filling are able to retranslocate stored photosynthates to grain, thus maintaining certain amount of stability.

20.3.4.3 Utilization of solar energy: The basic principle for increasing yield is harvesting more solar energy. All the management practices like optimum time of sowing, plant population, fertilizer application, irrigation, etc. are aimed at increasing the interception of solar radiation by the foliage so as to get the more yield.

20.3.4.4 Sensitive stages for solar radiation: In broad terms, leaves are considered as source for supplying carbohydrates to grains while storage organs are called sinks. In some crops like rice, wheat, etc., source is not limited but yield potential is less due to less number of storage organs. For such sink limited crops, amount of solar radiation in sufficient quantities is necessary during the period of formation of storage organs. i.e. from panicle initiation to flowering.

20.3.4.5 Conversion efficiency: Efficiency of conversion of absorbed radiation to dry matter decreases with the age of crop for the production of one gram of dry matter 3700 to 4100 calories of solar energy is utilized.

20.4 TEMPERATURE

20.4.1 Cardinal temperature: For each species of plants there are upper (maximum) and lower (minimum) limits of temperature at which growth is nil or negligible and optimum temperature at which growth is maximum. Most of the crop plants grow best at 15 to 30  ^ºC. Many crop plants die at temperature of 45 to 55 ^º C. There are also optimal temperatures for different growth stages.

20.4.1.1 Cool season crops : The crops which grow best in cool weather period are called cool season crops and are generally grown in winter season ( November to February). Most of the cool season crops cease to grow at an average temperature of 30 to 38 ^ºC. The important cool season crops are wheat, barley, potato, oats, etc. These crops are also called temperate crops. The cardinal temperature ranges for cool season crops are maximum Temperature 30-38 ^ºC. Minimum temperature 0-5 ^ºC and optimum temperature 25-30  ^ºC.

20.4.1.2 Warm season crops: The important warm season crops are rice, sorghum, maize, sugarcane, pearl millet, groundnut, pigeon pea, cowpea, etc. These crops are also called tropical crops. These crops are generally grown in monsoon and some also in summer season. The cardinal temperature ranges for warm season crops are maximum temperature 45-50 ^ºC, minimum temperature 15-20 ^ºC and optimum temperature 30-38 ^ºC.

20.4.2 Influence of temperature on growth

20.4.2.1 Biochemical reaction: Any chemical reaction increase with increase in temperature. This rate of increase in reaction for every 10 ^ºC increase in temperature is called quotient 10 or Q10 . 

          Where t is temperature in  ^ºC.

20.4.2.2 Uptake of carbon dioxide: The optimum temperature for net carbon dioxide uptake is about 24^ºC for wheat and barley. As the temperature increases above the optimum, C₂ uptake is decreased due to increase in stomatal and mesophyll resistance.

20.4.2.3 Enzymatic activity: Temperature increases the activity of certain enzymes important in the reduction of carbon compounds including ribulose diphosphate dehydrogenase and glyceraldehyde dehydrogenase.

20.4.2.4 Rate of photosynthesis: Rate of photosynthesis is reduced due to reduction in temperature. When maize plants are subjected to cold temperature of 10^ºC for 10 days, the rate of photosynthesis is reduced by 33 per cent of that of untreated plants.

20.4.2.5 Development of photosynthetic infrastructure: Temperature has considerable influence on chlorophyll synthesis and leaf area development. Temperature enhances the production of chloroplast. At low temperature leaves become yellow due to degradation of chlorophyll. Temperature governs rate of leaf emergence and leaf expansion.

20.4.2.6 Influence on growth substances: At optimum temperature, the activity of auxin, gibberellins and cytokinins (growth promoters) are high and activity of abscisic acid (growth regulators) is low with the result that growth rate is increased.

20.4.2.7 Dry matter production: The response of dry matter production to temperature depends on the stage of the crop and optimum temperature. Higher temperature during maturity of maize depressed the dry matter accumulation, while higher temperature over the normal, increased growth during tasselling and silking.

20.4.3 Influence of temperature on development

Temperature has greater influence on development rates like rate of germination, leaf initiation, tillering, flowering, spikelet initiation and grain filling. All these development processes proceed at a faster rate at higher temperature.

20.4.3.1 Yield formation and yield

Effect of temperature on grain formation, grain filling and grain yield is complex. Low temperature during panicle initiation stage to flowering results in formation of higher number of grains per plant due to prolongation of this period. Yield therefore, increases though the duration is more. Grain filling is faster with increase in temperature and this decreases duration of grain filling period in several crops. In wheat, average temperature more than 19⁰ C during grain filling period reduces duration of grain filling.

20.4.3.2 Growing Degree- Days

The heat unit or growing degree- day concept was proposed to explain the relationship between growth duration and temperature. A degree-day or a heat unit is the mean temperature above base temperature. Mathematically, it can be expressed as, 

Where, T_max is maximum temperature, T_min is minimum temperature and Tb is the lowest temperature at which no growth which is also called base temperature.

20.4.3.3 Photothermal Units

In photothermal units, the degree-days are multiplied by length of night in case of short day plants and length of day for long day plants. The basic principle is that flowering is hastened as the length of night increases in short day plants, while in long days plants, flowering is delayed as the length of night increases. It can be expressed mathematically as

20.4.4 Extreme temperatures

Excess or deficit of any growth factor is called stress. High or low temperature causes stress on crops.

20.4.4.1 High temperature stress

High temperature stress adversely affects mineral nutrition, shoot growth and pollen development resulting in low yield.

Mineral nutrition: High temperature stress causes reduction in absorption and subsequent assimilation of nutrients.

Shoot growth: High temperature, even for short period, affect crop growth especially in temperate crops like wheat. High air temperature reduces the growth of shoots and in turn reduces root growth.

Pollen development: Higher temperature during booting stage results in pollen abortion. In wheat, temperature higher than 27^ºC cause under development of anthers and loss of viability of pollen.

20.4.4.2 Low temperature stress

Low temperature affects several aspects of crop growth viz., survival, cell division, photosynthesis, water transport, growth and finally yield.

Survival : Temperate crops like wheat and barley have high resistance to low temperature damage especially at very early stage.

Cell division and cell elongation : Low temperature results in retardation of cell elongation than cell division.

Photosynthesis: When C₄ plants like maize and sorghum are subjected to low temperature of 10^ºC, the activity of pyruvate dikinase is reduced, resulting in less photosynthesis.

Water transport: Low temperature cause moisture stress. Entry of water into the plant is restricted due to low permeability of cells. The active transport of water from roots to the shoot is stopped at low temperature.

Vegetative growth: Temperate crops prefer low temperature during vegetative growth, while tropical plants require high temperature. In maize, seedling growth is reduced by 50 per cent at 10^ºC.

Reproductive growth: Low temperature causes high spikelet sterility in rice. It ranges from 3.6 to 96.8 percent depending on variety. The critical temperatures for spikelet sterility are 15-17 0^ºC. The main reasons for the failure of fertilization are (1) uneven pollen (2) indehiscence of anthers, and (3) abnormalities in micropores.