STT 1043 Plant Physiology LU7 Light Interception PDF

Summary

These lecture notes cover light interception in plant physiology. The document details how factors like nutrient availability, water availability, temperature, and weed/insect control affect crop yield. It discusses light interception by the leaf canopy and how plant growth is influenced by the leaf area index (LAI).

Full Transcript

STT 1043 PLANT PHYSIOLOGY LU 7 Interception of Solar Radiation by Canopy Light interception Providing that nutrients and water are not limiting, weeds and insects are controlled and crops are grown under optimal temperature, the maximum yield of...

STT 1043 PLANT PHYSIOLOGY LU 7 Interception of Solar Radiation by Canopy Light interception Providing that nutrients and water are not limiting, weeds and insects are controlled and crops are grown under optimal temperature, the maximum yield of a crop is largely dependent on the availability and amount of light intercepted as PAR by the leaf canopy. The key feature of light energy capture is that incoming PAR is absorbed or reflected by the first surface it contacts. If it is not intercepted by actively growing leaves, its energy is not made available for plant growth. Light interception Thus, the size of leaf area in a crop canopy determines how much PAR is captured by the leaf and consequently influences canopy photosynthesis and crop yield. Typically, there is a strong linear relationship between the amount of light intercepted and the amount of crop dry matter produced. Leaf area expansion The canopy size of a growing crop at a given time is determined by: 1. time of crop emergence 2. phyllochron 3. leaf expansion rate and duration 4. rate of axillary leaf production 5. rate of leaf senescence Each of these factors can be under genetic, environmental or agronomic control 1. Time of crop emergence The date of sowing is important as it determines the time of seedling emergence. The time of crop emergence and its subsequent growth is largely affected by the photoperiod at the location of planting. For long-day plants, crops that emerge at a decreasing photoperiod will extend its’ vegetative growth (keep producing leaves much longer) and thereby produce a larger canopy of leaves. 1. Time of crop emergence In contrast, crops that emerge into an increasing photoperiod (spring-summer) will become reproductive quicker and therefore produce a smaller canopy growth. In annual crops, the accumulation of biomass is affected by the duration of vegetative growth which is controlled by the time of flowering. 1. Time of crop emergence Later flowering crops have prolonged vegetative growth and therefore are expected to produce more biomass than early flowering crops. However, the time of flowering in annual crops is dependent on the date when the crop is sown which requires consideration of photoperiod. 2. Phyllochron In most annual crops, the rate of leaf appearance (phyllochron) and then their expansion on the primary stem affects canopy expansion. Leaf appearance rate varies with the time of planting, depending on the duration of photoperiod at emergence. For eg., long-day annual crops that emerge into a decreasing photoperiod will have a slower rate of leaf production but an extended vegetative growth period. 2. Phyllochron Spring/summer sown crops that emerge into an increasing photoperiod will produce leaves at a faster rate than autumn sown crops because they flower quickly. However, for day-neutral crops, photoperiod does not affect the rate of leaf production. Leaf production rate of day-neutral crops is solely driven by temperature. 2. Phyllochron The rate of leaf production accelerates with increasing temperature and vice versa. 3. Leaf expansion The rate of leaf expansion determines the rate of increase of photosynthetic leaf area. For each crop species, there is a distinct ontogenetic trend in leaf size. The number of leaves produced on the primary stem can vary with growing conditions. 3. Leaf expansion Plants exposed to water stress can have and adverse effect on the rate of leaf expansion. Leaves that expanded under water stress tend to be smaller in size due to a reduction in the rate of expansion. Apart from sowing time and irrigation, nitrogen nutrition plays an important role in determining the size of leaves. 4. Axillary leaf The rate of leaf appearance determines its success in establishment and competition with other species. Specifically, the time to the first axillary leaf production is a key development phase in seedling establishment. It indicates the point in time when the plant begins to increase its leaf area exponentially and starts to form a leaf canopy where it can compete strongly for light. 4. Axillary leaf Therefore, plants that reach axillary leaf production earlier will have an advantage to capture more incoming radiation through the expansion of the canopy. In short, the success or failure of a species can depend on the time taken to reach axillary leaf production. Annual species produce axillary leaves much quicker compared with perennial. 5. Leaf senescence Senescence and death are an integral processes in the life history of a leaf, being primarily under genetic control, but also dependent on available nutrient and environmental resources. Senescence involves a decrease in photosynthetic potential. The first signs of leaf senescence are yellowing of the leaf tip (due to breakdown of chlorophyll), as nitrogen is mobilised out of the oldest leaf and into the younger leaves at the top of the crop canopy, or to other storage organs such as grains. 5. Leaf senescence The senescence of older leaves results in dead material that becomes litter at the base of the canopy. Leaf Area Index (LAI) The canopy leaf area is quantified as the leaf area index (LAI), which is the area of green leaf per unit area of ground. When a crop is first sown, there is no leaf area, because of the absence of leaf. Hence LAI is zero. As seedlings emerge, they produce new leaves and begin to intercept available light. Leaf Area Index (LAI) Eg., a 1 m2 of green leaf per 1 m2 of ground equates to a LAI of 1.0 To maximize photosynthesis, a canopy of leaves requires a leaf area index (LAI) higher than 1.0. This is because a canopy with a LAI of 1.0 will capture less than 50% of incident PAR due to large gaps in the canopy while the remaining light will strike on the ground surface and be lost from the photosynthetic system Leaf Area Index (LAI) Leaf Area Index (LAI) Even at high LAI, there are gaps in the canopy, so 100% light capture is usually unattainable. Hence, the LAI at which 95% of incident PAR is intercepted is defined as the critical LAI (LAIcrit). As the LAIcrit is reached, 95% of the available light will be captured but net herbage accumulation is minimal because of high senescence loss. Critical Leaf Area Index (LAIcrit) 1.0 Proportion of intercepted PAR 0.8 0.6 0.4 0.2 Critical LAI 0.0 0 2 4 6 8 10 Leaf area index Leaf Area Index (LAI) Leaf area of a crop in an intensively farmed area can be influenced by the following factors: 1. Temperature – determine crop emergence, leaf and branch production, leaf expansion and senescence. 2. Nitrogen status – determine size and longevity of leaves Leaf Area Index (LAI) 3. Plant population density – influence the efficiency of light interception 4. Water supply – modify leaf size and longevity Beside these four factors, leaf area of a crop is also dependent on secondary constraints imposed by environmental stresses and hazards such as frost, high temperature, herbivory, disease and etc. Canopy architecture and light interception The value of LAIcrit differs among crop types and species. These differences in LAIcrit can be attributed to the orientation of leaves towards the sun. Species that have erect leaves will have more light penetration through the canopy, and are therefore able to intercept more light. Thus, they have a higher LAIcrit compared with species with horizontal leaves. Canopy architecture and light interception The differences in LAIcrit can be attributed to leaf arrangement and leaf angle in the canopy which determines how they intercept the incoming PAR. The intercepted PAR by the crop canopy is distributed among the leaves at different height and arrangement. This means, some leaves (especially those at the top) are exposed to full sunlight while some (below the canopy) are illuminated by transmitted and reflected light. Canopy architecture and light interception Canopy architecture and light interception The arrangement of leaf area and its effectiveness in the capture of light energy is quantified by extinction coefficient (k). This reflects the transmission of light down through the canopy and is affected by leaf orientation and angle. Values of k range from 0.2, from canopies of erect leaves, to a maximum of 1.0 for crops with a horizontal canopy. Canopy architecture and light interception Erect leaf Horizontal leaf Canopy architecture and light interception For some species (eg. alfalfa), leaf angle alters from near vertical at the top to more horizontal at the lower levels in the canopy. The relationship among LAIcrit, k and crop growth rate is complex, but can be interpreted by assuming crops with a high LAI have a high area of photosynthetic green leaf per unit of ground area. The critical point is that the plant material intercepting the radiation must be green and capable of Practical applications In practical terms, manipulating a crop canopy to maximize this green leaf area is the key to maximizing biomass yield. Maximizing initial ground cover and then maintaining green leaf area, and avoiding premature leaf senescence, are the main goals of crop husbandry practices. Practical applications In general, small sized plant species (which is controlled by genetic factor) tend to produce small canopy of leaves. Therefore, crop management recommendation: Increase the plant population density to: 1. increase the interception of solar radiation 2. increase competition between adjacent plants Practical applications To maximize crop yields, it is important to manage the crop canopy to ensure the top leaves, that are intercepting the majority of the incoming radiation, are healthy and maximizing their photosynthetic rate. Therefore, crop management should NOT NEGLECT nitrogen fertilization!! And always irrigate your crops, otherwise your crops will have a reduced leaf size.

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