Harvesting the sun: Canopy & Field Scale

Explain each part of the crop growth rate equation.

Crop growth rate (kg ha -1 d-1 ) = [(Light interception) x RUE] – respiration

Light interception is the amount of the solar radiation intercepted by the crop. It has units of MJ (or μmol photons) per unit area (m 2 ) per unit time (s). RUE (radiation use efficiency) is the amount of CO 2 fixed by photosynthesis per unit of light absorbed. Respiration is the crop loss of CO2 due to respiration by non-photosynthetic parts and at night.

What does radiation use efficiency (RUE) account for?

RUE is affected by numerous factors which affect photosynthetic efficiency – factors such as whether a plant is C3 or C4, stress effects on leaf function (e.g. nutrients, water, pests, etc.), and light saturation effects.

In the first half of a crop’s growth, what is canopy-scale photosynthesis tightly

correlated with?

Canopy-scale photosynthesis is tightly correlated with light interception, which increases as the plant grows more leaves and a higher percentage of light is intercepted. In contrast, RUE remains fairly constant in non-stressed plants, and respiration plateaus beyond a modest value.

What does crop growth rate (CGR) measure?

CGR is a measure of net canopy photosynthesis (photosynthesis in excess of canopy respiration).

In non-stressful conditions, what is the biggest determinant of solar energy use in

a crop canopy (a community of crop plants in the field)?

The total amount of solar radiation intercepted.

What is Leaf Area Index (LAI)?

Leaf area per unit land area.

Why is it important that crops rapidly develop a canopy of leaves in the early

season?

Typically a lot of solar radiation is not captured during the rather long time it takes for a crop to completely cover the land with leaves and reach 100% interception. Early development of leaf canopy will increase the amount of light utilized for crop growth.

What can be done to hasten the development leaf area in a crop?

One can hasten the development of crop leaf area by planting a higher number of seed per unit land area (higher population density). Also, one can use seed of relatively large size and seed that has high vigor. Other approaches used in certain crops is to first grow seedlings in a protected seedling bed or in pots that are later transplanted into the field (this is used in rice paddies and in some vegetable crops).

The majority of canopy photosynthesis takes place in what part of the canopy?

In the upper layers of the leaf canopy, however this can vary based on leaf orientation. Canopies with more vertical leaf orientation have a more even distribution from top to bottom of the canopy.

Name two advantages of vertically oriented leaves (Supplemental information not

in the lecture)

1) Allows light to penetrate canopy and illuminate more leaves, (2) each spot on a leaf is exposed to lower light intensity so the photosynthesis system does saturate and the leaf is not heated up so much.

Name two advantages of horizontally oriented leaves (Supplemental information

not in the lecture)

(1) Better light interception early in a crop season as canopy is being established (prior to canopy closure) or if population density is low, (2) better weed competition, (3) increased leaf temperature in a cool environment where photosynthesis is temperature limited (i.e. spring).

In canopies with vertically oriented leaves, the light intensity at any particular spot

is lower. Why is this an advantage?

Light is spread out so it does not saturate a particular spot on the leaf, and by not saturating, a) light is used more efficiently, and b) it lowers leaf temperature, which avoids thermal damage and decreases transpiration (increases water use efficiency). Also, by spreading out the light, there is a larger leaf surface area in the canopy that gets exposed to light.