Page 152 - FRUTAS DEL TRÓPICO

152 Frutas del trópico

This information can be very useful for developing pruning strategies that maximize leaf
photosynthesis by allowing sufficient light into the canopy. In fact, many of the newer pruning

strategies for tropical fruit crops take into account the light level within the canopy in relation to

the light response curves for that species or cultivar. It is important to point out, that the light

environment in which a leaf develops affects its photosynthetic light response. For example,

leaves that develop in the sun on the outside of the canopy (sun leaves) are often less pho-

tosynthetically efficient than shade leaves which develop on the inside of the canopy, although,

in situ, sun leaves have a higher actual photosynthetic rate because that are exposed to high

light levels. This information can be useful to quantify shading effects on plants and develop

artificial or natural shading strategies for shade-adapted tropical tree species such as coffee.
The slope of the initial, linear portion of the light response curve is the quantum yield,

which is an indication of moles of CO fixed by the plant per quanta of light. For subtropical and
2
tropical fruit trees, there is generally a correlation between temperature and quantum yield.

For example, Figure 5 shows the quantum yield of ‘Hass’ avocado leaves in an orchard in Aus-

tralia during the winter and spring and autumn. During the cold temperatures in winter, quan-

tum yield is reduced. This information is useful for determining the low temperature range of a

species with respect to maximizing its photosynthetic potential. Even within their temperature
ranges, quantum yields often vary seasonally due to temperatures. For example, in Australia,

avocado leaves have a lower quantum yield and thus a lower photosynthetic efficiency during

winter when temperatures are cooler than during autumn (Figure 4).

Another useful physiological measurement to help determine the optimum temperatu-

re range for a particular species or cultivar is the use of chlorophyll fluorescence. During light

absorption by chlorophyll, a small amount of light (approximately 15%) is re-emitted from

chlorophyll. This is called fluorescence. If a leaf is put in the dark, as soon as light strikes it, a

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