Page 147 - FRUTAS DEL TRÓPICO

intRoduction to teRminology and techniques foR studying enviRonmental
physiology of subtRopical and tRopical fRuit cRops 147

Leaf gas exchange
A common method to measure physiological stress to plants is measuring leaf gas exchan-

ge. Leaf gas exchange includes measurements of net photosynthesis, transpiration, stomatal

conductance and internal CO concentration also called the internal partial pressure of CO .
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Measuring these variables often provides a very good indication of plant vigor or plant stress

prior to any visible signs of damage or improved vigor. About 20 years ago, portable leaf gas

exchange systems became commercially available allowing for easy measurement leaf gas ex-

change in a greenhouse or in orchards. Figure 1 shows a typical portable infrared gas analyzer

for measuring leaf gas exchange. Simply put, a leaf or portion of a leaf is enclosed in a cuvette

and an air stream is passed through a cuvette at a constant known flow rate. The analyzer
measures the amounts of CO taken from the air stream into the leaf for photosynthesis and
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the amount of H O leaving the leaf during transpiration. Prior to the development of portable
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gas exchange systems, there was not much information about leaf gas exchange of tropical fruit

crops, particularly in orchards, because the instruments used to measure it were very cumber-

some and generally weren’t available in tropical areas. However, since these instruments have

become commercially available, there has been an increase in published information about

the effects of abiotic on biotic factors on plant stress in subtropical and tropical fruit orchards.
Net photosynthesis (Pn) often referred to as net CO assimilation (A) or carbon exchange
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rate (CER) is calculated as the rate at which CO is taken out of the atmosphere by the leaf.
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Single leaf photosynthesis measurements provide good indications of a plants photosynthetic

potential, however, may not be good indicators of actual photosynthesis because of mutual

shading of leaves and variable microclimates within a canopy. Therefore, there has been incre-

asing interest in measuring whole plant photosynthesis. However, for orchard trees, this can

be quite challenging, involving the use a very large whole plant chambers.

Universidad Autónoma de Chiapas