“Defoliations had significant effect on fruit yield per plant. Result revealed that fruit yield increased over control (zero leaves cut) up to 6 leaves defoliated plants and decreased thereafter significantly… The higher fruit yield was recorded in control (zero), 3 and 6 leaves defoliated plants (18 and 36% leaf loss of the total) and the highest fruit weight was recorded in 3 and 6 leaves defoliated plants (1.70 kg per plant).
In contrast, the lowest fruit yield was recorded in 12 leaves defoliated plants (1.19 kg per plant).
The result indicates that tomato plant can tolerant up to 6 leaves loss during flowering. The fruit yield per plant increased under 3 and 6 leaves defoliated plants was due to greater number of fruits per plant and larger fruit size compared to control.
”[4]
[End Quote]
What this really tells us is that if defoliation is handled correctly, yield gains occur; however, if handled incorrectly the opposite is true. This really comes down to the percentage of leaf material that is removed from the plant.
It has been concluded that Leaf area distribution is an important determinant of rates of photosynthesis in the canopy.
“When the leaf area index (LAI; leaf area per unit ground area) increases, photon flux density (PFD) captured by the canopy increases, leading to higher photosynthetic production in the canopy. However, when leaves at the bottom receive PFD that is lower than the compensation point of photosynthesis, further increase in LAI decreases canopy photosynthesis. There is an optimal LAI that maximizes rates of photosynthesis.”[5]
Therefore, at some point, too much canopy receiving too little light results in lower than optimal photosynthesis.
Iqbal et al (2012) note that:
“the photosynthetic potential of lower leaves on a plant axis is less than that of the upper leaves. Thus, one aspect of crop improvement is to maintain a critical leaf number and leaf area for the greatest photosynthetic capacity and most efficient metabolism. The critical leaf number or leaf area may be maintained by the partial removal of leaves. The removal of leaves, partial or complete, has been defined as defoliation... It provides an opportunity for the photosynthetically active younger leaves to grow, efficiently utilize available water and mineral nutrients and influence source-sink relations. Defoliation (removal of leaves) influences growth and photosynthetic capacity of plants, remobilizes carbon and nitrogen reserves and accelerates sink metabolism, leading to improved source-sink relations. The response of plants to defoliation could be used to manipulate source-sink relations by removing lower and senescing leaves to obtain greatest photosynthetic capacity and efficient carbon and nitrogen metabolism under optimal and stressful environments.”[6]
On “source-sink relations”. Sources are plant organs such as leaves that produce sugars for photosynthesis. Sinks are plant organs such as roots, flowers and fruit that consume or store these sugars. In simple terms, sources produce sugars for sinks. Therefore, the source-sink relations relate to the areas of the plant that produce sugars and the areas of the plant that consume or store these sugars. The fact that leaves act as sources (i.e. they manufacture and provide the energy of/for photosynthesis) while fruit and flowers act as sinks (I.e. they consume and store the energy created by sources) is important to understand. Too little in the way of leaves (sources) can compromise the development of sinks (flowers and fruits). Therefore, while some careful defoliation may improve growth, too much defoliation will result in a lack of sources, typically resulting in reduced yields.
Another thing to be aware of is that studies have shown that root elongation essentially ceases within 24 hours after removal of approximately 50% or more of the shoot system and root mortality and decomposition may begin within 36-48 hours. Root respiration and nutrient acquisition are also reduced following defoliation, but to a lesser extent than root growth. Root respiration begins to decline within hours of defoliation and it may decrease substantially within 24 hours. In line with the reduction in root respiration following defoliation is a rapid reduction in nutrient absorption. Experiments conducted with perennial ryegrass growing in nutrient solution demonstrated that the rate of nitrate (NO3-) absorption began to decline within 30 minutes following removal of 70% of shoot biomass. NO3- absorption decreased to less than 40% of the pre-defoliation rate within 2 hours following defoliation. In these experiments, NO3- absorption continued to decline over the next 4-12 hours until it became negligible for 2 or 7 days before recovery began under high and low light intensities, respectively… Rapid reductions in root respiration and nutrient absorption following plant defoliation are proportional to the level of defoliation.
[7]
Basically, following defoliation a plant focuses its energy towards regrowth in the canopy, while sacrificing what is occurring in the below ground parts of the plant (I.e. root growth, root respiration and nutrient absorption/uptake is compromised).
Other studies have concluded that defoliation alters biomass allocation and chemical defence through the carbon–nutrient balance at the plant and at the leaf level. Plant nitrogen concentration, a measure of the carbon–nutrient balance in the plant, significantly decreases immediately after defoliation because leaves have higher nitrogen concentrations than stems and roots.
This means that nutrient considerations need to be made when practicing aggressive defoliation techniques.
You’re perhaps starting to see how complex all of this is, which is why defoliation is a high-risk practice. Handled correctly it can improve yields; however, if handled incorrectly, yield losses can result.
[1] Mabry. C.M. and Wayne. P. W. Defoliation of the annual herb Abutilon theophrasti: mechanisms underlying reproductive compensation. Oecologia July 1997, Volume 111, Issue 2, pp 225-232
[2] Briske, David D., and James H. Richards. "Plant responses to defoliation: a physiological, morphological and demographic evaluation." Wildland plants: physiological ecology and developmental morphology. Society for Range Management, Denver, CO (1995): 635-710.
[3] Politud ER (2016) Effects of artificial defoliations on the growth and yield of okra (Abelmoschus esculentus (L.) Moench.) cv ‘Smooth Cayene’ under mid-elevation condition
[4] A.F.M. Saiful Islamet al (2016) Effect of Defoliation on Growth and Yield Response in Two Tomato (Solanum lycopersicum Mill.) Varieties
[5] Hikosaka. K. Leaf Canopy as a Dynamic System: Ecophysiology and Optimality in Leaf Turnover. Ann Bot 2005 Feb; 95(3): 521–533.
[6] Iqbal. N. Massod. A, and Khan. N. A. Analyzing the significance of defoliation in growth, photosynthetic compensation and source-sink relations. PHOTOSYNTHETICA 50 (2): 161-170, 2012
[7] Briske, D. and Richards, J. (1995) Plant responses to defoliation A physiological, morphological and demographic evaluation
