Five Ways To Increase Harvest Index

• Improving the harvest index is crucial for economic and ecological reasons.
• The means to increase the harvest index can vary with the crops.
• Plant breeding is more critical for maize, while irrigation practices are vital for rice.
• Growth regulators that increase source activity and strengthen sinks are needed in pineapple, while growth retardants to limit vegetative components are used in tomatoes.

The harvest index is a parameter used to estimate improvements in crop yields for many decades. Research and development have increased the harvest index for cereals and fresh produce. Find out some methods scientists have used in different crops to improve harvest index and increase food security.

Harvest Index

Harvest index (HI) is the ratio of yield to total plant biomass. It measures how efficiently a plant can partition resources to make harvestable products, which can be reproductive yields, such as seeds, fruits, and flowers, or vegetative yields, like tubers/roots, leaves, and stems.

The parameter was initially used in the breeding of cereals and has spread to improve yield in fresh produce, too. The benefits of increasing the harvest index are economic and can have a positive ecological impact.

The harvest index can increase by altering crop intrinsic and extrinsic factors.

• Intrinsic factors: Scientists improve and change targeted plant traits by altering the genotypes to increase yield.
• Extrinsic factors: Crop management that controls growing conditions can influence harvest index since environment, nutrition, water, and stresses also affect harvestable yield.

Some plant aspects changed to increase the harvest index are discussed below for various crops.

1. Dwarfing Rootstocks

The harvest index is increased in fruit trees by enhancing the useable fruit weight yield ratio to the total plant weight. Atkinson and Else (2005) reviewed the benefits of using dwarfing rootstocks in temperate fruit trees, apples, cherries, and pears to increase their harvest index.

Grafting is an ancient practice used to get trueness-to-type that is impossible from seeds or due to difficulties in the vegetative propagation of scions. Grafting was later used to increase the HI, and it was found that dwarfing boosts yield efficiency.

Dwarfing rootstocks increase HI by improving perocity, yield, fruit quality and size, and tolerance to abiotic and biotic stresses.

• Improved water and nutrition absorption: Rootstocks can have varying hydraulic capacities but usually increase water and nutrient absorption by scions, which boosts yield and HI.
• Better stress tolerance: Rootstocks are better adapted to climate and soil conditions; however, tolerances vary among rootstocks. They can increase the drought tolerance of a graft. Rootstocks with known tolerance to disease can confer the same benefits to the scion, improving yield.
• Control of size: Reducing the vigor and size of temperate trees through dwarfing rootstocks increases yield and is essential for economic fruit production. Reducing size reduces total plant biomass to improve the HI. The benefits from dwarfing of scions are many, such as:
  • The tree can divert dry matter to harvestable fruits instead of maintaining and growing a sizeable vegetative structure.
  • Smaller trees are easier to manage during pruning, spraying, and harvesting than large traditional trees. The costs of these operations are also reduced.
  • Dwarfed trees require fewer chemicals to control pests and diseases since all trees are easily reached, reducing the negative environmental impact of spray drift and contamination.
  • Dwarf trees require less water and nutrition, thereby saving resources and reducing the negative impact of chemical fertilizers on soil biota.
  • Tree density can be increased when trees are smaller to increase productivity per unit area of a farm.

Besides dwarfing mechanisms, chemical and physical methods exist to reduce shoot size and increase the harvest index in fruit trees.

2. Plant Growth Regulators

Table I. “Effect of gibberellin and cytokinin on the fruit weight, crown length and harvest index of pineapple. Fruit treatments were 0 (GA 0), 100 (GA 1) or 200 (GA 2) ppm of gibberellic acid and 0 (CK 0), 24 (CK 1) or 48 (CK 2) ppm of cytokinin,” Suwandi et al. (2016). (Credits: https://fruits.edpsciences.org/articles/fruits/pdf/2016/04/fruits150282.pdf

The development of pineapple fruits from the inflorescence sees a twenty-fold increase in weight. Leaf starch is directed to the fruit to meet the demands of the developing fruit. Agricultural management can increase fruitlet numbers and size in pineapples.

Plant growth regulators can improve source-sink movement and sink strength. Gibberellin, a growth regulator, improves assimilate-partitioning by activating enzymes involved in sugar metabolism. It has been shown to increase fruit number and size in grapes, delay maturity in citrus, and extend banana postharvest shelf life. However, the gibberellin application could not increase the pineapple harvest index in Indonesia. It increased fruit size, but the quality of the fruit was not good.

Hence, sometimes, it is necessary to combine growth regulators to support fruit cell division, enlargement, and nutrient mobilization and allocation. However, growth regulators like gibberellin (GA) and cytokinin (CK) can interact positively and negatively. GA and CK have been proven to have positive interactions to improve fruit weight in Hass avocado, some apple varieties (Golden Delicious’ and ‘Jonagold’), and Indonesian pineapples.

The amount of growth regulator used will influence results. For example, GA and CK increased HI, fruit weight, and quality in ‘Smooth Cayenne’ pineapples (see Table 1. GA applications of 100 or 200 ppm with 24 or 48 ppm CK increased HI, fruit weight, crown length, and delayed maturity. However, the specific combination of 100 ppm GA and 24 ppm CK produced good fruit quality in terms of soluble sugar, firmness, fiber, and vitamin C content compared to 200 ppm GA and 48 ppm CK.

Hence, it is vital to establish the optimum combination and application rates for both regulators.

3. Reducing Tomato Height and Side Shoots

In tomatoes, production efficiency for indoor farming can be increased if the volume of growing space is reduced. The plant architecture is reduced by shortening the internode and limiting side shoot production to improve the harvest index.

In an experiment, tomato plants were treated with a growth retardant- paclobutrazol (PAC). Then, the plant was topped after the second flower cluster. A 29% height reduction and a 28% reduction in vegetative shoot growth were achieved. Hence, even though the harvest was reduced by 12%, the harvest index improved from 0.53 to 0.58 on a fresh weight basis. Yield per cubic volume of space increased by 23% due to height reduction.

The side shoots of tomatoes were removed by a chemical Off-Shoot-O (OSO) and hand pruning. Hand pruning increased the HI to 0.71. However, OSO reduced side shoot numbers significantly and increased yield by 12%, giving an HI of 0.77. Since hand pruning needed three rounds of manual operations compared to only one for OSO-treated plants, labor charges were also reduced in the latter.

Hence, reducing vegetative shoot growth and canopy cover can increase tomato yield and HI. Authors Gianfagna et al. 1998 who performed this experiment, recommend these operational management techniques to increase tomato yield for planetary and lunar space stations!

4. Crop Breeding for Maize

Figure 1: “The evolution of maize harvest index over a century of breeding. Numbered grey lines refer to literature-era studies, and the blue and orange lines refer to data from this study. The red line represents the multi-dataset overall model adjusted with mixed models, considering the study and experiment within study as random, shaded areas represent the 95% confidence interval from the model, Ruiz et al., (2023). (Image credits: https://www.sciencedirect.com/ science/article/pii/S0378429023001843)

Genotype, environment, and management practices can all influence maize yield and harvest index.

Historically, there was little change in HI, which remained at 0.48-0.51 between 1920 and 1980 (see Figure 1). A study that tracked the progress of HI for 54 hybrids, plant density, and nitrogenous fertilizer shows that maize HI has changed by 17% over the last 56 years. These changes have resulted from plant breeding, not agronomic management practices like density or fertilizer treatments.

New data from 13 environments showed a 0.26% increase each year in HI due to genetics since 1964, adding up to a 15% HI increase in the US Corn Belt. The gains in yield have come from reduced barrenness, more photosynthesis, kernel number and weight, and extended maturity time.

There are physiological limits to HI increase. However, the maize HI increase has been lower compared to the 30% increase in rice and wheat during the same time. Moreover, the maize HI has been increasing only over the last few years, so the trend will likely continue and provide food security.

Future studies should focus on higher plant densities, fertilizer levels, and extreme weather to improve management contribution to maize HI.

5. Crop Management Techniques

Increasing water use efficiency (WUE) in rice is essential to maintaining and increasing yield. Crop WUE is the biomass accumulated per unit of water. Increasing WUE without affecting yield should be possible by manipulating the harvest index.

HI is variable and is associated with WUE and cereal yield.

It is possible to increase WUE by following several post-anthesis practices, such as controlled soil drying, non-flooded straw mulching, and alternate wetting and soil drying.

• Soil drying is helpful if the rice cultivar is too vigorous or if nitrogen fertilizer application levels are high.
• Non-flooded straw mulching is suitable for rice-wheat rotations in water-scarce areas.
• Alternate wetting and moderate soil drying are recommended for lowland farms.

These practices improve crop canopy cover, source-sink activity, and remobilization of stored carbon in vegetative organs to grains to increase HI in China.

These practices improve HI and yield and save water, making rice cultivation sustainable.

Different Pathways Increase Harvest Index

The five crops considered show that widely varying means can increase the harvest index. It involves studying shoots, roots, and leaves. Scientists will need precision tools for harvest index research. CID Bio-Science Inc. has instruments that cover these needs, such as the following:

• CI-110 Plant Canopy Imager,
• CI-340 Handheld Photosynthesis System,
• CI-602 Narrow Gauge Root Imager and CI-600 In-Situ Root Imager.

These tools can assist in increasing the harvest index to improve food security without overusing resources, especially water, and without any negative environmental impact.

Contact us to find out how CID Bio-Science Inc. can meet your toolkit needs for harvest index research.

Sources

Atkinson, C. J., & Else, M. A. (2003, October). Enhancing harvest index in temperate fruit tree crops through the use of dwarfing rootstocks. In Proceedings of the International workshop on cocoa breeding for improved production systems (pp. 118-131).

Gianfagna, T. J., Logendra, L., Durner, E. F., & Janes, H. W. (1998). Improving tomato harvest index by controlling crop height and side shoot production. Life Support & Biosphere Science, 5(2), 255-261.

Gur, A., Osorio, S., Fridman, E., Fernie, A. R., & Zamir, D. (2010). hi2-1, a QTL which improves harvest index, earliness and alters metabolite accumulation of processing tomatoes. TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik, 121(8), 1587–1599. https://doi.org/10.1007/s00122-010-1412-8

Ruiz, A., Trifunovic, S., Eudy, D. M., Sciarresi, C. S., Baum, M., Danalatos, G. J., … & Archontoulis, S. V. (2023). Harvest index has increased over the last 50 years of maize breeding. Field Crops Research, 300, 108991.

Suwandi, T., Dewi, K., & Cahyono, P. (2016). Pineapple harvest index and fruit quality improvement by application of gibberellin and cytokinin. Fruits, 71(4), 209-214.

Yang, J., & Zhang, J. (2010). Crop management techniques to enhance harvest index in rice. Journal of experimental botany, 61(12), 3177-3189.