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Alternate bearing on yield components, biochemical characteristics, and antioxidant enzyme activities of Iranian commercial date palm cultivars

This study demonstrates that the alternate bearing phenomenon in Iranian date palm cultivars significantly influences yield components, biochemical properties, and antioxidant enzyme activities, with 'ON' bearing trees exhibiting higher yields but lower physiological quality compared to 'OFF' trees, while the 'Khare-Khosh' cultivar consistently outperformed others in most measured traits.

Original authors: Mehdi Khayyat, Mehdi Zarei

Published 2026-07-14

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Mehdi Khayyat, Mehdi Zarei

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a date palm tree as a hardworking athlete who has to decide between two very different training schedules every year. One year, they go "all-in" on the big game, producing a massive pile of fruit. The next year, they take a break, producing very little fruit but getting stronger for the future. This rollercoaster ride is called alternate bearing, and it's a huge headache for farmers because it makes income unpredictable.

Scientists in Iran decided to investigate why this happens in three popular date varieties: 'Kabkab', 'Khare-Khosh', and 'Zahidi'. They wanted to know what was happening inside the tree's "body" (its leaves and flowers) during these high-yield years (called ON years) versus the low-yield years (called OFF years).

The Great Yield Trade-Off

Here is the big surprise: When the trees went into ON mode, they produced a huge amount of fruit—up to 63.65 kg tree⁻¹. But there was a catch. Because the tree was so busy feeding all those fruits, the individual fruits were smaller and less sweet.

In contrast, during OFF years, the trees didn't produce much fruit at all. However, the fruits that did grow were the champions of the bunch: they were heavier (15.55 g), longer (61.60 mm), and packed with more sugar (35.92 °Brix). It's like the tree was saying, "I can't feed a whole army this year, so I'm going to make sure the few soldiers I have are super-powered."

The Tree's Internal Battle: Heat and Stress

The study took place in a hot, dry part of Iran where summer temperatures can get brutal. The researchers found that ON years were physically tougher on the trees.

The Sweat Test: In ON years, the leaves got much hotter, reaching temperatures as high as 53.26°C in the 'Zahidi' variety. In OFF years, the leaves stayed cooler, dropping to 36.52°C in the 'Khare-Khosh' variety.
The Leaky Bucket: When plants get too hot, their cell membranes can get damaged and start leaking. The trees in ON years had much higher "electrolyte leakage" (38.90%) compared to OFF years (21.69%). Think of this as the tree's skin getting a bit cracked from the heat stress.
The Thirst: Because the tree was working so hard to make fruit in ON years, its leaves were drier, with a relative water content of only 65.77%. In OFF years, the leaves were plump and hydrated, holding 74.06% water.

The Tree's Emergency Toolkit

So, how did the trees survive these tough ON years? They pulled out their emergency chemical toolkit.

The Stress Alarm (Proline): The trees in ON years pumped up their levels of a substance called free proline to 151.56 µmol g⁻¹ FW. This acts like a cellular shield, helping the tree hold onto water and protect its cells from the heat.
The Cleanup Crew (Antioxidant Enzymes): The heat and stress created dangerous "rust" inside the tree (called reactive oxygen species). To fight this, the trees in ON years activated their cleanup crew enzymes:
- Peroxidase: 432.17 units mg⁻¹ FW min⁻¹
- Catalase: 498.98 units mg⁻¹ FW min⁻¹
- Glutathione reductase: 1022.73 units mg⁻¹ FW min⁻¹
- Phenylalanine ammonia-lyase: 15.01 units mg⁻¹ soluble protein

These numbers were significantly higher in ON years than in OFF years. The tree was essentially running a marathon, so it had to work its immune system overtime to keep from collapsing.

The Star Player: 'Khare-Khosh'

Not all trees handled this stress the same way. The 'Khare-Khosh' cultivar was the MVP of the study.

It produced the highest total fruit yield (81.88 kg tree⁻¹ in one ON year).
It kept its leaves cooler and held more water than the others.
It had the highest levels of protective chemicals (like 176.46 µmol g⁻¹ FW of proline and massive amounts of antioxidant enzymes).
It also managed to keep more potassium (a vital nutrient) in its leaves (9.20 mg g⁻¹ DW in OFF years) compared to the other varieties.

The 'Zahidi' cultivar, on the other hand, struggled the most. It had the smallest fruits, the hottest leaves, and the lowest levels of protective chemicals.

What the Science Says (and Doesn't Say)

The researchers measured all of this directly in a real orchard over three years (2021, 2022, and 2023). They didn't just guess; they measured the fruit, weighed the leaves, and tested the chemicals in a lab.

The study suggests that the tree's ability to handle the stress of making a lot of fruit depends heavily on its genetics. The 'Khare-Khosh' tree seems to have a better "engine" that allows it to produce a lot of fruit without burning out as quickly as the others. However, the paper notes that this is just one study on three specific varieties. There are many other date palm types in Iran, and scientists suggest we need to study those too to see if they have similar superpowers.

In short, making a lot of fruit is a stressful job for a date palm. It burns more energy, heats up the leaves, and requires a massive chemical defense system. Some trees, like 'Khare-Khosh', are just built better for the job.

Technical Summary: Alternate Bearing in Iranian Commercial Date Palm Cultivars

Problem Statement
Alternate bearing, characterized by cyclical fluctuations between high-yield (ON) and low-yield (OFF) years, represents a significant economic constraint for date palm (Phoenix dactylifera L.) cultivation. This phenomenon leads to severe fruit abscission and yield instability. While the causes are often attributed to nutritional imbalances, hormonal shifts, and environmental stressors like high temperatures, the specific physiological mechanisms—particularly the role of biochemical properties and antioxidant defense systems—remain insufficiently understood in Iranian commercial cultivars. High temperatures, a prevalent stressor in arid regions, induce oxidative stress via reactive oxygen species (ROS), potentially damaging photosynthetic apparatus and reducing productivity. This study aims to elucidate the interplay between bearing status, biochemical characteristics, and antioxidant enzyme activities to explain the physiological basis of yield fluctuations.

Methodology
The research was conducted as a factorial experiment in a Randomized Complete Block Design (RCBD) with five replications. The study spanned three consecutive growing seasons (2021, 2022, and 2023) on a commercial plantation in Nehbandan, South Khorasan Province, Iran.

Factors: The experimental factors were bearing status (ON vs. OFF) and cultivar ('Kabkab', 'Khare-Khosh', and 'Zahidi').
Plant Material: Twenty-year-old trees were selected, with 25 trees per cultivar. All trees received uniform agronomic management, including manual pollination, fruit thinning, and irrigation with specific electrical conductivity and pH levels.
Measurements:
- Yield Components: Total fruit yield, fruit weight, length, pulp weight, and total soluble solids (TSS) were assessed at the Tamar stage.
- Physiological & Biochemical Traits: Leaf temperature, relative water content (RWC), and electrolyte leakage were measured. Leaf tissues were analyzed for total chlorophyll, free proline, total soluble carbohydrates, total soluble proteins, and potassium (in both leaves and inflorescences).
- Enzyme Activities: The activities of key antioxidant enzymes—peroxidase (POD), catalase (CAT), glutathione reductase (GR), and phenylalanine ammonia-lyase (PAL)—were quantified in leaf homogenates.
Statistical Analysis: Data were analyzed using two-way ANOVA followed by Tukey's HSD test at $P \leq 0.05$ .

Key Results
The study revealed distinct physiological and biochemical differences between ON and OFF bearing years, as well as significant genotypic variations among the cultivars.

Yield and Fruit Quality: The ON bearing status produced the highest total fruit yield (63.65 kg tree⁻¹). Conversely, OFF trees exhibited superior fruit quality metrics, including higher fruit weight (15.55 g), length (61.60 mm), pulp weight (13.16 g), and total soluble solids (35.92 °Brix). The 'Khare-Khosh' cultivar consistently demonstrated the highest total fruit yield, while 'Zahidi' showed the lowest yield and smallest fruit dimensions regardless of bearing status.
Leaf Physiological Status: OFF trees displayed significantly higher leaf relative water content (74.06%), total chlorophyll (3.01 mg g⁻¹ FW), and lower leaf temperatures compared to ON trees. ON trees experienced higher leaf temperatures and electrolyte leakage, indicative of greater heat stress and membrane damage.
Biochemical Accumulation:
- Stress Markers: ON trees accumulated higher levels of free proline (151.56 µmol g⁻¹ FW), suggesting a response to the combined stress of heavy fruit load and high temperatures.
- Nutrients and Solutes: OFF trees maintained higher concentrations of total soluble carbohydrates (179.53 mg g⁻¹ FW), total soluble proteins (8.20 mg g⁻¹ FW), and leaf potassium (8.37 mg g⁻¹ DW). In contrast, ON trees showed higher inflorescence potassium, reflecting translocation to the developing fruit.
Antioxidant Enzyme Activities: A notable finding was that ON bearing status triggered significantly higher activities of antioxidant enzymes, including peroxidase (432.17 units mg⁻¹ FW min⁻¹), catalase (498.98 units mg⁻¹ FW min⁻¹), glutathione reductase (1022.73 units mg⁻¹ FW min⁻¹), and PAL (15.01 units mg⁻¹ soluble protein). This suggests an upregulation of the defense system to counteract oxidative stress induced by high temperatures and heavy crop loads.
Cultivar Performance: The 'Khare-Khosh' cultivar emerged as the most resilient, exhibiting the highest yield, RWC, chlorophyll, proline, carbohydrates, potassium, and antioxidant enzyme activities, alongside the lowest leaf temperature and electrolyte leakage. 'Zahidi' consistently performed the poorest across most parameters.

Significance and Claims
The paper claims that the yield components, biochemical properties, and antioxidant enzyme activities in date palms are strongly dependent on both the bearing status and the specific cultivar. The study demonstrates that:

Physiological Trade-offs: High yield (ON years) comes at the cost of reduced fruit size, lower soluble solids, and increased physiological stress (higher temperature, electrolyte leakage, and proline accumulation), necessitating a robust antioxidant response.
Genotypic Variation: There is significant genetic heterogeneity among Iranian cultivars. The 'Khare-Khosh' cultivar possesses superior adaptive mechanisms, including better osmotic adjustment (high proline and RWC) and a more active antioxidant defense system, allowing it to sustain high yields under harsh, hot conditions.
Screening Potential: The identified biochemical markers (e.g., antioxidant enzyme activities, leaf potassium, and RWC) and physiological traits (leaf temperature) can serve as effective tools for screening heat-tolerant and high-yielding cultivars in breeding programs.

The authors conclude that while the study provides valuable insights into the physiology of alternate bearing in these three cultivars, further research is required to explore these mechanisms in the broader diversity of Iranian date palm cultivars. The work does not propose new management protocols but rather highlights the physiological basis for yield fluctuations to inform future selection and breeding efforts.