Citrus is an important fruit crop in the world which is one of the main sources of vitamin C. The global annual revenues of citrus industry account for more than 9.5 billion dollars. Important commercial citrus species include oranges, mandarins, lemons, grapefruit, and limes. Israel exports around 160-170 thousand tonnes of citrus fruits every year and especially European nation serve as major destination. In 2014-2015, 75% of Israel’s citrus exports were exported to European markets (USDA Foreign Agriculture Service, 2015). Some of the major citrus producing countries in arid and semi-arid regions such as Spain, Italy Egypt and Israel are subjected to high solar radiation and scarce water resource. Further, citrus trees bear large canopies which represents a great evaporative surface, their transpiration rate often exceeds root water uptake rate. Israeli scientists are exploring new techniques to produce better yield with higher quality citrus fruits under high solar radiation and water scarcity. Citrus trees exhibit a relatively long juvenility phase (lasting 2 to 5 years, depending on the cultivar) before trees reach the mature stage to produce flowers.
Light is one of the most important environmental factors for plants, as it provides the source of energy for plant growth. It is therefore not surprising that plants can sense light signals such as light quantity, quality, direction and duration. Continuous efforts of light manipulation have been made in horticultural techniques to improve crop performance, such as the use of shade netting and light-emitting diodes. Among these methods, netting is the most commonly used technique. Black and reflective nets are widely used for shading purpose, while clear and transparent nets are often used for pest control and for protecting plants from environmental hazards, e.g. hail, wind and frost. But these types of nets have only reduced the light quantity, they do not alter the light spectral composition. In recent years, an innovative agricultural technology was developed based on regular shade netting technique called photoselective netting. It refers to the use of special netting products into which chromatic elements are incorporated during manufacturing. Photoselective netting refers to covering crops by nets which have the capacity to selectively filter the intercepted solar radiation, in addition to their protective function. Each photoslective net might differentially absorb UV, Blue, Green, Red, FR or IR spectral regions, and at the same time enrich the scattered and diffused light, which would improve light penetration into the inner canopy, thereby increasing radiation use efficiency of plants. This approach is mainly aimed at better utilization of the solar energy. Surprisingly, photoselective nets also revealed pronounced benefits in pest and fungi control.
Since colored nets could bring many benefits, Israeli scientists applied it on young orange trees to see whether they could shorten the juvenile phase, fasten the tree establishment and enhance fruit yield by evaluating the root growth and development. There was no study addressed the effects of colored nets on roots of orange trees before. In their study, the two year old grafted orange trees grown under three different photoselective nets (red, pearl and yellow) and control trees were grown without nets.
Root photographs were taken during different developmental stages of the trees during the year using a mini-rhizotron system (MR), a system which is designed to slide down a transparent tube fixed with a live mini-camera in the ground and to take pictures of roots belowgound continously. Thus, it helps us observe the root growth over time and trace the single roots in the course of their development.
After two years of data collection from MR system in orange orchard, Israeli scientists revealed that orange trees grown under colored nets not only significantly changed the root distribution and root production compared to trees grown without nets, thereby increased the shoot growth and fruit yield. The physiological reasons behind this is still not exactly understood. But it can be inferred that shoots of trees could sense the light changes and then transfer the light signals to roots. Further, the roots of trees under photoselective netting may promote higher water and nutrient use efficiency in the semi-arid region. Therefore, the roots act like the brain of plants and change the growth pattern according to these light signals. Moreover, photoselective nets reduced the canopy temperature in the month of august, which is the hottest month in the region.
In summary, this study showed that all three kinds of photoselective nets modified both light quantity and quality, thereby promoted rapid establishment of trees and early high-fruit yield in young orange trees. Further, this research study confirmed that yellow nets might be the optimal option for growing orange trees because they showed remarkable yield of orange fruits and they could enable plants to develop deeper root system. Moreover, photoselective nets improved the orange trees physiological performance (photosynthesis rate, vegetative growth and yield). Further studies were carried out to understand the mechanism of how photoselective nets affect shoot and root growth at the physiological and genetic level.
Work done as a master student in irrigation in the Albert Katz International School for Desert Studies, Ben Gurion University of the Negev.
Currently, working as a staff in Ningxia Academy of Agriculture and Forestry Sciences, China. Part time editor of a public account in Wechat media platform: Agriculture of Dryland in Israel. Personal website: http://zhoukaining.weebly.com
(This article was originally published as ‘Special Guest article’ in ‘The Agraria’ e-Magazine. You can Subscribe it here)