Researchers: Lu Zhang, Srijana Panta, Josh Massey, Saleh Taghvaeian
Pecan nut growth has a special stage, water stage, which derives its name from the status of its liquid endosperm (Fig.1). Endosperm is the tissue providing nutrients for the growth of the kernel. Water stage occurs from the end of July through August. Trees require a large amount of water when nuts are at water stage. It was suggested that drought stress during the liquid endosperm stage exacerbates fruit drop and poor kernel fill. Drought stress in summer could also cause the death of the tree as pecan leaves are too thick and rigid to wilt to conserve water under water stress.
Eight inches of irrigation water per month is recommended for mature pecan orchards in Oklahoma (Upson et al., 2011) during the summer months. However, most native pecan growers, as well as some improved cultivar growers, in Oklahoma practice minimal irrigation management due to the expensive cost of installing irrigation systems or due to non-uniform tree lines that prevent the easy installation of pipes. Meanwhile, some growers may waste water and valuable nutrients in the root zone by over irrigating their trees.
At the OSU Cimarron Valley Research Station in Perkins, Oklahoma, we installed WATERMARK soil moisture sensors at 6’’ and 30’’ depths in our pecan experimental orchard. These sensors provide estimates of soil water tension, which is a measure of how tightly water clings to the soil. The drier the soil, the greater the value. The obtained data showed the values for August were less than 30 centibars, which is fairly wet as a value over 40 indicates drought. This finding leads us to consider the following question: can we use the minimum required water to feed nuts at the critical nut development stage in summer?
We conducted an interesting preliminary experiment last August to watch tree physiological reactions (i.e., leaf water potential) to full-irrigation, half-irrigation, and non-irrigation trials. In August, irrigation duration was 48 hours each week applied with micro-sprinklers at 0.73 gallon per minute. The location received 1 inch of rain on August 30. Nuts of each trial were collected at harvest for grading.
Measuring plant water potential by using a pressure chamber is a direct method in evaluating fruit tree water status. Generally, 97 percent of the water in plants is carried through the plant and evaporates from the leaf surface while the other 3 percent is used to support growth and metabolic processes. Transpiration in leaves acts as a force which drags water all the way from root to leaf and then out of plants through the stomata, causing negative pressure inside the xylem. By measuring leaves’ water potential with a pressure chamber, the water status in the plants can be detected. Low water potential reveals that plants have difficulty dragging (transpiring) water from the roots, which means the plants need to be watered. Upon obtaining water potential data, irrigation can be scheduled.
Our data showed that the stem water potential of the no irrigation trial was extremely high (14 bars) compared to the half-irrigation trial (12 bars). In contrast, the water potential found in the full-irrigation trial was 10 bars when tested in the third week of August. After the rain on August 30, the water potential decreased to between 8.5 and 10. This observation reveals an inch of rain in summer can largely alleviate drought stress. It also gives us a clue as to how one or two supplemental irrigation events could make a reasonable difference in nut production and tree health.
The results of nut grading showed the number of brightly colored nuts in the full-irrigation trial was twice as the number of nuts in the non-irrigation trial. Drought conditions at the nut’s water stage is one of the most important factors determining the nut quality. Trees with no-irrigation treatment showed an observably poorer yield than trees under full- and half- irrigation schedules.
This experiment highlights the importance of summer irrigation and how it facilitates nut quality and production, but since the yield of trees in half-irrigation did not decline, proper water reduction could be viable for some orchards. This research also shows growers with native rain-fed groves supplemental irrigation’s potential benefits.
Literature Cited: Upson, S., Rohla, C., Locke, J., and Springer, J. 2011. Pecan Production 101. Establishing and Managing an Improved Variety Pecan Enterprise in the Southern Great Plains. The Samuel Roberts Noble Foundation Agricultural Division. pp. 5.