A simple nondestructive method to continuously measure plant water content has long been sought in the study of both soil-water-plant relations and the impact of environment on plant growth. A number of methods have been proposed to measure plant water content, and most of these require indirect measurement of other variables, such as leaf water potential and/or leaf water content, to infer plant water content. These methods are destructive, consuming plant tissue and providing only intermittent and localized measurements. In practice, a non-destructive, frequent measure of water content of a whole plant is needed. One way to quantify the water content of a whole plant is to measure the change of stem diameter, which has proven successful for fruit trees. In this paper we experimentally investigate the responsive change of stem diameter of the eggplant to plant water content and soil moisture in a greenhouse.
The experiment was conducted at Xinxiang, Henan Province, China (latitude 35.19° N) in a greenhouse of 40 m long and 8.5 m wide. It is a sub-humid area, susceptible to drought. Eggplants (Solanum melongena) were cultivated in pots and a small plot respectively, both in the greenhouse. The soil property in the pots and the plot are: organic matter 18.85g/kg, total-N 1.10g/kg, total-P 2.22g/kg, available-N 15.61mg/kg, available-P 72.0mg/kg, available-K2O 101mg/kg, soil bulk density 1.38kg/cm3, and a field water capacity of 24%. The experiment was designed using a two-factor randomized-block method by taking soil moisture content and growing stages as variables. The soil moisture content was controlled at 80%, 70%, 60% and 50% of the field water capacity, and growing stages that were chosen were seedling, flowering and fruit-forming, and harvesting stages; each treatment having three replicates. The pots were weighed and watered daily to minimize soil moisture change, and the soil water content of the small plot was monitored using Time Domain Reflectrometry and drying-weight method respectively at five-day intervals. The change of stem diameter was measured continuously using a stem diameter sensor (DD-type) linked to a date logger. The measurement started when the stem diameter was large enough to hold the stem diameter sensor. The sensor was attached to the stem of each plant approximately10-15 cm above the soil surface, and the measurements were taken automatically at an interval of either 10 min or 30 min. The data stored in the data logger was downloaded to a microcomputer after three or five days. The leaf water potential was measured at hourly intervals with a pressure chamber (ZLZ-4), each measurement was taken on two leaves. The leaf relative water content was measured with the weighing method. Transpiration rate and stomatal conductance were measured by a Portable Photosynthesis System (CIRAS-1 type). All the measurements were made simultaneously at sunny days from 08:00 to 18:00 to minimize the diurnal variations. Air temperature, relative air humidity, net solar radiation and other atmospheric factors were taken from a standard meteorological station installed in the greenhouse during the growing season from 2002 to 2004.
The results showed that on all sunny days the stem diameters shrank in the daytime and returned to their original size at nights, regardless of plant water content. The degree of shrinking and swelling of the stem diameter was closely related to plant water content and soil moisture content. Results from the small-plot experiment indicated that the change of stem diameter was positively related to vapor pressure deficit in the greenhouse with a correlation coefficient of 0.8938. The diurnal change of stem diameter was closely related to the diurnal change of leaf water potential and relative leaf water content with correlation coefficients 0.867 and 0.965 respectively (p=0.01). These results suggest that the change of stem diameter can be seen as an indicator of plant water content and hence be used as a simple non-destructive method to continuously measure plant water content.