Submitted to: Subtropical Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2011
Publication Date: N/A
Citation: Interpretive Summary: Giant reed is a non-native, invasive giant grass in the dry climate of the Lower Rio Grande Basin of Texas and Mexico. This harmful grass weed outcompetes native vegetation and removes water along the Rio Grande and associated rivers and reservoirs, thereby, depriving agricultural and domestic water uses of millions of dollars worth of water per year. Giant reed also hinders international border enforcement and fuels wildfires along the Rio Grande. Scientists at the USDA-ARS in Weslaco, Texas, have released two insects to feed on giant reed to reduce its ability to grow and reproduce, a technique called biological control of weeds. One way to measure the impact of these insects is to take the weed's 'pulse' by using a light meter clamped to the leaf blade, or by using airplanes. The meter or airplane determines how much light the giant reed plant is absorbing and how much it is reflecting. Light absorption is a good measure of the plant's ability to convert light energy into building blocks for growth through photosynthesis. Giant reed plants that are being fed upon by the biological control insects are expected to absorb less light and reflect more light than healthy plants, and can thus be expected to grow less or not at all compared to healthy plants. The light meter clamped to giant reed leaves at ground level helps to verify or 'calibrate' the readings taken from the airplane. A limitatiion of this approach is that the light meter equipment is fragile and bulky, and thus difficult to carry into the thick stands of dense giant reed found in remote areas along the Rio Grande, some of which are accessible only via rough roads requiring 4-wheel drive vehicles. A possible solution is to cut leaves off of the giant reed plants, quickly take them on ice back to the lab, and use the light meter clamp there. This study demonstrated that giant reed leaves can be transported in plastic bags, but not paper bags, and may be transported and stored at either room temperature or on ice for up to 96 hours after cutting, with little change in the light reflectance readings compared to giant reed leaves that were stored under similar conditions and were still attached to a plant in the laboratory the whole time. The results validate the approach of sampling giant reed plant health via leaf detachment in remote field locations.
Technical Abstract: Research was conducted to evaluate the effects of leaf excision and sample storage methods on spectral reflectance by foliage of giant reed, Arundo donax, an invasive weed which has caused extensive damage in many areas of the Rio Grande Basin in Texas and Mexico. Within 24 hours of excision, A. donax leaves exposed to ambient laboratory conditions (room temperature under natural lighting conditions) exhibited two trends indicative of physiological stress: 1) small but significant increases in reflectance of blue and red wavelengths (400-500 nm and 600-700 nm, respectively) and 2) a substantial reduction in reflectance of near-infrared (NIR) wavelengths (700-1,100 nm). A similar but less pronounced trend was evident among leaf samples held within conventional paper sacks. Leaf samples held within sealed plastic bags under two types of lighting conditions (natural light and artificial darkness) and temperature regimes (room temperature vs. artificially cooled) exhibited slight but significant increases in both visible and NIR wavelengths (a trend that was also evident in attached foliage), although no evidence of physiological stress was detected during a 96-hour observation period. These trends indicate that accurate spectral measurements may be obtained from samples of A. donax foliage for periods up to 72-96 hours following excision if such samples are transported and maintained in suitable containers designed to minimize effects of desiccation.