HEAT STRESS TEST BRASSICA SPP. Region IV Southwest 2014

Introduction Region IV, Southwest, conducted a referee that was a vigor test in the form heat stress testing on Brassica sp., specifically Broccoli. Heat stress testing is a fairly narrow assessment of seed vigor, which has a practical application for many crops being produced in hot dry climates.

Objectives The purpose of the study was to was to see if different labs could conduct heat stress tests using the same methods and arrive at similar results, across several different temperature regimes. This would hopefully lead to pinpointing a 'breakout' temperature that would indicate an area where seed lots reach a tipping point between acceptable and unacceptable germination. It could result in developing a standard vigor assessment temperature for Brassica spp.

Materials and Methods Two samples of Broccoli seed were sent to eight laboratories. Both samples were known to have high germination under optimum conditions. One was known to perform very well under heat stress conditions, while the other was known to perform questionably under heat stress. Both samples were tested under the standard procedures (20-30°C), as well as at several stress temperatures, 31°C, 33°C, and 35°C

Only one stress temperature was mandatory for this exercise, 33°C Only one stress temperature was mandatory for this exercise, 33°C. Two other temperatures were optional, 31°C and 35°C. The tests were done in rolled or folded germination paper or blotter paper. This is favorable for this kind of test because it allows for good elongation of the roots, which tends to be a critical area of evaluation for Brassica stress tests.

The tests were all prepared in the same manner and treated like normal germination tests. They were evaluated according the AOSA seedling evaluation standards for Brassica spp. Only the final counts of normal seedlings were used in the assessment.

Test Results Sample A Lab 5 Lab 7 Lab 8 Lab 9 Lab 10 Lab 11 Lab 12 Standard test, 20-30°C 99 98 97 96 91 Stress test 31°C   Stress test 33°C 89 79 85 46 3 93 Stress test 35°C 25 59 74 20 62 Sample B Lab 5 Lab 7 Lab 8 Lab 9 Lab 10 Lab 11 Lab 12 Lab 14 Standard test, 20-30°C 99 97 94 Stress test 31°C 96   Stress test 33°C 88 60 71 63 6 90 93 Stress test 35°C 18 54 23 72

Sample A All tests

Sample B All tests

Sample A 20-30°C and 33°C Only

Sample B 20-30°C and 33°C Only

Sample A

Sample B

Discussion 8 of 8 participating labs completed the standard tests and the 33°C stress tests. 5 labs additionally completed the 35°C tests, and 2 labs were able to complete all four. All of the 8 labs got very similar results for the standard tests of both samples, showing them to be of high germination, as expected. The two labs that tested at 31°C also got similar high results.

The 33 and 35°C stress tests were highly variable, which leads to a few observations and topics of discussion. Predictable variability The stress temperatures of this study are in a range where there is a tipping point between acceptable germination and development, and crop failure. The object was to try to pinpoint this critical area, so it was expected that there would be some variation of test results near this point.

Germination chamber precision Using a germinator that is capable of maintaining very precise and accurate temperatures is critical to heat stress testing. A slight variation in temperature can lead to a wide variation in results. Many standard germination chambers will fluctuate too much in temperature, or may not be perfectly calibrated. It was not required to log and report the temperatures in these tests. This would be very helpful in future studies.

Evaluation Brassica seedlings grown under heat stress conditions are subject to several developmental challenges, particularly with the roots. The evaluation standards do not fully address the issue of minimum root length. There will naturally be some differences in judgment in this area. This may point to the necessity of adopting a standard minimum measurement, not only for the roots but for overall growth, as in the cotton cool test.

Although the results of the stress tests at the critical temperatures were highly variable, it was a valuable exercise in that it highlighted areas where this requires improvement. Future such referee tests should include mandatory monitoring and logging of germinator temperatures. Germination chambers with a range of more than +.3°C should probably not be used for heat stress tests. All labs should run all of the stress temperatures, and perhaps other temperatures should be included. Having some standards of seedling development, such as overall length or root to hypocotyl ratio, would also be helpful.

Questions, Comments? Scott Taylor, RST scott@ammseedtesting.com AMM Seed Testing, Inc. 2064 Alameda Padre Serra, Suite 110 Santa Barbara, CA 93103 805.564.2155