Pollen viability under Delhi conditions
--B.M. Prasanna

Interesting observations on extended viability of pollen under specific conditions were made in MNL by Barnes and Walbot (MNL 67:106) and Burnham (MNL 67:101). I have attempted to ascertain the viability of maize pollen under Delhi conditions. The pollinating season in Delhi is characterized by medium to very high humidity (60-90%) with field temperatures ranging 32-40 C during the peak pollination period (last fortnight of August to first fortnight of September), with frequent occurrence of mild to moderate showers due to the southeast monsoon. Pollen viability under these conditions was analyzed by two methods: (i) pollinations in vivo of selected inbred lines and genetic testers; and (ii) in vitro germinability of pollen collected from the field.

(i) in vivo: Two sets of material were used for this purpose: (a) Two inbred lines (6914, an unreleased line, and MCU 508, a released line) carrying y (white endosperm) marker and CM 111, an inbred line with Y (yellow endosperm) as a male parent; (b) homozygous R-mb:cc lines as female lines in crosses with lines carrying R-Navajo (R-nj) marker in homozygous condition. In both cases, the female lines were detasseled, pollen was collected from the male parents at different timings of the day (8:00 AM, 11:00 AM, 3:00 PM and 6:00 PM) and fractionated in a glassine bag to exclude debris, and generous amounts of the sieved pollen were used for pollinations on at least 5-6 detasseled plants each (with similar silk age) for each treatment (day timing). Since R-nj displays full penetrance when transmitted through the male parent in crosses with R-mb:cc (unlike R-mb:cc which shows a drastic reduction in penetrance and expressivity when used as a male parent), R-mb:cc x R-nj crosses were expected to yield kernels with either nj+mb:cc or Navajo phenotypic expressions.

The results obtained (data not shown) from pollinations done in the last week of August indicated that the pollen of CM 111 retained a considerable amount of viability till the evening hours even on days with considerably high mid-day temperatures (35-40 C). This viability was further extended on days (August 28 and 29) which coincided with showers during the normal pollination period, since pollinations carried out even at 6:00 PM resulted in good seed set (with 281 av. kernels/ear vs. 368 av. kernels/ear control for pollinations done at 8:00 - 8:30 AM). In the case of R-mb:cc x R-nj crosses, extended pollen viability was observed on rainy days. However, on the normal days (clear sky with high temperatures and humidity), the average seed set was almost negligible when pollinations were done during the evening hours. The presence of the Navajo marker in the male parent aided in excluding the possibility of contamination.

(ii) in vitro pollen germination ability: Pollen germinability of CM 111 and MCU 508 was evaluated to ascertain the extent of germination in pollen collected at regular intervals (8:00 AM, 11:00 AM, 2:00 PM and 5:00 PM) from the field-grown plants. Although both CM 111 and MCU 508 germinate well in vitro, under normal conditions, in Walden's in vitro pollen germination medium, pollen germinability cannot be considered as an accurate measurement of the capacity to bring about fertilization. Therefore, this assay has been used only to supplement the observations made in vivo. The in vitro study was carried out on three successive days in the first week of September. Pollen germinability at different day timings differed significantly in the two inbred lines. A comparison of the percent change in pollen germination at successive timings (in comparison with the observation at 8:00 AM, when the pollen is expected to be fresh and mostly viable) revealed that a considerable amount of viable pollen can be obtained even at 2:00 PM and 5:00 PM in the case of CM 111, unlike MCU 508. The decline in pollen germination in MCU 508 was also sharper than CM 111. The above observation suggests that under certain climatic conditions, maize lines might show considerable differences with respect to pollen viability. Definitive reasons for this are difficult to offer. Probably, a combination of factors such as the genetic constitution of the line, tassel form (compact or lax) and, plausibly, anthocyanin coloration in plant parts may have a role to play. 

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