<br><div><section class="_135editor" data-role="paragraph" style="font-size: medium; max-inline-size: 100%; margin: 0px; padding: 0px; border: 0px none; z-index: 0; overflow-wrap: break-word !important; outline: none 0px !important;"><p style="max-inline-size: 100%; margin-bottom: 0px; clear: both; min-height: 1em; cursor: text; line-height: 1.75em; overflow-wrap: break-word !important; outline: none 0px !important;"><span style="max-inline-size: 100%; margin: 0px; padding: 0px; cursor: text; font-size: 15px; overflow-wrap: break-word !important; outline: none 0px !important;">　　棉花连续多年自留种一般导致品种退化，使种性降低、变劣，从而降低产量和品质。首先，棉花是常异花授粉作物，天然异交率一般为２％～１５％，个别情况甚至超过５０％。异交必然使基因发生重组，改变品种群体的遗传型，导致品种退化。其次，棉花吐絮期长，需要分次采摘，专门加工，程序和技术环节复杂，容易发生机械混杂，而机械混杂可加速品种的退化。退化一般使品种变劣，产量降低，品质变差，抗逆性变弱。具体表现为：棉株或高大松散，或矮小细弱，或枝叶繁茂结铃性差，脱落率常比正常棉株高；铃形往往不典型，由圆变长，铃嘴变尖，且株间不一致，铃变小，五室铃比例减少，铃重下降，铃壳变厚，吐絮欠.等；常出现异形子，如多毛大白子、绿子、灰褐子、稀毛子、光子、小子等，而且子指变幅很大，种皮一般稍厚；衣分下降明显；绒长往往变短，变幅多在１～２毫米之内，纤维整齐度变差；成熟期个体间则往往表现早晚不一；抗逆性降低，病株率上升，病情指数增高，抗虫品种的非抗虫株率增加，综合抗虫性下降。</span></p><p style="max-inline-size: 100%; margin-bottom: 0px; clear: both; min-height: 1em; cursor: text; overflow-wrap: break-word !important; outline: none 0px !important;"><br style="max-inline-size: 100%; margin: 0px; padding: 0px; overflow-wrap: break-word !important; outline: none 0px !important;"></p><p style="max-inline-size: 100%; margin-bottom: 0px; clear: both; min-height: 1em; cursor: text; line-height: 1.75em; overflow-wrap: break-word !important; outline: none 0px !important;"><span style="max-inline-size: 100%; margin: 0px; padding: 0px; cursor: text; font-size: 15px; overflow-wrap: break-word !important; outline: none 0px !important;">　　由于以上原因及结果，生产上应当定期更换由科研单位或专门企业经过一定科学程序提纯复壮的原种或原种三代以内的优良品种。通常情况下，原种三代后不应再用作种子。</span></p><p style="max-inline-size: 100%; margin-bottom: 0px; clear: both; min-height: 1em; cursor: text; overflow-wrap: break-word !important; outline: none 0px !important;"><br style="max-inline-size: 100%; margin: 0px; padding: 0px; overflow-wrap: break-word !important; outline: none 0px !important;"></p></section><section class="_135editor" data-role="paragraph" id="autoparagraph" style="font-size: medium; max-inline-size: 100%; margin: 0px; padding: 0px; border: 0px none; z-index: 0; overflow-wrap: break-word !important; outline: none 0px !important;"><p style="max-inline-size: 100%; margin-bottom: 0px; clear: both; min-height: 1em; cursor: text; overflow-wrap: break-word !important; outline: none 0px !important;"><br style="max-inline-size: 100%; margin: 0px; padding: 0px; font-family: 微软雅黑, &quot;Microsoft YaHei&quot;, sans-serif; font-size: 17px; caret-color: rgb(255, 0, 0); overflow-wrap: break-word !important; outline: none 0px !important;"></p></section></div>